Hello everyone.

I'm in the middle of planning for a residing of my house.  I live in the Detroit area of Michigan (so southeast of michigan - zone 5) and I'm trying to figure out the best way to reside and improve the house energy efficiency.

I've been reading through a bunch of articles and forums, but was hoping I could get some advice.  It seems there are some very good posting here and love the dialog that goes on about insulation trends and condensation issues.

So I am going to be residing and taking off the existing boards and trim.  My house was built in 1993 and has normal 2x4 construction.  Due to the age of the building I'm not sure if there is a homewrap on there.  I'm planning on putting then homewrap, some level of rigid foam, furring strips and then installing the fiber cement siding.    Since I will not be doing anything to the interior of the house, I am left with dealing with the fact there is low density R13 kraft faced batt insulation in the 2x4 walls.

My issues are that I don't want to create a moisture problem due to rigid foam and kraft paper causing a vapor sandwich.  I had been thinking of putting 1" of polyiso, but then remembered the kraft paper and so am thinking of switching to just 1" or 1.5" of XPS.    I see that the minimum R value is R5 for my zone to deal with condensation issues.  I'd like to be over the minimum, but have to deal with retrofitting and extending the window sill and moulding.  I also have to deal with the lower half of the wall being brick so the interaction between the top siding and the bottom brick needs to look clean at that joint, meaning I really can't go too much beyond 1" of insulation, 1" of furring strips or the siding will come out over the brick ledge.

I was wondering your thoughts on this install and mainly for just adding 1" of XPS to the outside and what to expect due to the kraft faced batt insulation, my climate area and the risks of condensation for this project.

Thanks much.

Alan

1-1.5" of XPS and a rainscreen cavity between the foam & fiber-cement siding will give it plenty of drying capacity without creating a moisture trap.

Per IRC 2009 just with the rainscreen gap you'd have enough exterior drying that would have allowed a type-III vapor retarder on the interior.  Kraft facers are more-restrictive type-II vapor retarders, so it does allow at least SOME drying toward the interior.   The permeance of 1" XPS is about 1.2 perms, making it a type-III vapor retarder, but at 1.5" it becomes a minimal type-II (a rather arbitrary distintion of being just under 1 perm).  At 1" XPS  allows ~3x the moisture transfer of a kraft-facer, so as long as you have a vented rainscreen the sheathing to pass that moisture into the sheathing can still dry relatively rapidly to the exterior come spring, and with the R5/R13 foam/fiber ratio AND kraft facers on the interior the amount of moisture absorbed in winter would be minimal anyway.  You'd only be at some risk if you went with foil faced, but even that risk low with kraft-faced compared to foil-faced batts or poly vapor barriers. Foil + poly = classic  moisture trap, but not kraft + 1-2" XPS.

XPS has at least some shrinkage issues over time (it was a lot worse 30 years ago than it is now), so using ship-lap XPS keeps that shrinkage from creating stripes of R0 in 20 years.  Seal the edges with 1-part gun foam, seal the seams with housewrap tape or duct-mastic.

Dana-

You said: "Per IRC 2009 just with the rainscreen gap you'd have enough exterior drying that would have allowed a type-III vapor retarder on the interior. Kraft facers are more-restrictive type-II vapor retarders, so it does allow at least SOME drying toward the interior." Did you mean to say, "... Kraft facers are less-restrictive type-II vapor retarders..."?

I was interested in your response to the OP as I have a daughter's house in Connecticut that we may want to eventually add esterior foam insulation to. It is a 1940's era house, and I think that it has fiberglass insulation of some type in the walls from what she has reported.

Posted By Lee Dodge on 12 Mar 2012 12:11 PM
Dana-

You said: "Per IRC 2009 just with the rainscreen gap you'd have enough exterior drying that would have allowed a type-III vapor retarder on the interior. Kraft facers are more-restrictive type-II vapor retarders, so it does allow at least SOME drying toward the interior." Did you mean to say, "... Kraft facers are less-restrictive type-II vapor retarders..."?

I was interested in your response to the OP as I have a daughter's house in Connecticut that we may want to eventually add esterior foam insulation to. It is a 1940's era house, and I think that it has fiberglass insulation of some type in the walls from what she has reported.

Yep- you got it! Kraft facers are Type-II vapor retarders, MORE restrictive to vapor diffusion than Type III vapor retarders (eg: latex paint) that would be allowed with just a rainscreen, but far less restrictive than poly or foil (Type-I vapor retarders.)

Asphalted kraft facers run ~0.4 perms...

