2x6 outside walls, Tyvek DrainWrap over sheathing, Hardi/Cement Fiber siding.  North Calif Coast (Trinidad/Eureka/Arcata area), 2 story over a vented crawl space

While I still have plenty of time to decide (foundation footings just now being formed) I'd like to firm up how I'm going to insulate this home. 

I'd like to decide on something I can do myself, but spray foam may make that tough.

I do want the inside of the outer walls air sealed. 

I've read through numerous threads here and here... seems to be so much info about the pros and cons of open cell vs closed cell.  Can someone confirm the differences.  Is it open cell that can also acts as an air seal?

Options I'm considering:

1. Air seal it myself (similar to that DIY mastic that kogashuko used on his remodel project to air seal) and then hire a firm (or figure out a way to do it myself) to net and blow in damp cellulose in the walls. That would be appx R30?

2. Either hire a firm (or again, figure out a way to do it myself) to blow in foam, if it can air seal in one step. 

Why do people consider open vs. close cell foam in their outer walls?  I've heard about certain applications gassing toxic fumes, etc.

I do plan on hiring a firm to blow foam under the sub floor (1st floor) in my vented crawl space, as clearly that will make a huge difference.  From what I've read, it is critical to air seal AND insulate in this process.  So, should I consider open or close cell for that application?  I will probably hire a firm to perform this, as it can be dangerous in a crawl space (fumes)... unless one has a rig that pumps air into a breathing helmet apparatus.

Dana...you've forgotten more that I'll ever know on the subject of insulation.  Are these questions you can tackle succinctly?

Are there structural reasons for building with 2x6s?  If not, then consider building with 2x4s and adding plenty of EPS outside the studs.  If termites are bad there, then think about steel studs, SIPs, etc.

I suggest good tape to air seal exterior sheathing. And still do some type of interior side air sealing.

Can't reinvent the wheel on the studs now. Lumber pack has been ordered.

I'll look into taping sheathing before Tyvek is installed.

More importantly, looking for answers to the open cell vs closed cell expanded foam questions and which one is an effective air seal

I think you'll find that closed cell is the better air seal. But keep in mind that it is not foolproof, and there can be voids and cracks in the foam which degrade that seal. That's the reason for the tape and good urethane caulking. It also does zero for stopping thermal bridging which is a large component of a heat loss. So, while it can be helpful, and is better insulation than fiberglass batts, it is not an all-in-one solution.

Neither open cell nor closed cell foam in wall cavities is sufficient as an air sealing strategy for walls, since it can only seal the cavity.  The stud plates & subfloors etc need treatment as well.

For air sealing wall cavities recent testing by the Building Science Corporation indicates that open cell foam is slightly tighter than closed cell foam (but comparable):



Equal or better air tightness of the sheathing can be obtained by caulking the studs to the sheathing in every stud bay.   This can also be achieved using a bead of construction adhesive between the sheathing  & framing at every framing element resulting in a more structurally rigid assembly, but that is likely going to be prohibited in your area for seisimic resilience reasons. (A less flexible wall assembly is more likely to separate from the foundation during an earth quake.)   Acoustic sealant caulk stays flexible forever (or at least more than 5 decades), and will still likely be sealed after an earthquake.  (Powered caulking guns that take the larger sized tubes can make quick work of it too.)

Almost all closed cell polyurethane foam in the US is blown with HFC245fa, which has a global warming potential (GWP) of about 1000x CO2, and in a high-R assembly the full lifecycle climate impact would never be recovered by the lower carbon footprint of the lower energy use.  Both Honeywell and DuPont have released HFO1234yf blowing agents (GWP less than 5x CO2) for closed cell foam (trade names Solstice, and FEA-1100), but they do not yet have huge market penetration.  Icynene has two ~2lb density products that are blown with water (negligible GWP), MD-R-200, and MD-R-210, that run about R5/inch (compared to ~R6-7 inch for 2lb foams blown with HFC245fa.)

Almost all open cell polyurethane foam is blown with water at the blowing agent.

The vapor permeance of 2lb density HFC-blown ccSPF is about 1 perm (give or take 0.2 perms) @ 1".  Icynene MD-R-200 is about 5 perms @ 1"/R5.2.   MD-R-210 is about 2.4 perms @ 1"/R4.9.

Half pound density open cell foam runs about 30-40 perms @ 1".

Whether open or closed cell, there is some risk of outgassing issues- which is almost always installer-error on temperature or mix.

Whether open or closed cell there is some risk of fire as is cures, if installed in lifts thicker than the manufacturer's spec. (That's typically 2" per lift for closed cell, 5" for open cell, but it varies- read the specs.)

Interior moisture drives (and thus the vapor permeance of the cavity fill) make very little difference to the moisture content of the susceptible sheathing in your climate zone, especially if you provide at least 1/4" of rainscreen gap between the Hardie and the housewrap.

