I have a small house project where we are using double stud exterior walls with blown-in fiberglass, and, in general, making the house very energy efficient.  The client wants Siga Majvest housewrap system as the infiltration barrier and house will be sheathed in cedar sidewall shingles.  In Oregon you are required to have a rainscreen, but we are intalling a sill pan flashing which allows us to forgo the requrement. The client has just found some Mira Drain (dimple board) at the local Habitat for Humanity re-sale store very cheap – the material costs for the whole house will be $350 - and wants to use this as a rainscreen just for due-dilagence.  This material is designed as a foundation drain mat/protection board and it does not breath. 

There will be allowance for some air movement from the gap at the bottom of the mat at the foundation, but the client does not feel it’s necessary at the top of wall or under windows, etc.  Our walls are 12’ high beneath the eaves and higher than that on the gable ends. 

The general theory on walls is that you put your impermeable barrier on the inside and the exterior is allowed to breath. My question is, should this layer be permeable, or will the fact that it is open at the bottoms suffice?   Does anyone know what code says about this type of waterproof rainscreen system? 

Thanks

 

Well what are you waiting for, just go in and try out what yo want to go for and implement. Do keep sharing your expereince as to what ever results you get after such hardship and effort.....

Posted By esger on 28 May 2011 12:20 PM

I have a small house project where we are using double stud exterior walls with blown-in fiberglass, and, in general, making the house very energy efficient.  The client wants Siga Majvest housewrap system as the infiltration barrier and house will be sheathed in cedar sidewall shingles.  In Oregon you are required to have a rainscreen, but we are intalling a sill pan flashing which allows us to forgo the requrement. The client has just found some Mira Drain (dimple board) at the local Habitat for Humanity re-sale store very cheap – the material costs for the whole house will be $350 - and wants to use this as a rainscreen just for due-dilagence.  This material is designed as a foundation drain mat/protection board and it does not breath.  <!--?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /-->

There will be allowance for some air movement from the gap at the bottom of the mat at the foundation, but the client does not feel it’s necessary at the top of wall or under windows, etc.  Our walls are 12’ high beneath the eaves and higher than that on the gable ends. 

The general theory on walls is that you put your impermeable barrier on the inside and the exterior is allowed to breath. My question is, should this layer be permeable, or will the fact that it is open at the bottoms suffice?   Does anyone know what code says about this type of waterproof rainscreen system? 

Thanks

 

That general theory is only valid for cold/very cold climates, and a true vapor barrier (on ether side) would be a mistake in all but the coldest high-altitude parts of OR.  It's generally better to design a wall stackup to be vapor semi permeable but air tight, yet resilient to minor air leakage from either side for the local climate conditions.

Rainscreen ventilation adds resilience by increasing the capacity of the assembly to dry toward the exterior.  To be fully effective the mere presence of the gap isn't sufficient- exterior air has to be able to convect through the gap to dry both the cedar siding AND the structural sheathing.  If using a vapor-impermeable dimpled plastic like MiraDrain, it has to be the type that is dimpled both sides, and it must be installed in a way that outdoor air will convect on both the interior & exterior sides of the material. Otherwise the less-vented side will be susceptible to rot, since moisture can't move through it via vapor diffusion..

When in doubt, leave it out.  This would be a mis-application of a highly impermeable product.  There's some chance that it could be made to work, but unless it's done right it's more likely to become a disaster in short years.

In an OR climate (or just about any US climate) you can stiget away with an exterior vapor barrier if:

 A: you put enough R-value on the outside of the structural sheathing as rigid or closed cell spray foam.

...and ...

B: use only semipermeable materials on the interior (no poly, vinyl, or foil sheeting.)

The ratio of foam-R to fiber-R necessary is local-climate specific.  The minimum ratio that works in Bend could be woefully inadequate in Fargo, yet overkill in Sacramento.

In a cold climate (say like Minnesota), if you have some type of external vapor barrier then you better have an internal one too. See here (bays 1 and 12) for an example of what happens if you don't. http://www.forestprod.org/woodprotection06huelman.pdf

You can also reference another actual study which concludes "Vapor retarders with a dry cup perm rating less than 1 were important in the Pacific Northwest climate" and "The lack of a vapor retarder resulted in excessive moisture accumulation in the wall cavity in this study". It also talks about the importance of rainscreen ventilation.

http://www.ewpa.com/Archive/2006/aug/Paper_296.pdf

Thanks Dana, what you say makes a lot of sense.