... to latex paint's ~2-4 perms... (4-10x faster drying than a kraft facer)

...or 6 mil poly/ 0.35 mil foil's ~0.05 perms... (only 1/8 the drying capacity of a kraft facer)

...or 1 mil foil's 0.001 perms  ("Drying? Fuggedaboudit! Not in your lifetime!)

Most of CT is on the warm edge of US climate zone 5, and with air-tight construction and dense-packed fiber you really can get away without vapor retarders (even though not code-sanctioned), but on brick/stucco/stone clad buildings you would absolutely need a vented cavity.  If you're going so far as stripping the siding it's worth pounding in cellulose at 3lbs+ density into the cavities, even if you leave the existing batts in place, since that will limit conditioned-space air leaks into the cavity and buffer any moisture accumulation in the cellulose rather than the framing or the sheathing.

When retrofitting exterior foam you need to know if the existing fiberglass batts had foil facers and/or if a separate poly or foil vapor retarder was installed. If there are none you can use whatever you like for exterior foam, but somewhat-permeable on the exterior is generally preferable than impermeable.   A kraft facer on the fiberglass isn't much of  problem, but foil or poly is.  Still, no matter what, under vented cladding the more vapor-open the exterior foam is, the better.  The thermal performance difference between 1" XPS (R5) and 1" foil-faced iso (R5.6, when derated for sub-25F average winter temps) but the  difference in drying capacity is more than 100x, so in the O.P.s case XPS is the safer choice by quite a bit.  With only paint as the interior vapor retarder it would be a tossup- the wintertime moisture loading would be slightly higher, with the more-open interior but the drying rate to the interior would be at least 5x higher when the weather got warmer- either way it would be just fine.

If the siding is unvented/barely-vented siding keeping it less than 5 perms on the exterior is a good idea to resist moisture drives of sun on dew/rain wetted siding.  If masonry or stucco it's better keep it under 2 perms unless you have at least 3/8"  of rainscreen cavity, vented top & bottom.

Some of the very high density (~8lbs/cubic foot) rock wool panels look pretty good for higher-R exterior insulation retrofits where poly or foil is on the interior, since they're comparable or less air-permeable than dense packed cellulose, yet orders of magnitude more VAPOR permeable than even Type-I EPS, with a comparable R/inch to EPS. At 4" most EPS is well under 1 perm, but the rock wool is still 10+, which means you can load up the exterior R and still have a stackup that dries freely to the exterior. 

XPS is only good up to R10 before it's too vapor-tight, at which point best-practices dictates a more vapor-open interior.   EPS is good up to R14-16. Single-layers of some (read the specs) fiber-faced iso of any thickness is fine, but doubling up can create a problem.  With high density rock wool panels the sky (or the mechanical weight issue) is the limit.  Also, unlike foam they can't shrink, deform with heat, or burn.  I haven't seen pricing info on them yet though.
 

Up to now I've been recommending single-layer 2-4" of fiber faced roofing iso in retrofits with existing Type-I interior vapor retarders in a zone-5 climate.  One pair of fiber facers is about as vapor-tight as 1.5-2" XPS, but with iso you're slightly higher R/inch, and the perm rating is the same at 4" as it is at 1".  With 4" of iso on a 2x4 fiber insulated wall the fractional R and average temp of the iso is higher than in the 1" case, so it'll perform better than a derated R5.6 inch- in US zone 5  the 4" iso performs at about R24, and on a 2x4 studwall with 20% framing fraction you're looking at R33-R35 for whole-wall R, cutting the heat loss through walls by 2/3 from where it started.  But just that first 1" of XPS or iso cuts the conducted heat loss through a typical 2x4 wall by about 1/3 from where it started, and at 2" it's been cut in half.  Whether the next 2"+ is worth it depends on what the design goals are, and where lower-hanging fruit might be.  At 2" of exterior iso you have an R20 wall, at which point dealing with an uninsulated foundation or window upgrades etc. may start looking like better investments than taking it up to 4"+ thicknesses on the exterior foam.  

Regionally/locally in MA/CT there are several sources for reclaimed roofing foam though, which can shift the economics toward thicker foam than if going with virgin-stock.  Typical pricing is 1/4-1/3 of new goods, but the deals come & go, and you can't necessarily rely on being able to pull your truck up at any random time and get the thickness/quantity/type that you want. It's always going to be worth calling around when you know your construction schedule though. Insulation Depot in Framingham MA is probably the biggest and best-stocked, but there are others, some closer to the CT/MA line than that.

Thanks for the detailed information. Yeah I've read about those shrinkage issues. I'm contemplating using two layers of foam, just to limit that seam issue.