HFC-blown closed cell foam thicker than 2" in the wall cavities all but prevents drying toward the the interior. The more vapor open Icycene water blown foam (MD-R-200) still has about 1 perm of drying capacity at 5" (the thickest practical application in 2x6 cavities), and would be the preferred closed cell foam, for either a full fill or a 2" flash & batt in your climate.  If only 1" flash & fill, other closed cell foams would be fine from a moisture control point of view.

The difference in whole-wall at a 25% framing fraction between 5" of R6.5/inch ccSPF and 5.5" of R3.7/inch ocSPF is R2 (R14.4 vs. R16.5).  Adding 3/8" fan-fold XPS underlayment detailed as the weather resistant barrier instead of drainwrap (or in addition to a housewrap, and using open cell would hit the same performance point at both lower cost and lower environmental damage.  If the framing is 24" o.c. the framing fraction is probably closer to 20%, and the difference is a whole-wall R of ~15.4 vs 18.3, or nearly R2 (an expensive R2), a difference that could be made up with 1/2" of unfaced EPS under the siding.

It may be both cheaper and better to go unvented on the crawl space with 2-3" of closed cell (code min= R13) on the foundation wall all the way up to the sub-floor and installing a ground vapor barrier than going with a vented crawl with the code-min R19 open cell on the floor joist insulation. see: http://publicecodes.cyberregs.com/i...sec002.htm  Closed cell foam runs about 17-18 cents per R- per square foot to open cell at 13-14 cents, and experience tells us that it's far easier to air seal at a foundation wall than under a floor.  So the unvented crawlspace wall area (+ band joist area) at code min would run about R13 x $0.18= $2.35 per square foot,  to a code min sub-floor R19 x $0.14= $2.65 per square foot.   If the crawlspace wall area is less than the footprint of the house, it's a no-brainer, since it's fewer square feet at a lower cost per square foot, and usually higher air tightness to boot.

Thanks Dana...

It sounds like I might be better off spraying a mastic on the inside of the outer walls and airs sealing - and then either filling the inside of the walls with spray foam or netted blown cellulose. I think I'm circling back to my original plan.

Under the first floor sub floor, spray mastic to air seal and then either open or closed cell foam. Foundations have already been poured and vented, so I'd have to plug those up and put down a vapor barrier to make it a conditioned space and not sure I want to go down that road.

I find this Building Science Corp quote interesting:

Lstiburek said, “In houses, the big holes matter. Plain walls don't contain the big holes. I can make a house insulated with fiberglass just as tight as a spray-foam home or a cellulose home if I seal the big holes.

That may be overstated, but the numbers suggest that the choice of open cell, closed cell or cellulose is likely to be responsible for only 5% of the total building air leakage. I'd think more about double air barriers, tape vs caulk, detailing, electrical boxes, interior ducts, completely balanced hvac systems and smoke tests.

I don't have nearly the info Dana does but I have found that with the double studded wall I can almost completely coat the studs and the bays with 1inch closed cell foam. I have also found that I have to tape around the windows or the liquid will squeeze in the cracks around the studs and then expand with the possibility of it getting on the window. I would not be surprised if something similar would happen if a 2x6 cavity was filled completely and the excess trimmed back to allow the drywall to still fit on. The stuff gets into everything in its liquid state and then expands. This isn't possible with the 2 part propane type tank setups because the stuff is already expanding in flight. I can't see how you could possibly make the wall as tight with tape or some spots with chalk.

As for the global warming potential in my case I am only using one inch of what is supposed to be low voc foam. I don't completely buy into the global warming stuff but if you can avoid the blowing agent falling under the category you can negate the debate. I would love to know if this info about the low voc blowing agent on my stuff is incorrect. As for the smell of the stuff I used it is noticeable at first. The parts of the second floor I did had the smell go away within 24 hours with the Windows open. The first floor still has the window and door with plastic and taped. I still faintly smell the polyurethane smell. The kitchen is on the other side of the wall and I can not smell it there. Makes me think after more air changes I will be ok.

I have seen a foamed MgO cement panel as insulation, which relieves a lot of concerns like gassing and fire proofing. This product is light in weight, 150kg/M3 and it is close celled. The ingredients are magnesium oxide, magnesium chloride and EPS beads. It is non flammible. The size can be 4'*8' or 4'*10'. The thickness is 50mm to 100mm. It is easily tooled. It is cement slurry friendly. However, it has not been certified as most Chinese building materials. The manufacturer has just developed it. I don't intend to ask John to use this. Just share a product info.

