"double vapor barrier"
Last Post 01 Mar 2010 10:09 AM by Dana1. 11 Replies.
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jerkylipsUser is Offline
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20 Jan 2010 10:20 AM
I've read a few posts here warning against "double vapor barriers" - that is, a vapor barrier on each side of a wall.  It sounds like the issue is that moisture can be trapped between the 2 layers & can't get out, causing damage, mold, etc.

My question - it's looking like SIPs are out of the budget for our new house.  I've been working closely with our builder to find alternatives that will be comparable.  I told him that I want something that the exterior foam board is a must - that seems to be a key to creating a really tight envelope.

He recommended doing at least 1" of spray foam inside the stud cavities, then fiberglass batt, PLUS at least an inch of foam board outside.  This will give me the r-value I'm looking for, but should I be concerned about having foam on both sides of the OSB?
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20 Jan 2010 10:57 AM
Are you using open cell or closed cell spray foam on the inside?

If you use xps or similar on the outside you should be fine. Foil faced doesn't allow vapor to pass through.

Instead of 1" of spray foam plus fiberglass batts, consider dense pack cellulose... get both priced out. As long as you air seal the framing along with the foam board on the outside w/ seams taped and sealed, this way be a cost effective way to go.
I built my home with the help of Pierson-Gibbs Homes, "The Hands on House". They build the shell, you finish it. www.p-ghomes.com
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20 Jan 2010 11:12 AM
Posted By jerkylips on 01/20/2010 10:20 AM
I've read a few posts here warning against "double vapor barriers" - that is, a vapor barrier on each side of a wall.  It sounds like the issue is that moisture can be trapped between the 2 layers & can't get out, causing damage, mold, etc.

My question - it's looking like SIPs are out of the budget for our new house.  I've been working closely with our builder to find alternatives that will be comparable.  I told him that I want something that the exterior foam board is a must - that seems to be a key to creating a really tight envelope.

He recommended doing at least 1" of spray foam inside the stud cavities, then fiberglass batt, PLUS at least an inch of foam board outside.  This will give me the r-value I'm looking for, but should I be concerned about having foam on both sides of the OSB?

An inch of 2lb foam or XPS/EPS foam board is not a vapor barrier, it's a class-III (semi-permeable) vapor retarder, which means it can pass water vapor and dry out.

Foil & poly facers on foam boards are class-I vapor retarders (highly impermeable), and must be used with caution, taking care that it's on the correct side of the assembly for the climate zone, and not to be used in assemblies with other class-I vapor retarders lest you create a moisture trap.

It takes at least 2" of 2lb foam or XPS (or a FOOT of half-pound foam) to become a class-II (semi-impermeable) vapor retarder.  If the stackup is

exterior siding|1-2" XPS or foil-faced iso|OSB|1" of 2lb foam|4" fiber|gypsum|interior air

...you're still good to go, since the OSB can dry toward the conditioned-air interior, and the bulk of the R-value is on the outside.  In most lower-48 heating dominated climates the dew point of the interior air in winter occurs in a foam or wood layer, but with the interior foam the air can't circulate to the OSB.  In a cooling dominated climate it's fine, since the higher vapor retardency is on the hot-humid exterior, and exterior air can't get into the fiber level to condense.  Just be sure that the batts aren't foil-faced, and no interior poly vapor retarder is used.  If the outer layer of foamboard is under 2", an interior class-I vapor retarder might be worthwhile in 8000HDD+ climates, and the exterior foam needs to be facer-free.

So, what's your zip code, and how thick is the fiberglass layer?