It's interesting to see different viewpoints. I keep going back to a comment post by Ted Clifton over on the GBA blog titled 'Musings of an Energy Nerd' about the overview of the 2012 energy code. I'm not entirely sure who Ted is, but he seemed to have his own viewpoint:
"I have run a number of calculations, using real-time data, that show clearly that using just 1" of XPS foam (R-5) on the outside of a 2x6 wall in a cold moist climate (like that around the Great Lakes) will cause moisture to condense on the inside of the wall sheeting. It will remain as condensation for months at a time, especially on the north side of the house. The Washington State Energy Code made this same mistake two years ago. They ran all their tests in Western Washington (coastal zone 4), where the winter temperatures are much milder. Of course it worked fine here, but in Eastern Washington (zone 5, a little bit of zone 6) the first condensing surface is inside the fiberglass portion of the wall. History has proven that OSB can be a very good petri dish."

It's hard to keep it all straight. He doesn't specifiy what is on the inside so I guess i could discount his measurements in that they may have included any number of factors. As I'm in the great lakes area it made me worried though. I guess I should go back and see really how much difference in the wall it would be with 1.5" of XPS.

Posted By alang on 12 Mar 2012 03:37 PM
Thanks for the detailed information. Yeah I've read about those shrinkage issues. I'm contemplating using two layers of foam, just to limit that seam issue.

It's interesting to see different viewpoints. I keep going back to a comment post by Ted Clifton over on the GBA blog titled 'Musings of an Energy Nerd' about the overview of the 2012 energy code. I'm not entirely sure who Ted is, but he seemed to have his own viewpoint:
"I have run a number of calculations, using real-time data, that show clearly that using just 1" of XPS foam (R-5) on the outside of a 2x6 wall in a cold moist climate (like that around the Great Lakes) will cause moisture to condense on the inside of the wall sheeting. It will remain as condensation for months at a time, especially on the north side of the house. The Washington State Energy Code made this same mistake two years ago. They ran all their tests in Western Washington (coastal zone 4), where the winter temperatures are much milder. Of course it worked fine here, but in Eastern Washington (zone 5, a little bit of zone 6) the first condensing surface is inside the fiberglass portion of the wall. History has proven that OSB can be a very good petri dish."

It's hard to keep it all straight. He doesn't specifiy what is on the inside so I guess i could discount his measurements in that they may have included any number of factors. As I'm in the great lakes area it made me worried though. I guess I should go back and see really how much difference in the wall it would be with 1.5" of XPS.

"Around the Great Lakes" covers everything from  warm to mid-range  US zone 5 up through US zone 7, so that's not very useful information, even if we assumed a 2-3perm paint-only interior.  According to the IRC Table N1102.5.1 you'd need R7.5 on the exterior of his 2x6 walls even in zone 5, and WELL under the R11.25 minimum spelled out for 2x6 construction in zone 6 (let alone the R15 needed for zone 7).  So in the MI-UP or WI you BET  R5 wouldn't cut it using only latex paint on the interior!

Yet you have 2x4 construction and kraft facers.  It's the ratio of foam-R to fiber-R and the mean wintertime outdoor temps that will determine whether your sheathing is capable of loading up with moisture with a fairly vapor-open interior (and you don't even have that vapor-open interior.) This is why only R5 is required for 2x4 in the same zones where R7.5 is spelled out for 2x6 construction. R5(foam)/R13(fiber) is about the same ratio as R7.5/R20. So R5 really IS just fine for you, but that doesn't mean going for more foam R isn't a good idea, even if it's not essential:

The mean winter temp in Detroit is about 25F, and the dew point of 30%RH/70F interior air is about 38F. As long as the sheathing is at or above 38 F in those conditions, it won't accumulate winter time moisture even from minor air leaks. Assuming R13 cavity insulation and R5 foam , the total center-cavity R is R18, and the average winter temp at the sheathing if 5/18 of the way between 25F and 70F.  That's 25F + (70-25)*5/18=37.5F, so R5 is really the absolute minimum.  If you bump it to 1.5" or R7.5 the center-cavity total is R20.5, and the mean temp at the sheathing is 25F + (70-25) *7.5/20.5=41.5F, and you have some real margin.

In my own designs I tend to use the January mean temp for the zip code (which is a degree or three lower than the winter-average), and assume a 40F dew point, which builds in a rock-solid margin without going too nuts on foam-R.  R7.5 on 2x4 framing is PLENTY of margin for Detroit.