Well, I'm probably settling on that roof mastic air sealing the interior of the outer walls, following up with a DIY of 5.5" open cell SF. Between the 2, I should be completely air sealed (arguably wouldn't need the first stage spray mastic for air sealing). I have about 3800 sq' of total wall space - both stories, all 4 walls (house is a rectangle) from the 1st floor rim joist to the 2nd floor top plate. By my calculation this should give me close to an R20 wall (using Dana's calcs above, 5.5" of R3.7/inch ocSPF = R20.35).

Crawl space will be vented. I am not interested in changing that for several reasons. So, I will need to somehow foam the and air seal under the first floor. That DIY foam gun is too large to wield around in a 36-42" crawl space, so I'll either need to hire that out or find a different DIY solution for the crawlspace. And fumes are a concern under there to, so I'd need some sort of oxygen mask breathing system if I take a DIY approach. I have about 1780 square feet of floor.

Since I will own the equipment to spray foam the walls, I can eventually spray foam the attic floor (top of the 2nd floor ceiling). My attic will also be vented. If I spray 5-6" on the attic floor (appx R20) I would still need to blow in some cellulose, correct? What type of R value should I be shooting for in my vented attic in my area? R30?

2x6 outer walls, Tyvek Drainwrap, Cement Fiber lap siding.

Well, I'm probably settling on that roof mastic air sealing the interior of the outer walls, following up with a DIY of 5.5" open cell SF. Between the 2, I should be completely air sealed (arguably wouldn't need the first stage spray mastic for air sealing). I have about 3800 sq' of total wall space - both stories, all 4 walls (house is a rectangle) from the 1st floor rim joist to the 2nd floor top plate. By my calculation this should give me close to an R20 wall (using Dana's calcs above, 5.5" of R3.7/inch ocSPF = R20.35).

Crawl space will be vented. I am not interested in changing that for several reasons. So, I will need to somehow foam the and air seal under the first floor. That DIY foam gun is too large to wield around in a 36-42" crawl space, so I'll either need to hire that out or find a different DIY solution for the crawlspace. And fumes are a concern under there to, so I'd need some sort of oxygen mask breathing system if I take a DIY approach. I have about 1780 square feet of floor.

Since I will own the equipment to spray foam the walls, I can eventually spray foam the attic floor (top of the 2nd floor ceiling). My attic will also be vented. If I spray 5-6" on the attic floor (appx R20) I would still need to blow in some cellulose, correct? What type of R value should I be shooting for in my vented attic in my area? R30?

edit: It looks like my HEERs audit wants R21 walls and #38 ceiling, so maybe I would be better off air sealing the ceiling with a mastic and just blowing in cellulose to an R40 value? Can one achieve an R21 (2x6) wall with blown cellulose?

2x6 outer walls, Tyvek Drainwrap, Cement Fiber lap siding.

Here is a graph that I stole from insulRight (am crediting them and their add http://insulright.com/R%20Value%20and%20Economic%20Thickness.pdf)

Looks like right at around 5-6 inches is the way to go. I would think that after that you are probably better off to focus on thermal bridging and other ways to keep make your house green. I do understand that this is only a 40deg F temp difference and would be very interested to see a greater temp difference like that of somewhere with sub zero temps or under a roof deck. My thoughts is that the loss and gains would be greater but I dont think the differences in thickness to loss gain would change much. I could be totally wrong. I just know that I can hold a McDonnalds coffee cup full of high liability hot coffee for hours and not only not get burned but have the coffee stay so hot I cant drink it. Those cups are made out a very small chunk of styrofoam. Turn to your coffee pot in your house. I dare anyone to try to hold the glass pot for five minutes much less hours right after it has been taken from the heater. Now, compare that to how long it takes that same coffee in that pot to cool. Now, that is what you are doing to prevent heat loss and gain to your house.

That's an interesting article/pdf Matt. Obviously it seems a little bit self serving, as they sell spray foam, but the fact that they advocate stopping at 5" seems their motivations are possibly altruistic.

So there is a good argument to spray open cell 5" in the attic and augment to R38 with blown cellulose.

It doesn't appear I can achieve R21 in my 2x6 walls with open cell, and I don't want to screw the budget with closed cell. In my heart, I know a well air sealed 2x6 wall filled with open cell foam will perform perfectly well in my climate zone, and I doubt an inspector would catch that I'm only R20 (or would even care), so that is tentatively what I plan to do. Although I have probably 12 months before I will actually be insulating the walls.

If you think about it that the R21 requirement is the same for foam, cellulose, or anything else. Just imagine how much better sealed your house is going to be with the foam.

