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Will Water Re-Vaporize?

We are having a very difficult time figuring out a WRB for our home in Massachusetts. After reading a great deal about house wraps and learning that condensed vapor trapped behind Tyvek rotted sheathing, we became afraid to use anything but 30-lb asphalt felt, feeling that it was "smart" and "forgiving".

But as we were shopping for Siga tape we learned about Majvest house wrap. We were told that its high permeability would let out trapped water (condensed vapor) because the water would vaporize and escape. Is this so?

We are fixing up a tiny cottage on a very limited budget, trying to get the most energy efficient value for our money. The 1 x 4 wall assembly is to be fiberglass siding (with built-in rain plane), WRB, air-sealed plywood, dense-packed cellulose, air-tight drywall with 2 coats of latex paint (maybe vapor retardant paint because the building inspector demands a vapor retarder.)

This is a DIY project for everything except the cellulose and we are in our 60's so things move s-l-o-w-l-y and Majvest might hold up better waiting for the siding to be installed, but not if we are risking wet cellulose.

Regards, Deb Davis

Asked by Deb Davis
Posted Tue, 10/02/2012 - 12:38

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10 Answers

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Deb,
The best way to address the "cold sheathing" problem (and attendant worries about condensation or moisture accumulation at or near the sheathing layer) is to include a vented rainscreen between your siding and your WRB. That helps a wall dry quickly to the exterior.

Both Tyvek and similar WRBs from Europe are vapor permeable. As with any drying scenario, the main question is whether the drying rate exceeds the wetting rate or vice versa.

You want a high rate of drying; that's why you want a vented rainscreen gap.

You want a low rate of wetting; that's why you want airtight drywall and air-sealed wall sheathing.

Your post is a little confusing. I'm not sure what you mean by a "1 x 4 wall assembly," and I have never heard of fiberglass siding.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 10/02/2012 - 12:48

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Sorry, meant 2 x 4 construction.

Fiberglass siding is new from Apex, parent company Marvin as in windows and doors. Installation clips keep siding -- about 98% -- from the sheathing/WRB, providing a drainage plane and is vented top and bottom.

The wall assembly, albeit modest, will have the elements you call for -- though the local building inspector is talking plastic or kraft paper as a vapor retarder. I'm hoping he'll accept retardant paint (which, I think, Joe Lstiburek says is "iffy" regards actual performance).

Our confusion is that we thought that once vapor condensed into water (on the wrong side of the "plastic" WRB's) the water resistive barriers prevented it from escaping, rotting the walls. I think what you are saying is that it is not necessarily so. That water can escape, in the form of vapor, as long as the trapped water has the opportunity to re-vaporize without more water getting in. So if a wall structure is basically sound and there is a mighty Nor'easter and water gets trapped between the sheathing and the WRB it can make the return trip in the form of vapor once the sun comes out. Is that right?

I remember your article on WRB's saying Joe L. feels any WRB with a perm rate greater than 5 is overkill. That is due to the other elements of the wall assembly, right? But in the case of water trapped between sheathing and WRB more permeability would be better, right?

Answered by Deb Davis
Posted Wed, 10/03/2012 - 10:25

3.
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Deb,
As I explained earlier, virtually all plastic housewraps are vapor-permeable. That means that they allow any moisture trapped behind them to evaporate to the exterior.

Of course, if water is entering the wall assembly at a faster rate than it can evaporate, you'll get rot. The idea is to keep water out of your wall, and to encourage drying.

Most problems like you describe -- water behind the WRB -- happen because of flashing errors. If you install flashing correctly, and lap the housewrap over the flashing without any reverse laps, you'll go a long way towards keeping water on the exterior side of your WRB instead of the interior side of your WRB.

It's possible for water to accumulate at the sheathing layer during the winter due to air leaks through the wall assembly that bring humid interior air in contact with cold wall sheathing. The best way to avoid that problem is to pay attention to airtight construction methods.

In short, if you build you wall properly, you shouldn't have any "water trapped between the sheathing and the WRB." And if your sheathing ever gets damp, your WRB is vapor-permeable, and is designed to allow drying to the exterior.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 10/03/2012 - 11:19

4.
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Thanks very much. Clearly we were confused about this issue.

Answered by Deb Davis
Posted Wed, 10/03/2012 - 12:02

5.
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Martin tells it true- most Tyvek products are over 30 perms, and dry quite rapidly. Typar is in the 10-12 perm range and would still be able to release any moisure permeating from the interior that is aborbed by the sheathing, so long as there is some exterior breathing room to dry into. (Interior latex paints run 2-5 perms, after all.)

If you have moisture between the WRB and sheathing 9x out of 10 it's from an improperly flashed window/door situation, and literally NEVER a condensation problem. The other 1 in10 is bulk water getting thorough the siding and finding it's way through poorly taped &/or mis-lapped seams.