It really doesn't matter if the moisture stays in the sheathing during the cold months, as long as it can leave quickly when it warms up, which is what you get with a rainscreen cavity to vent into. Under a low-perm or unvented siding that rapid drying can't happen, so the more foam-R you have, the lower its permeance, the slower the drying rate, but the more foam you have the higher the sheathing temp too. But in any cavity it's better to have 1-perms of total of drying capacity (adding up the permeance of both interior and exterior surfaces) to be able to tolerate minor leaks like wind-blown moisture getting around flashing, etc without causing problems, and that's about where you'd be with 1.5" exterior XPS and a kraft facer on the interior. (With foil-faced R10 iso and a 2-3 perm interior you'd have 2-3 perms of total drying capacity, and essentially zero condensing hours at the sheathing, but it's not cost-effective to rip out the kraft facers, eh?)

BTW: I mis-stated the permeance of XPS- it depends somewhat on density, but the most commonly used XPS sheathing is specified at 0.8 perms @ 1" (most 2lb spray foam is on the order of 1.2 perms, which is where my caffeine-starve memory was taking me :-) )  At 2" the vapor diffusion rates through XPS is about the same as that of a kraft facer, at 1" it's ~2x (and not ~3x.) So at 1.5" you're at ~ 0.6 perms on the exterior, plus 0.4 perms on the interior for about a 1 perm total. Going lower than that becomes ever more like a moisture trap.)

If you retro fit the wall cavites from the exterior with cellulose before putting up the foam, any wintertime moisture that gets stored in the wall cavities stays in the cellulose, not the structural wood, and it drastically reduces the amount of air-transported moisture that can get in via leaks from the interior.  There are homes in central Saskatchewan that were insulated with cellulose as far back as the 1920s that are still standing, with no vapor barriers and no exterior foam.

HI Alan,

I'd look into injecting foam into the 2x4 cavity which compresses and replaces the fiberglass and give you a net increase in R and a greatly increased air drift barrier as well. I was reading that fiberglass is the most overly rated and promised insulation on the market due to the way it is measured. It is measured in zero wind and zero dampness conditions which are unrealistic. When temps get below 30 d F and or Winds are 20 mph fiberglass R value disappears almost completely.

Cellulose loses when it gets wet but it stays effective at super cold temps although it is only slightly better than fiberglass in wind.

I think the only real solution is open or closed cell foam. I'd go closed cell if I had the option otherwise Open cell. I would take the money saved by not paying the siding contractor to insulate your house and put it towards injection insulating the walls, then put the nice new siding up without the cost of the wrap nor the insulation outside Unless you can afford to do both.

I got a bid to do a 1100 sqft brick house and it worked out to $1 per inch foot installed. So $1x3.5" x 8' wall x 1'=$28 so about $28 per linear foot around your outside walls. My house was 120' so it came to about $3000 to do just the walls. Thing is since it is the highest r value I can have, I never have to do it again.

Posted By Dana1 on 12 Mar 2012 06:23 PM


In my own designs I tend to use the January mean temp for the zip code (which is a degree or three lower than the winter-average), and assume a 40F dew point, which builds in a rock-solid margin without going too nuts on foam-R.  R7.5 on 2x4 framing is PLENTY of margin for Detroit.

It really doesn't matter if the moisture stays in the sheathing during the cold months, as long as it can leave quickly when it warms up, which is what you get with a rainscreen cavity to vent into. Under a low-perm or unvented siding that rapid drying can't happen, so the more foam-R you have, the lower its permeance, the slower the drying rate, but the more foam you have the higher the sheathing temp too. But in any cavity it's better to have 1-perms of total of drying capacity (adding up the permeance of both interior and exterior surfaces) to be able to tolerate minor leaks like wind-blown moisture getting around flashing, etc without causing problems, and that's about where you'd be with 1.5" exterior XPS and a kraft facer on the interior. (With foil-faced R10 iso and a 2-3 perm interior you'd have 2-3 perms of total drying capacity, and essentially zero condensing hours at the sheathing, but it's not cost-effective to rip out the kraft facers, eh?)

Yeah too much to do on the inside at least.  I did that in one room that had some window issues, but it was necessary.  I replaced that area with closed cell foam last year.  This area is behind the brick venear area, so I was careful to keep the foam in the brick area so in case I did retrofit the outside siding portion it wouldn't clash by having a foam/foam combo.   I had one area that I also did upstairs, but replaced it with similar batt insulation about 8 years back.  In hindsight I should have done that differently.

Posted By Dana1 on 12 Mar 2012 06:23 PM

BTW: I mis-stated the permeance of XPS- it depends somewhat on density, but the most commonly used XPS sheathing is specified at 0.8 perms @ 1" (most 2lb spray foam is on the order of 1.2 perms, which is where my caffeine-starve memory was taking me :-) )  At 2" the vapor diffusion rates through XPS is about the same as that of a kraft facer, at 1" it's ~2x (and not ~3x.) So at 1.5" you're at ~ 0.6 perms on the exterior, plus 0.4 perms on the interior for about a 1 perm total. Going lower than that becomes ever more like a moisture trap.)