There is a spot in my addition where I would have only been able to get 1/2inch polyiso on. It is the engineered truss between the first and second floors. The builder placed the stairs right up against it which is common practice. However, it would have been a GIANT source of heat loss gain over the live of the structure. My solution, with the whole wall too since above it is a 2x4 construction, was to shim it out with 2x3s parallel to the roof and floor. This will give me about 4.5 inches on the walls and on the offending area about 2.5 inches to play with filled with closed cell. Filling the whole thing up with closed cell will actually make code. With the shimming of the wall I will be cutting the code pretty close for required stairway width. If 10 years from now I ever have problems with any code enforcement I could just put in a spiral staircase.

Posted By kogashuko on 27 Apr 2014 05:59 PM
If you think about it that the R21 requirement is the same for foam, cellulose, or anything else. Just imagine how much better sealed your house is going to be with the foam.

There is a spot in my addition where I would have only been able to get 1/2inch polyiso on. It is the engineered truss between the first and second floors. The builder placed the stairs right up against it which is common practice. However, it would have been a GIANT source of heat loss gain over the live of the structure. My solution, with the whole wall too since above it is a 2x4 construction, was to shim it out with 2x3s parallel to the roof and floor. This will give me about 4.5 inches on the walls and on the offending area about 2.5 inches to play with filled with closed cell. Filling the whole thing up with closed cell will actually make code. With the shimming of the wall I will be cutting the code pretty close for required stairway width. If 10 years from now I ever have problems with any code enforcement I could just put in a spiral staircase.

It's possible to build assemblies as-tight or tighter using caulk + fiber insulation at a much lower overall price point than closed cell foam (and often but not always, lower net cost than open cell foam.)

The foam can only insulate the framing cavity it is in, and has no effect on the amount of leakage under the bottom plates, of framing, or between doubled up top plates, etc.. Very few PassiveHouse certified homes are built using spray foam as the primary air-sealing method, and many are built without any spray foam at all, and test at no greater than 0.6 ACH/50 (the max leakage allowed.)

My point exactly, the R21 requirement is blind of the air sealing. Rather you do it with foam or calk doesnt matter since the code sees it no differently from a drafty house with batts jammed in there.

In my case I will definitely be calking on the bottom plates of mine in addition to the spray foam in the cavities, and the latex mastic sprayed in the gaps at the top and bottom. Unfortunately, my addition was built before I looked into passive house design. I got my house resided at the same time. I asked the builder if they would add foam under the siding and he said it didnt need it. Had I known, the week the house was without siding I would have added 4 inches of rigid foam to the outside of the entire house. I trust what you are saying that it is possible to get the house as tight without foam. However, doing all the research I have done I feel that at least an inch of closed cell is the best for my applications. This might not be the same for everyone.

I also had some serious doubts about the DIY approach machines at lowes for the cellulose and being able to dense pack the stuff with that approach at the proper density as to not settle. The price of getting the pro job was almost as much as the spray foam. Sure the spray foam was only an inch and the quote on cellulose was for three but it lacked a solution for under the roof deck. So my solution will be a flash w/ cellulose on top or possibly the addition of some open cell foam and a layer of fiberglass.

As for the rest of my house, I plan to eventually redo a good portion of drywall in my living room, kitchen, and a couple of bedrooms. This is simply because I plan to replace half of the wall that has wood panels with straight drywall. There is also serious damage to one of my bedroom walls due to old wallpaper removal. I can not pack cellulose into these walls because of the 1/2inch polystyrene used on the outside of the house instead of plywood or osb. 3.5 inches of closed cell foam only make sense in this application since I only have 2x4 construction. Again, according to sprayez all of the foam I am using has no VOCs even in the closed cell applications. With using 1x4 shimming I can lose only one inch of space in each room and totally eliminate thermal bridging from the 2x4s if I go with a total of 4inches of closed cell. This would bring the total R value of the wall up to around R25 - R28 with the poly on the outside. Again this is the best solution for me and would easily seal up all the holes in the polystyrene from the siding.

As a DIY, caulking the framing to the sheathing and installing rock wool or high density fiberglass batts is pretty easy, and as a homeowner you're more likely to be obsessive about tucking the edges, splitting the batts rather than compressing behind wiring, and trimming tightly to air-sealed air-tight electrical boxes, not leaving any gaps, etc. It's possible to do a good job with batts (easier with high density batts than with spoogey-floppy low density junk) even if the average hack doesn't always deliver the best result. Rock wool or HD fiberglass with a smart vapor barrier detailed as an air barrier on the interior side is well withing DIY-er territory, and can be a decent peformance highly resilient assembly.

I did notice that the new R13 batts that I put on my master closet walls (between the addition and existing house since the addition wasnt insulated at the time) was much better than the old R11 batts I pulled out of the existing walls. Rock wool would not be bad either if I could find somewhere that has it in the south.

Posted By kogashuko on 01 May 2014 08:21 PM
...Rock wool would not be bad either if I could find somewhere that has it in the south.

You should be able to order it (Roxul) at www.lowes.com and pick it up at your local store.