Have you considered wet-sprayed fiberglass or cellulose instead of batts? (Either of which will have much fewer voids, and less intra-insulation convection.)  Cellulose would also offer considerable hygric buffering capacity, wicking water away from structural material, whereas fiberglass wouldn't, but fiberglass dries far more quickly from bulk-water incursion events, and the newer finer wools will yield a slightly higher R-value than cellulose.  Still, the performance of sprayed goods will uniformly be higher than batts (which are impossible to install perfectly), often for similar money up front.
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20 Jan 2010 11:28 AM
OK, according to your profile you're in Green Bay, which is ~8000HDD, which means if all of the foam is on the outside of the fiberglass layer, it needs to be at least ~50% of the total R-value of the assembly to get away without an interior vapor retarder (60% is better.) An inch of 2lb foam is ~R6, an inch of XPS sheathing is R5, which means if the fiberglass is more than R11 you're edging into winter-condensation potential without an interior vapor retarder. But if you boost the XPS sheathing to 2" you can put in R16 of fiber in there.

If it's 2x6" construction, the cavity starts out as 5.5" depth, but shrinks to 4.5" after foam, which isn't a standard batt thickness (another reason to go with sprayed fiber). You could compress R19/R21 batts in there, which would make them ~R16 as-installed. If it's 2x4" framing the after-foam depth is 2.5", which is a standard R7/R8 batt depth, in which case you're fine even with only 1" of exterior foam.
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20 Jan 2010 12:01 PM
Posted By Dana1 on 01/20/2010 11:28 AM
OK, according to your profile you're in Green Bay, which is ~8000HDD, which means if all of the foam is on the outside of the fiberglass layer, it needs to be at least ~50% of the total R-value of the assembly to get away without an interior vapor retarder (60% is better.) An inch of 2lb foam is ~R6, an inch of XPS sheathing is R5, which means if the fiberglass is more than R11 you're edging into winter-condensation potential without an interior vapor retarder. But if you boost the XPS sheathing to 2" you can put in R16 of fiber in there.

If it's 2x6" construction, the cavity starts out as 5.5" depth, but shrinks to 4.5" after foam, which isn't a standard batt thickness (another reason to go with sprayed fiber). You could compress R19/R21 batts in there, which would make them ~R16 as-installed. If it's 2x4" framing the after-foam depth is 2.5", which is a standard R7/R8 batt depth, in which case you're fine even with only 1" of exterior foam.


good stuff, thanks so much!

yep, we're in GB.  It's going to be 2x6 exterior wall, & I'm really leaning toward doing the 2" of foam outside the sheathing.  That seems to give the best bang for our buck, from what I can tell.


The builder gave us 2 options, to start -

3" closed cell foam (R18) + 3" fiberglass (R11) + 1" foam outside.  According to my calcs, R value would be around 30

1" closed cell foam + 5.5" fiberglass + 2" foam outside.  My numbers gave me R32, but I didn't factor in the fiberglass being compressed.  If that truly gives me 16 vs 19, the number changes to about 29.5. 

I'm not sure if those are overkill, and now I'm wondering how much I'd gain w/the 1" of foam if it reduces the r-value of the fiberglass.  If we did 5.5" of fiberglass + 2" of foam board, that comes out to about R27.  Considering the price of the spray foam, I don't know if the extra money would be worth gaining R2.5.  Of course, I don't know how much better the air-seal issues will be with spray foam + foam board vs. foam board only..

What do you think?

What do you think?
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20 Jan 2010 12:23 PM
What to you plan on using to insulate the bond(s)... spray foam, fiberglass batts?
If using TJI's for your floorsystem, I strongly suggest spray foaming the bonds. Getting fiberglass batts to properly fit between the I-joist without gaps and air leaks is next to impossible.
I built my home with the help of Pierson-Gibbs Homes, "The Hands on House". They build the shell, you finish it. www.p-ghomes.com
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20 Jan 2010 12:39 PM
Posted By Jere on 01/20/2010 12:23 PM
What to you plan on using to insulate the bond(s)... spray foam, fiberglass batts?
If using TJI's for your floorsystem, I strongly suggest spray foaming the bonds. Getting fiberglass batts to properly fit between the I-joist without gaps and air leaks is next to impossible.


sorry for being dumb, but I'm not sure I know what you mean by "bonds".  My builder spray foams all of his box sills, if that's what you're referring to.  If it's something beyond that, please let me know.  thanks!
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20 Jan 2010 01:28 PM
Spray foam as  a seam-sealer in the cavity (not a full 1" over everything) and 5.5" of wet-spray cellulose, with 2" of XPS and an interior vapor retarder would also give you R30 and a very tight wall.  If wet-spray fiberglass instead of cellulose  would increase it to ~R32.  Air-sealing with foam is a lot cheaper than insulating with foam.  If you install all windows & doors when its just sheathing & bare studs, using a blower door to pressurize the shell you can get a pretty much PERFECT air seal running around with a foam gun and smoke pencil, finding & fixing the unseen gaps as you go.