Ok.  Good to know about the 1perm limit.

Posted By Dana1 on 12 Mar 2012 06:23 PM


If you retro fit the wall cavites from the exterior with cellulose before putting up the foam, any wintertime moisture that gets stored in the wall cavities stays in the cellulose, not the structural wood, and it drastically reduces the amount of air-transported moisture that can get in via leaks from the interior.  There are homes in central Saskatchewan that were insulated with cellulose as far back as the 1920s that are still standing, with no vapor barriers and no exterior foam.

Hmm.  I did talk to one contractor about doing this.  I was afraid the cellulose wouldn't blow in right and I would be left with a weird crushed batt/ partially filled wall.  And my effective insulation would be hard to predict.  Have people had luck blowing in cellulose from the outside?  I'm assuming they leave the existing batt's in  there, but I guess it might be possible to remove the batts from the outside as well.  I can't see that being done most times though.

Posted By Tony_Scarpelli on 12 Mar 2012 08:30 PM
HI Alan,

I'd look into injecting foam into the 2x4 cavity which compresses and replaces the fiberglass and give you a net increase in R and a greatly increased air drift barrier as well. I was reading that fiberglass is the most overly rated and promised insulation on the market due to the way it is measured. It is measured in zero wind and zero dampness conditions which are unrealistic. When temps get below 30 d F and or Winds are 20 mph fiberglass R value disappears almost completely.

Cellulose loses when it gets wet but it stays effective at super cold temps although it is only slightly better than fiberglass in wind.

I think the only real solution is open or closed cell foam. I'd go closed cell if I had the option otherwise Open cell. I would take the money saved by not paying the siding contractor to insulate your house and put it towards injection insulating the walls, then put the nice new siding up without the cost of the wrap nor the insulation outside Unless you can afford to do both.

I got a bid to do a 1100 sqft brick house and it worked out to $1 per inch foot installed. So $1x3.5" x 8' wall x 1'=$28 so about $28 per linear foot around your outside walls. My house was 120' so it came to about $3000 to do just the walls. Thing is since it is the highest r value I can have, I never have to do it again.

Injecting the foam from the inside or the outside?  I would loose some stud cavities from the inside.  Is this the slow rise stuff?  I was worried as well that the spray foam (like the cellulose spray I mentioned to Dana) wouldn't totally compress the batt insulation.  I guess your saying it does and then I'd be left with  3.5*7 = R24 right?  I have 1500 sq feet of exterior walls that will be touched.  So assuming 10' sections, I guess that's about 150 linear feet = $4200.  I'm not totally sure of costs, but it seems no matter what I do during this re-siding it will cost about the same to do it.   So I guess I have to figure out what works best.

Numbers I'm using for the rigid foam....
Super Tuff-R was $18 for 1" at 32sql feet
1500 sq feet/ 32 sq feet = 47 pieces. = $846 and then furring strips and installation.

1.5" XPS is $24 for 1" + .5" to it's about the same as the polyiso cost.


I agree with you on the batts.  First off the builder cut corners ( I bought the house after two previous owners) . The batts in the places I opened up weren't installed right and then a security system was installed and it seems that it was done after the insulation was put in.  The security installers seem to have used some weird wire tool that spun some batts,  effectively giving an R0 in places near the windows.  I'm sure the whole house is iffy.  I have a HERS energy audit being done tomorrow.  We'll see what that shows, but in the winter is certainly feels as though the walls are barely insulated. 

I liked the idea of external insulation to get rid of the thermal bridging as well as take care of the flooring/roof area ( I don't know the construction term) just above and below the 8' wall.

Blowing cellulose from the outside over batts isn't a big deal, but it may take more than  two 1-1/4" holes per cavity if they can't snake a dense-packing hose in there.  With multiple holes they can tell by the insulation blowing out of the next hole in the series that  it fully made it.  It's SUPPOSED to crush the batts, and fill in all voids.  If the blower backs up and stalls, and no material was coming out the top hole there's a fire stop, bunched-up batt or some other obstruction, and they'd just have to probe and drill another hole above the obstruction, or maybe not, depending on just what it is.

