Posted By jzinckgra on 30 Aug 2012 10:54 AM
Well it will cost us an additional $3K if we want to go with the double stud wall. Our builder wanted $2500 for labor and $3500 for XPS to do the ext foam board, so we can't afford that.

If you don't use XPS and use reclaimed iso, that $3500 shrinks to under a grand.

Hey Bob ,I think I have extensive training in the use of cellulose and what is possible and what is fantasy. I'm going to take back my statement that that it can't be done. The only way you can dense pack a double wall with enough pressure is to install OSB on the interior walls and blow the cavities with thw most pressure your machine can generate. I started installing blown in insulation in 1975 so I have used every method, in every situation. I know the limitations of cellulose, It's a great prodict but not when your trying to pack it in a 12" cavity. If you look at a 30lb bag of cellulose when it is opened it just sits there. It goes thru a machine to fluff and seperate the fibers, then it is installed in the cavity to be repacked. If it starts out as a much smaller bundle, that means it has the potential to become compressed again, leading to settling. In a retrofit situation you have wires,nails and in the case of a older home, plaster and lath. all of the hold the cellulose in place. When you a 10-12" cavity you are dealing with smooth surfaces and the wiring is of limited value. The weight of that amount of cellulose wants to naturally settle. The second problem is that when you blow lets say a 24' wall section the cellulose will spread thru the whole area with inconsistent pressure. The areas furthest from the point of installation will have a much lighter density. As soon as you turn off the blower the material willl settle. When you go to pack the new area that already has cellulose in it the material will resist packing thereby leaving inconsistent densities in the wall. Point of entry areas will be tight, areas away from the installation point will have a lighter density. This is the reason I developed the clear tube in the wall method of cellulose and fiberglass installation back in 1975 to even out the uneven densities. There is a gentleman at National Fiber that I meet 3years ago, an old timer like me and when we compared notes I found that I beat him using this method by a couple of years. So Bob I can always learn somthing new but not about double wall cellulose. If I was going to use this method, I would install a batt in the exterior wall and then blow cellulose in the remaining cavity. The batts would act as a spring pushing back against the cellulose and would provide an uneven surface to hold the cellulose in place. Sounds like your a responsible contractor just offering my opinion based on 37yrs of experience. Of course this is a moot point since the original poster is using a different wall system.

What are your thoughts on stabilized cellulose (glue) or horizontal netting to support the cellulose? With a new build, one has lots of flexibility as to how the cellulose is put in / sprayed on.

Smartwall - i'm not a cellulose contractor so I can't counter your 37 years of experience. Dana1 wrote a long post discussing that subject in detail so I won't replow the same ground. Your mention of the 24' wall section leads me to wonder if you understand that each stud bay area is sealed of from other areas by the fabric, so that the area you are filling would be 12" x 16"(or 24") x 96", not 12" x 288" x 96". I do understand that it has to be a limited, defined area, that blowing into a 24' area cannot be dense packed. Getting it dense enough to be stable without having it so dense that it buckles the sheetrock is an art, and that is where the installers experience comes into play. National is a big player in our area, and there are several excellent insulation contractors who have been dense packing these double wall houses for over a decade - MoreTite and Murphy's Cell Tech are two of the oldest & best. Are you involved in NESEA? If not you need to be - excellent conference coming in March where even us old guys can learn new things.
Bob

Well our builder has given us an option, as we've decided not to do the double stud wall. He will do the glass/spray foam as originally proposed, but will also add 1" XPS to the exterior of the house. The tradeoff, is we leave the garage unfinished (no insulation or drywall). My thought is it's much easier to finish the garage at a later time and probably a lot cheaper then the 6K he's claiming it will cost. If we don't add the XPS now, there's no going back and doing it later. OTOH, I really would like the garage finished, since it's nice to get into a fairly warm car in the winter, although we won't be heating the garage initially. I guess we got spoiled in our old house which was finished and heated. We can't go with the 2" foam board, as it exceeds our budget. If you had the option of going with the 1" board vs having the garage finished, what would you do? Seems like a no brainer to do the XPS.