In your above calc, 3" of cc foam + 3" of R11 batts doesn't add up to 5.5". The batts will compress to ~ R9.5, so it's really closer to R28.5 before calculating the thermal bridging of the studs.  (Your clear-wall R-values will actually be in the mid-20s when framing is factored in.)

Band joists & foundation sills should still be given at least an inch or two of interior 2lb foam to avoid condensation on the structural wood.

Have you pored over the Building Science High-R Walls study?

Your proposed wall looks pretty much like combination of this:

http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/high-r-wall-09-flash-and-fill-hybrid-wall-construction/images/highr_wall_09_web_rev02.jpg
http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/high-r-wall-09-flash-and-fill-hybrid-wall-construction/


and this:

http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/advanced-frame-wall-construction/


http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/advanced-frame-wall-construction/images/highr_wall_02_web_rev.jpg

Note the commentary in the artical about vapor and air infiltration control.  I'm thinking you get better value out of the latter, only using interior spray foam for sealing the cavities as-needed, and insulating the rim joist & sill (as shown).  In the case study document on p 11-12 they point out that with 4" of XPS on the exterior (R20) and R19 fiberglass in the studwall you can skip the interior vapor retarder. 

This would be functionally the same if you used 3" of iso (taped & foam-sealed) instead of 4" of XPS on the exterior, which may be both less expensive and more effective than the 1" spray + 2" exterior XPS you were considering, and would achieve a true R35+ clear-wall R-value. (see the R-value comparison table 3 on p.9- look at 2b.)  Using iso instead of XPS it would only be 1" thicker than the stackup you were considering, but ~R10 higher clear-wall R value- a 40% performance boost.  (Price it out, anyway! Last time I priced it out R20 fiber-faced iso in 4x8 sheets were ~$1.70/square foot.  Even with installation costs factored in it's only about 2/3-3/4 the cost of doing 3" of 2lb foam for ~R18.)


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23 Feb 2010 05:28 PM
After three years in the sun, I went over my house wrap with lapped 30# felt under cedar shingles.  My plan is:

cedar shingles / 30# felt / 0.5" ply / 5.5" fiberglass batts / 1" foil faced / strapping / sheetrock

Am I about to make the double vapor barrier mistake with the felt and foil faced?  Any suggested fixes?  The felt and shingles are on...
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23 Feb 2010 05:51 PM
Posted By tcole on 23 Feb 2010 05:28 PM
After three years in the sun, I went over my house wrap with lapped 30# felt under cedar shingles.  My plan is:

cedar shingles / 30# felt / 0.5" ply / 5.5" fiberglass batts / 1" foil faced / strapping / sheetrock

Am I about to make the double vapor barrier mistake with the felt and foil faced?  Any suggested fixes?  The felt and shingles are on...
Short answer:

You don't have a double vapor barrier in that stackup, just one (the foil), and is nothing to worry about in a heating-dominated climate. 30 lb felt is semi-permeable when dry (~5perms), but becomes highly permeable when wet, and shouldn't be thought of as a vapor barrier.

Longer answer with stuff you didn't ask about:

The half-inch plywood in your stackup has even lower dry permeance than the felt, but it too becomes highly permeable when wet.  The permeance of OSB is similar to plywood when dry, and while it's permeance increases with humidity, it's never highly permeable, but behaves more like a class-III vapor retarder when wet (semi-permeable.)