With empty-cavity installs the lower-density 2- hole method is common, one hole about 12" from the bottom and another 12" from the top. They blow the bottom first, initially directing it downward a bit, then blow upward until they see it coming out the top. Then they move up to the top hole, fill down/up and call it done. That results in something like ~2.5lbs density, sometimes a bit more.  When "dense-packing" it's one larger hole (usually near the bottom), insert a flexible narrower hose up to the top of cavity, blow until it stalls the blower and no material is moving,  pull back a foot or so, wait until it stalls, repeat.  That results in 3+ lbs. density, determined by the blower settings & power.  With 1.5" of foam on the exterior 2.5-3lbs is "good enough", since the cellulose stays warmer, and has lower seasonal humidity cycling (the humidity cycling is what causes dry-blown to settle over time.)  It's often possible to dense-pack over batts by setting the blower for air-only while inserting the dense-packing hose.  If the interior walls can take the pressure (the installer should assess this) dense-packing to 3.5lbs removes any possibility of settling, and makes the cavity fairly air-tight- nearly as tight as foam.

"Cellulose loses when it gets wet but it stays effective at super cold temps although it is only slightly better than fiberglass in wind."

Uh, where'd you get that? It's substantially better than R11/R3 batts under wind-washing, and densed-packed can be nearly as effective as open cell foam.  So is Optima or Spider fiberglass at 1.8lbs+ densities.


"I think the only real solution is open or closed cell foam. I'd go closed cell if I had the option otherwise Open cell. I would take the money saved by not paying the siding contractor to insulate your house and put it towards injection insulating the walls, then put the nice new siding up without the cost of the wrap nor the insulation outside Unless you can afford to do both."

Closed cell foam in a 2x4 studwall with a 20% framing fraction come in with a whole-wall R (thermal bridging of the framing factored in), at about R11-R12, provided you can get at least 3" of it in there with the batts, without blowing out the wall.  Blowing in cellulose from the exterior to ensure a complete cavity fill and lower the convection/infiltration factors delivers a whole-wall  of ~R10.

Then, adding R5-R7.5 on the exterior you get the benefit of the thermal break over both the cavity and studwall, for a whole wall R of R15-R17.5, for a LOT less money &  risk than a 2lb pour or non-expanding injection foam.

The moral of the story is, save the foam budget for outside the sheathing.  Closed cell foam is wasted in wall cavities due to the framing dominating the heat transfer.  Detail either the sheathing or the housewrap as the primary air barrier, and seal the seams on the rigid foam, and it'll be TIGHTER than a cc foam cavity fill, since the cavity-fill doesn't address air leakage at the band joists, subfloor, studwall plates.

First, let me say I am not a professional insulator so my opinions are just that....what conclusions I have come to after reading all I can find on the matters at hand. Despite my preference for closed cell foam as an insulator in 'finite' spaces which cannot be enlarged, I do really like cellulose for it's green properties (recycled paper and low toxicity), very low cost and health reasons and believe it to be about the best all around product out there. I believe it to be one of the most green products available maybe next to old shredded cotton blue jeans but cellulose has the cost factor going for it in spades. That might change as less people subscribe to newspapers /other paper products of which are recycled to make same. The toxin that people are referring to in cellulose is basically Borates' -Borax the household cleaner/disinfectant and water softener sold at wal-mart. Borax is a type of salt. I have used borax out of the box (7 years or more) to spread on my floors and around my foundation (country home and city house)and it is way cheaper and safer than anything that Pest-B-gone will use in your home. It is extremely inert and very versatile you can use it to feed your growing Tomatoes. It is also used as a base for many cosmetic face creams and cosmetics, pharmacies use it to make medicated packs for women to put inside them to control yeast infections or put on their toes for a low tech anti-nail fungus. So how toxic could it be? These same properties protect your house which occasionally gets wet where it should not. Borax is a disinfectant similar to low % hydrogen peroxide and you can use it to disinfect carpets, clothes that you don't want to ruin with Clorox bleach. I add it to my bath water some times. It is even a laxative if you put a tease-spoon in a glass of water (check the internet for the exact measurement on that). If you keep a box of borax in your kitchen below the counter you can use it to put out a flash oil fire. It's fire retardant properties are why it is in cellulose as well as to keep it from being a meal to bugs and other pests. If a coach a roach, ant, termite or any bug with a thorax comes in contact with borax they will die shortly afterwards as its crystal structure punctures their thorax and they dehydrate to death. If you have an ant problem it will disappear after a single treatment. Boric acid on the other hand (industrial concentrate of Borax) makes them sterile so if they eat the borax they die off as well as they cannot repopulate. It has no such effects on humans. If you mix toothpaste and borax in a paste and put it under your counters it acts like a roach bait and ant bait, they will just disappear. It kills the worst fire ants and Brazilian ants too. You never have to worry about poisoning as you will automatically know right off if you ingested too much of the stuff. (I've been using it as carpet powder to disinfect rental carpets for years and I probably got a bit of dust from time to time when I vacume the rentals carpets). BTW it is a major ingredient in all carpet disinfectant and deodorizing powders. How will you know of you got a bit too much? It makes your balls hurt like you have been kicked hard. So its not really and issue of accidental poisoning that damages your health and you never know it. Its more like you took a teaspoon of salt and it tastes nasty and they your balls hurt so you go to the doctor and he says well do not eat that stuff dummy. It prevents the paper from spreading a fire and bugs will become sterile if they eat it. It is automatic protection from termites, ants and other crawl bugs. It prevents mold growth(might grow on the walls but not in the insulation). Even the much more concentrated Boric acid which is the main product in Roach Proof has a toxicity of backing soda or salt so don't believe all the hype. So if you don't drink it won't kill you. As far as immeasurable dust getting into the house, I just haven't seen anything supporting that anywhere. On the other hand I do worry about fibers from fiberglass known to cause cancer like asbestos. A&B chemicals from Foam even after mixed and hardened it can be ingested as powder when foam walls are tampered with, moved, what have you. I am somewhat of a green type person and concerned for the health of my family. I have removed all fiberglass from my ceiling and walls and going back with foam. I am concerned about the foam but that is the way I have chosen as I see no other way to get R7 or 6.9 per inch. I made my walls 5 1/2" thick and I guess the next time I side my house I might add 1-2 inches more on the exterior. We shall see what this does for me now and decide later if I want anything else.