Putting at least 1/4" of rainscreen gap between the siding & WRB cures a lot of ills, up to and including some amount of air-leakage moisture transfer to the sheathing from the interior. Rainscreen gaps are now required by code in Canada- 10mm (~3/8") minimum.

With a rainscreened siding on the exterior there is no MA requirement for interior side vapor retarders. MA has adopted IRC 2009: http://www.mass.gov/eopss/consumer-prot-and-bus-lic/license-type/csl/8th... All of MA is in US climate zone 5, and in zone 5 vented cladding (rainscreen gaps qualify) per section N1102.5 the interior vapor retarder requirement are relaxed to class-III (standard latex paint) no matter what type of exterior sheathing or siding is used. It's pretty common to use 1x furring nailed or screwed at each stud on which to mount the siding, but there are mesh underlayments (John Obdyke, etc) that qualify.

BTW: IIRC in MA 2x4 construction requires R5 continuous exterior insulation (usually 1" XPS foam) to meet code min these days (and that amount of exterior foam would by-itself relieve the interior vapor retarder spec per IRC2009.) 2x6 still meets code-min without foam though.

Answered by Dana Dorsett
Posted Wed, 10/03/2012 - 18:35

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At a recent presentation by a retailer of high performance building products, the presenter said that the pore structure of Tyvek would "clog" with water if there was no capillary break--that is, if wet siding was installed directly against it. That matches something I've seen several times when tearing houses apart. Of course the presenter said that his products do not have this problem, and interestingly, neither does felt paper (or tar paper, I forget if he differentiated).

Answered by michael maines
Posted Thu, 10/04/2012 - 08:10

7.
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In terms of working within a tiny budget, the cost of dense packing may not cut it. Air-sealing the cavities and caulking between stud plates & subfloors etc as they go up is more effective at tightening up the place than dense packing, and using the sheathing the primary air barrier is more robust than relying on housewraps as the air barrier (however carefully detailed.)

Using reclaimed/recycled roofing foam from any number of MA vendors (search your local craigslist for "rigid insulation" or go to insulationdepot.com- their yard is in Framingham) usually has a lower $/R than batts. Air sealing with caulk & gun-foam, wet-spray cellulose for cavity fill and applyin 2" of exterior roofing foam outside the sheathing would probably come in at about the same cost as just dense packing the cavities, but would have more than 2x the whole-wall R value and need no interior vapor retarder, all at the same thickness as a 2x6 framed wall. If you used rigid polyisocyanurate (aka "iso") instead of EPS and went with 3" you'd have nearly 3x the whole-wall R value of the dense-packed 2x4/no-foam wall. And the warmer you keep the cellulose, the less seasonal humidity cycling it sees, and it's the humidity cycling that causes settling (not that damp-sprayed goods settle very much if at all in wall cavities, since it's bound together with water-activated adhesives.)

BTW: Roofing felt is a perfectly fine WRB, but going with 15# good is safer than 30# goods since the latter can be too vapor retardent (depends on the stackup.) The permeance of felt changes with it's moisture content, but even 15# felt only rises to ~5 perms when damp, lower when dry. The 30# stuff is sub-2 perms until it's a bit higher humidity than you'd want to see in the sheathing, and you'd have to make the interior side more vapor tight (or use exterior foam to keep the sheathing warmer.) Using reclaimed roofing iso is CHEAP and effective.

Answered by Dana Dorsett
Posted Thu, 10/04/2012 - 10:43

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Dana, we though the #30 was better because it has a lower perm rate -- though I think we read somewhere that it can clock in at 60 when wet -- because we are also trying to address inward vapor solar drive. I feel like Goldilocks.

Answered by Deb Davis
Posted Wed, 10/10/2012 - 16:44

9.
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Deb,
I think that #30 felt in this application is fine, although it is sometimes hard to install tightly in inside corners.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 10/10/2012 - 17:28

10.
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Deb: With fiberglass siding you have very little summertime inward vapor drive to worry about. If it were brick or stucco you'd need to pay that some heed since those claddings hold a huge amount of water to be release when baked in the sun. But fiberglass siding doesn't wick-up and hang on to water- it can't. Lower perm is not better, especially on the exterior layer in a MA (US zone 5) climate. Felt has to be totally soaked (and the sheathing would be soaked too) to hit anything like 60 perms (and I don't think it can ever reach that high.)

Since the siding is inherently back-ventilated any water that gets by it won't hang out for long. I'd still go with #15lb goods.

Do consider exterior foam though- the reclaimed stuff is cost-competitive in $/R with cheap low-density batts, and the warmer you keep the structural wood, the drier it will be. With only dense-packed cellulose cavity fill for insulation you'd be required by code to install an interior vapor retarder, and you'd be relying primarily on drying to the exterior. It doesn't take much foam to literally double the whole-wall R value of a 2x4 framed wall- about 2" of any foam insulation would get you there, but with polyiso you'd be there at ~1.5".

Answered by Dana Dorsett
Posted Thu, 10/11/2012 - 14:47

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