Have your builder call around to commercial roofing suppliers - for instance Beacon Sales has a store in Lewiston - they sell XPS for less than any residential supplier including the big box stores. You may be able to get 2" for close to what he's being quoted for 1" at the lumber yard or HD/Lowes.

Agree with your reasoning - you can insulate the garage anytime, but this is your one chance to insulate the house. You're going to appreciate the warmth a lot more than your car will.

what would you do?

I would drop the spray foam and use other methods to air seal. And use EPS instead of XPS (for cost reasons).

I understand that your builder wants to do what he knows, and that "flash and batt" is common with the commodity insulation companies - they can install the fiberglass they are comfortable with installing. That doesn't make it the best system, though. I know this method works - 2x6 with dense packed cellulose and 2" of XPS on the exterior. Someone- either you or your builder or the insulation company has to carefully air seal the framing, carefully install the XPS and tape the XPS. Shouldn't be any more expensive.
EPS has a lower GWF (global warming factor) than XPS, but it breaks easily and isn't as easy to use in this application as XPS. My opinion is that the overall installation will save enough emissions to offset the difference in the foam.

I agree with Bob. Your spray foam is increasing your budget. Plus you do not want to sandwich your sheathing between spray foam and exterior XPS. Drop the spray foam, go with a blown fiberglass or cellulose, and go with 1.5-2" of XPS on the exterior. Tape the joints of the XPS and and foam the box sill. This should be a wash compared to the flash/batt and get you a higher performing wall.
With that said, you are talking about 1-3k in differences here. Put the money in the exterior of your home first. Adjust your budget in other areas to come up with the money if needed. Put in cheaper tiles or bath fixtures, cheaper cabinets int he kitchen, etc. THese are all easy things to be changed later if needed. Your exterior walls....not so much. Your exterior shell is the only place in your house you will see payback. Invest there.

Also note if you do not drywall the garage, all exposed wiring in the walls for switches and outlets has to be in conduit to pass code.

What is the problem with sandwiching the sheathing between xps and spray foam?

conduit in the garage? not required by national code in an unfinished space.

Posted By jzinckgra on 04 Sep 2012 01:21 PM
What is the problem with sandwiching the sheathing between xps and spray foam?

Nothing, but it doesn't buy you (much of) anything either. The difference in whole-wall R between dense-packed cellulose cavity fill and 5" of closed cell polyurethane foam in a 2x6 wall cavity is less than R2.

You can air-seal the sheathing using other methods far more cheaply and completely than even a 1" flash-foam, and spend the difference in thicker exterior foam.

Bob: Unfaced EPS is easy to break, true, but foil-faced iso isn't too fragile, is easily air-sealed, and at 2" would meet IRC code for sheathing-R in US climate zone 6 without an interior vapor retarder where 2" of XPS would not.  The HFC134a used for blowing XPS has 200x the greenhouse gas potential of the pentane used for blowing iso.

At only 2" on the outside of a cellulose filled 2x6 wall in zone 6 the lifecycle GHG potential of XPS would be lower than the energy use it offsets, but the crossover point is measured in several decades.  With iso that crossover happens in under 10 years. 

In Europe blowing foam with HFCs is banned, and XPS is blown with CO2 (for an even lower GHG potential than pentane-blown iso) but those goods only run ~R4.3/inch rather than R5/inch.  SFAIK the relative ruggedness of European XPS is about the same as the US product though.

dana1: Joe Lsiburek recommends a permable wall until you get to about 50% of the R value being on the exterior. With 2" of XPS in this assembly, you're not there yet. So I've avoided polyiso for that reason. Your thoughts?