Tony: Polyiso is R6+ per inch under ASTM C 518 test conditions, and blown with pentane rather than HFC 245 or 143, giving iso FAR less greenhouse gas potential (a few percent of that of closed cell spray foam). Closed cell polyurethane foam is great for air-sealing, but as in insulation, at any significant R value it has a higher lifecycle greenhouse gas potential than the energy sources it's offsetting Unlike open-cell foam, closed cell foam also continues to outgas for decades.

Applied beyond air-sealing and moisture-control thicknesses, closed cell spray foam is the antithesis if "green". Dense-packing cellulose over your existing fiberglass achieves a comparable whole-wall R while dramatically reducing air-infiltration potential, with EXTREMELY little environmental impact. There are newer blowing agents coming on the market that will improve the greenhouse gas equation for ccSPF, but it's still a VERY long way away from iso or cellulose. See:

http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/new-blowing-agent-addresses-climate-impact-foam-insulation

Thanks Dana,

One Idea a contractor came up with was to spray 2" of closed cell to seal things up and then tight fit cellulose over the top of that. The question I have is, do we have to ventilate the cellulose afterwards and how do we ventilate the cellulose afterwards?

Posted By Tony_Scarpelli on 14 Mar 2012 02:22 PM
Thanks Dana,

One Idea a contractor came up with was to spray 2" of closed cell to seal things up and then tight fit cellulose over the top of that. The question I have is, do we have to ventilate the cellulose afterwards and how do we ventilate the cellulose afterwards?

Ventilation is the enemy of fiber-insulation. You want it to be air-tight, but sufficently vapor open (to water vapor molecules, not air) so that moisture-susceptible materials are able to dry.   If you have closed cell foam on the interior side of the structural sheathing on 2x6 studs it means you shouldn't use an interior vapor barrier or the cavity may not have sufficient drying capacity, depending on the stackup.  "Flash'n'batt" or "flash'n'fill" with 1-2" of cc foam on the sheathing works great for air sealing the cavity (but it doesn't air-seal bewtween stud-plate and subfloor, etc.), and provides a non-wicking condensing surface facing the fiber, so it won't pull condensation toward the moisture-susceptible sheathing even if the foam gets below the dew point of the interior air. (With the R-value of 2" of foam and 3.5" of cellulose condensing hours at the foam/fiber interface are pretty few, even in climate zone 6 & 7.)

At 2" most ccSPF is between 0.5 and 0.7 perms, which is fine- the sheathing can still dry toward the interior if need be, so long as you keep it vapor-open, which means latex paint finishes, no vinyl or foil wallpapers, no poly vapor barriers under the gypsum, etc.

With a 30% framing fraction 2x6 construction with 2" of R6.9/inch foam and 3.5" of R3.6/inch cellulose you end up with a whole-wall R of ~ R13, (about the same as 5" cc. foam) but still only roughly code-min for much of US climate zone 5: 

R13+5c.i. 2x4, or R20 center-cavity 2x6 is pretty typical of current codes in zone 5,  either of which is in the low to mid-teens for whole-wall R at typical framing fractions.

If you did the entire cavity with cellulose (R12, at 30% framing fraction), and did just ONE INCH of cc foam on the exterior (~R6.9) you'd be nosing up on R20 for whole-wall R, which is 50% higher whole-wall R for only 70% the high-cost foam volume, with a ~60% increase in the low-cost cellulose volume.   AND it would air-seal and thermally break the band joists & plates, etc (with a bit of detailing at the top & bottom edges of the sheathing.) Whether you need more exterior foam than that or a class-II vapor retarder on the interior to protect the sheathing from wintertime moisture drives depends on your local climate. (Where are you?)