If you live in the hot humid tropics, Florida, or the Gulf coast you might want to revise it by making the exterior layers less permeable, and the interior more permeable.

If you want to make the structure more moisture-tolerant, replacing the batts with blown cellulose (or dense-packing the cavity with cellulose and leaving the batts) will give you a significant amount of hygric buffering. Dense-packing cellulose to 3lbs/ft^3 or higher is even better, but standard 2-hole method is still more than half the buffer of dense pack.  With fiberglass he moisture that doesn't drain away or vaporize quickly ends up in the structural wood, wheras cellulose wicks it away from the structural materials.
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23 Feb 2010 06:31 PM
I live in Massachussetts, so I like the idea of the foil faced on the inside.

What’s the 2-hole method? Is that something like a hole to fill and a hole near the top of the wall to check for fillage? I have a general understanding of cellulose, but haven't converted yet. There's a couple things that have kept me in the fiberglass batt camp, including its water resistance, DIY application and hope that with careful application I can reduce convection issues. Also, with my impermeable side on the inside, my concern for the cavity insulation is water incursions rather than vapors. I though fiberglass could handle this better. Won’t cellulose settle?

Really appreciate your time and expertise on this!
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01 Mar 2010 10:09 AM
Posted By tcole on 23 Feb 2010 06:31 PM
I live in Massachussetts, so I like the idea of the foil faced on the inside.

What’s the 2-hole method? Is that something like a hole to fill and a hole near the top of the wall to check for fillage? I have a general understanding of cellulose, but haven't converted yet. There's a couple things that have kept me in the fiberglass batt camp, including its water resistance, DIY application and hope that with careful application I can reduce convection issues. Also, with my impermeable side on the inside, my concern for the cavity insulation is water incursions rather than vapors. I though fiberglass could handle this better. Won’t cellulose settle?

Really appreciate your time and expertise on this!

The 2-hole method of blowing cellulose is where they drill a hole a few feet up from the bottom of the cavity and another a few feet from the top. They insert a nozzle (preferably one with a big of an angle to it to direct it up down or sideways), and starting at the bottom hole, blow down for a bit, then up for a long while until fluff is coming out the top hole. They then move to the top hole and blow down for a bit, then up until the cavity is filled.  This results in a density of 2-2.5lbs/ft^3, and gives it ~R3.6-R3.8/inch of thickness.

But at those densities cycling of seasonal humidity year after year results in a 5-10% settling after a couple of decades, and it's loose enough that some small amount of convection and air infiltration still occurs.

Dense-packing cellulose involves drilling a single hole (somewhat larger than used in 2 hole method) and snaking a flexible blow tube to the end of the cavity (the bottom first, in walls), pulling it back a few inches then blowing until the blower begins to stall, drawing the tube back 8-10 inches and blowing until it stalls, repeat. When it's packed from the bottom up to where its at the drill hole, the tube is then snaked up to the top of the cavity (sometimes with just air blowing, but no fiber to make it easier to get it through the lower-density cellulose that is already there), and repeating the blow-until-stall, back it out, repeat.  This ends up using ~50% more material, for a density between 3.0-3.6lbs/ft^3.  At that density it's under spring-tension and won't sag or settle, and the air movement within the fiber is cut 95-99%.  The raw R-value is about the same as with 2-hole method, but the performance (especially long-term) is measurably better.

Batts can never conform perfectly, and compressions/voids create paths for larger scale convection & infiltration losses.  If you go with fiberglass, blown fiberglass is head & shoulders above batts in performance (both in fundamental R-value and convection losses with big delta-Ts.)  Blown fiberglass can settle too, but newer blowing wools with water activated adhesive (similar to wet-blown cellulose) probably won't.

Fiberglass will dry quicker from large bulk wetting events, but it can't redistribute minor wetting or condensation the way the hollow fibers of cellulose do, and more of the water will be absorbed by the wood elements, creating a mold hazard.  And because air moves much more readily through fiberglass, air transported moisture is better able to find it's way to a cold piece of structural wood to condense.
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