Posted By Bob I on 04 Sep 2012 01:23 PM
conduit in the garage? not required by national code in an unfinished space.

let me rephrase- our local code requires conduit over all exposed wiring within a wall space (basements and garages). Within a ceiling space you are fine. So, check with your local code.

Posted By Dana1 on 04 Sep 2012 02:08 PM

Posted By jzinckgra on 04 Sep 2012 01:21 PM
What is the problem with sandwiching the sheathing between xps and spray foam?

Nothing, but it doesn't buy you (much of) anything either. The difference in whole-wall R between dense-packed cellulose cavity fill and 5" of closed cell polyurethane foam in a 2x6 wall cavity is less than R2.

You can air-seal the sheathing using other methods far more cheaply and completely than even a 1" flash-foam, and spend the difference in thicker exterior foam.

Bob: Unfaced EPS is easy to break, true, but foil-faced iso isn't too fragile, is easily air-sealed, and at 2" would meet IRC code for sheathing-R in US climate zone 6 without an interior vapor retarder where 2" of XPS would not.  The HFC134a used for blowing XPS has 200x the greenhouse gas potential of the pentane used for blowing iso.

At only 2" on the outside of a cellulose filled 2x6 wall in zone 6 the lifecycle GHG potential of XPS would be lower than the energy use it offsets, but the crossover point is measured in several decades.  With iso that crossover happens in under 10 years. 

In Europe blowing foam with HFCs is banned, and XPS is blown with CO2 (for an even lower GHG potential than pentane-blown iso) but those goods only run ~R4.3/inch rather than R5/inch.  SFAIK the relative ruggedness of European XPS is about the same as the US product though.

what about the lack of drying ability of the sheathing when sandwiched between closed cell and low perm XPS?...

Posted By Bob I on 04 Sep 2012 02:25 PM
dana1: Joe Lsiburek recommends a permable wall until you get to about 50% of the R value being on the exterior. With 2" of XPS in this assembly, you're not there yet. So I've avoided polyiso for that reason. Your thoughts?

A 2x6 studwall with a 25% framing fraction, cellulose fill + gypsum + sheathing comes in at about R13 whole-wall, before siding & exterior foam.

Add 2" of iso + rainscreen gap + siding and that part of the stackup comes in at  about R13.

That's a 50/50  solution from a whole-wall R point of view (which is what I suspect J.L. is talking about, not center-cavity-only), and it works great in zone 6 even though the foil facers are vapor-impermeable, and meets IRC code with a bit of margin with just standard latex paint on the interior, with a total drying capacity of over 2 perms. 

With 2" of XPS instead of iso the exterior stackup is only R10.5, but won't cut it from a center-cavity dew-point point of view on the cold edge of zone 6 (but usually would on the warm edge of zone 6) and the IRC spells out a class-II interior side vapor retarder.    The foam is still somewhat permeable but comparable to kraft-facer vapor or "vapor barrier latex" vapor  retardency, at 0.5-0.6 perms.  Kraft & v.b. paint come in at ~0.4-0.5 perms on the interior so the total drying capacity is only about 1-perm, only half the total drying capacity of a 2" iso solution.

See:  http://www.greenbuildingadvisor.com...-sheathing

http://www.buildingscience.com/docu...uirements/

The moisture buffering capacity of cellulose in a stud cavity also provides significant protection for the sheathing, but we'll leave that out for now.  (WUFI simulations of the 2" iso + 2x6 cellulose fill w/ standard latex usually look pretty good.)

For US zone 7 or colder it's worth doing the dew point calc (or referring to the IRC), but if the sheathing averages 40F (about the dew point of wintertime conditioned space air) at the mean January outdoor temp using the center-cavity R values you're pretty golden.  

That would still violate code in Canada, which requires that the sheathing stay above the dew point at the outside heating design temperature, which takes MORE than a 50/50 split at center-cavity for most of the country, and would in much of US zone 6 as well, but to only allow condensing temps at the sheathing 1% of the time or less is a HUGE amount of margin, when there is ample evidence that it's not necessary (ergo the less restrictive IRC requirements.)