Most of the time it's cheaper to detail the exterior sheathing as the primary air barrier, and use taped/sealed rigid foam on the exterior to raise the exterior R high enough to protect the sheathing from wintertime condensation. And, it IS possible to get the house as tight or tighter than with typical foam cavity fills (which only treats some of the air-leakage issues.)

And talking about exterior foam board insulation.....
Would there be any disadvantages (other than cost - I have no idea what it would cost)and would there be advantages, to adding 1.5 inches of XPS on 2 walls - north and west - and not on the other two? My weather comes from the west northwest, and the east and south walls are window rich. The north wall has 2 windows and one door, and the west wall 6. By only redoing (removing the siding and windows, adding the foam, resetting the windows and door, rplacing the siding) these 2 walls, I would minimize the number of windows/doors that need to be removed and reset, and get the extra insulation on the walls that need it most. Any thoughts?

Thanks,
Rosalinda

I'm in Wichita, KS.

Damn, 8 years ago I put brand new 50 year siding on this place and it is perfect condition, even the paint is still in top condition. I hate to reside just to add 2" foam and wrap on the outside of the place. That seems like walking backwards. I wasn't even aware of zero net energy homes back then. I do remember my guy asking me if I wanted to pay $500 more to have 1" of foam added and I just blew it off. How short sighted of me.

Posted By Tony_Scarpelli on 14 Mar 2012 07:46 PM
Damn, 8 years ago I put brand new 50 year siding on this place and it is perfect condition, even the paint is still in top condition. I hate to reside just to add 2" foam and wrap on the outside of the place. That seems like walking backwards. I wasn't even aware of zero net energy homes back then. I do remember my guy asking me if I wanted to pay $500 more to have 1" of foam added and I just blew it off. How short sighted of me.

Ouch!!

I've been building mine the last couple of years, but since the main floor (top story) is a modular, and the modular company would not put foam on the outside of the sheathing, I didn't add it to the (stick built) ground floor either. It is the biggest thing I would have done differently if I could have.

It is probably not a cost effective retrofit for me, since in truth the house is tight, especially the ground floor, but to get the main floor up to the standard of the ground floor - I think it might be something I should at least look into, most particularly on the west wall.

-Rosalinda

Dana1,
You mentioned rock wool. I hadn't considered this option. In looking at the Roxul Comforboard IS it seems to be R6 at 1.5" and very high perms. There are some others I guess as well.

Reading again GBA, I see an article which is right on topic and says:

'I posed the question to Dr. Straube, who said, “If air is presumed to leak through the inner stud cavity, it can reach the sheathing, where it cools down and dumps its moisture load. But thinner Roxul insulation is not as risky as thin rigid foam, because when the OSB is wet, it can dry much more quickly to the outside through Roxul than through foam. But if you switch the OSB to fiberboard, it would truly change the story, because fiberboard can dry out at a rate that is 10 times faster than OSB.”

Builders excited about building walls that dry in both directions should be cautious, however, because vapor-open walls can perform poorly during the summer if the building is air-conditioned. “If you have a very vapor-open assembly, it’s good and it’s also bad,” Straube said. “In the summer, when rain wets wood siding, these vapor-open walls are the type of walls that taught us about inward solar vapor drive. But knowing that inward vapor drive is a risk, we should make sure there is a gap between the siding and the sheathing, and we should ventilate the gap and back-prime the siding, so water isn’t stored in the siding. Those steps are probably enough to remove the summertime risk from vapor-open assemblies.”

My house sheeting is OSB, just confirmed it around a bad window I was fixing. So this information is right on for me as well. I also saw that Building Sciences is saying that furring strips will work on the 8 pcf and that fiber cement ( i believe hardie is 3 psf) will work fine with it. They saw little deflection.

So maybe going with a rock wool product could be a good compromise without having to re-insulation the walls.

A couple of things though. It seems as though the rock wool product has a very small expansion coefficient which is good. It has a high perm rating. But the one thing I couldn't find whether it is an air barrier? Maybe I'm missing something in that the specs don't have to say that number since it's assumed to block air penetration.

Actually on another matter, this rock wool looks to be a nice sound insulator as well. I am refinishing my basement and I may put it in the rafters to keep the sound from travelling upstairs. Not quite sure if I would equally want to use it on the basement walls. It's R/inch value is smaller leader to need more of it to get to R10 as is code in my area at the moment.