Posted By lzerarc on 04 Sep 2012 03:13 PM

what about the lack of drying ability of the sheathing when sandwiched between closed cell and low perm XPS?...

2" of US-style XPS comes in at about 0.5-0.6 perms, a 1" flash of closed cell is around 1 (sometimes more), for a total drying capacity of around 1.5 perms balanced toward the drying-to-the-interior, which isn't bad.

With the flash-foam as the condensing surface less moisture ends up in the sheathing during  since the foam doesn't wick during those condensing hours, and the number of hours when the foam/fiber layer is at condensing temps is also much fewer.

With 2" of ccSPF in the cavity the total drying capacity shrinks to about 1 perm, and the permeance of the interior foam is about the same as a kraft facer. It's still not terrible, but half the drying capacity of a more vapor-open enterior and a slightly higher R impermeable exterior.

Looks like for our budget we will be able to go with 2" Iso and cellulose in the cavity. Spray foam will be used on the sill and around the foam board seams (on the inside). I always thought foam board went on the outside of the OSB followed by the siding. Our builder says he attaches it direct to the studs then uses additional bracing for the OSB to prevent the "racking effect". Is this an unusual method, as I've never seen or heard of attaching the foam board to the studs.

Posted By jzinckgra on 05 Sep 2012 10:33 AM
Looks like for our budget we will be able to go with 2" Iso and cellulose in the cavity. Spray foam will be used on the sill and around the foam board seams (on the inside). I always thought foam board went on the outside of the OSB followed by the siding. Our builder says he attaches it direct to the studs then uses additional bracing for the OSB to prevent the "racking effect". Is this an unusual method, as I've never seen or heard of attaching the foam board to the studs.

This is unusual, and it has some issues:

Bracing the studs is more effective on racking forces than bracing OSB that is hanging out there 2" from the stud on fasteners, and it's putting a greater shear force on the fasteners, so there will need to be more of them, all penetrating the foam.

Nailing the foam to the studs on a tight pattern thermally bridges the foam.

Nailing or screwing the sheathing to the studs through the foam with pan-heads or monster-nails every 12" adds even more thermal bridging of the foam.

Nailing siding to the sheathing without a rainscreen gap, you have 10,000 nails poking into the foam, reducing it's effective R value somewhat. 

The thermal conductivity of steel nails/screws is more than 100x that of foam. If you poke enough nails & screws through it the benefit begins to evaporate.

I'm not convinced using the foam as the primary air barrier has anywhere near the long-term integrity as using sheathing. (Especially XPS, which has a history of shrinking with age- a problem the industry claims to have overcome, but we'll know in another decade or two.)

With OSB wedged between the foam & siding it's drying capacity isn't nearly what it is with a rainscreen. It's not necessarily a problem, but it's just not as resiliant in the longer term.

I'm not sure what advantages the contractor is working toward with that stackup.

A tried & true method is to put the sheathing on the studs, sealing the sheathing to the framing as you go (with construction adhesive, lo expansion foam or acoustic sealant caulk), and using furring through-screwed 24" o.c. to the studs to fix the foam in place and provide something on which to hang the siding.  Depending on how you install & flash the windows the housewrap either goes over the foam or between the foam & sheathing.

I've seen drawings & pics of thinner foam (3/4", maybe 1") attached directly to the studs with cut-in steel bracing on the studs (or shear-panels cut into the corners) for racking, but you can't dense-pack the fiber in those assemblies, and there are still 10,000 nails bridging the foam if you hang the siding by nailing though the foam to the studs.

I don't know if it matters, but we're using Maibec clapboard siding, which requires a space (1/4"?) between the siding and whatever the next layer in the wall is. This supposedly is req'd for warranty against moisture issues related to the siding. I think they will be using furring strips or some other offset for this.