I guess you could put rainscreen ventilation at both the top and bottom of the siding for the convection you mention, but it still seems like a large area to properly ventilate. The owner is acting as their own contractor on this project, so the "unless it's done right" is the part is a big caveat here.

I'll pass on your comments to the homeowner and hope he'll drop the idea altogether.

Posted By esger on 31 May 2011 10:43 PM
Thanks Dana, what you say makes a lot of sense.

I guess you could put rainscreen ventilation at both the top and bottom of the siding for the convection you mention, but it still seems like a large area to properly ventilate. The owner is acting as their own contractor on this project, so the "unless it's done right" is the part is a big caveat here.

I'll pass on your comments to the homeowner and hope he'll drop the idea altogether.

The information shared by Dana surely is of key worth, would like to thank her too.....

I've been working on and building for forty years and have seen many different situations on new and old buildings. When I heard about the "rainscreen" ideas a few years ago it instantly made sense out of a number of questionable siding issues and became a "must do" on all my projects. Allowing the siding to breath, as it does naturally in pre-insulated houses and still does in barns and garages is the one sure way to prolong the life of wood siding and sheathing, as well as avoid many of the rot issues shown in the jonr's links above. Whether it is critical that it is continuously ventilated is another issue, since siding is inherently not airtight. Air will get behind siding; that is a known fact. When it brings water with it and when there is an insufficient space for ventilation, problems result. Continuous ventilation cannot hurt, though. As to the original question of using impermeable dimple board, that is simple - don't do it. Instead use 1x3 strapping, applied horizontally at the shingle spacing. If you're convinced you need continuous ventilation, you'll need two layers, but the experts on BuildingScienceAdvisor.com have discussed that issue and the consensus, which I agree with, is that the vertical layer is unnecessary. While there is a remote possibility of trapping water, shingles naturally shed water so the issue isn't really bulk water but moisture. With air surrounding the shingles they will dry out when they get damp, and give the sheathing some breathing room.

Posted By jonr on 31 May 2011 10:37 PM
In a cold climate (say like Minnesota), if you have some type of external vapor barrier then you better have an internal one too. See here (bays 1 and 12) for an example of what happens if you don't. http://www.forestprod.org/woodprotection06huelman.pdf

You can also reference another actual study which concludes "Vapor retarders with a dry cup perm rating less than 1 were important in the Pacific Northwest climate" and "The lack of a vapor retarder resulted in excessive moisture accumulation in the wall cavity in this study". It also talks about the importance of rainscreen ventilation.

http://www.ewpa.com/Archive/2006/aug/Paper_296.pdf

In that document they also state:

" Walls constructed with R-13 cavity insulation plus R5 foam sheathing provided better performance than a wall with R-21 cavity insulation only.  Combined with a smart vapor retarder the R-13+5 combination provided excellent performance."

They don't state explicitly where the test lab was located, but the climate near Pullman where WSU is significantly cooler than west of the Cascades in either WA or OR.

The CertainTeed MemBrain smart vapor retarder is only sub-1 perm when it's fairly dry (<25%RH)  at the membrane layer, but increases dramatically with RH (>10 perms@ 60% RH).  In a typical western OR climate 25% interior RH simply never happens except during the rarest of cold-snap events. Most of the season the interior RH is higher (30-40%).  They didn't compare the R13+ 5 under stucco construction with other interior vapor retarders in this study, but 2-perm paint would probably perform about as well in that stackup, in that location.

The also didn't specify the perm rating of the R5 foam, but assuming it was XPS it was likely ~1.2perms.  With more than 25% of the center-cavity R value as exterior foam, the average temp at the sheathing was likely above 40F. The mean January outdoor temp in Pullman is ~30F, so with a 70F 35%RH interior the average temp at the sheathing would be above the dew point of that air,  and resilient even to condensation from air-leaks. (And certainly resiliant to 2-5 perm vapor diffusion trough interior paint.) 

In Seattle/Portland climate the January OUTDOOR mean temp is ~40F, and the importance of interior vapor retarders drops significantly, with or without exterior foam, and a  sub-1-perm vapor retarder just blocks drying toward the interior, in a climate with plenty of exterior moisture drives from rainfall (much mitigated with rainscreen), and very low interior moisture drive hazard.  Interior poly would be a mistake, but with rainscreen & 1"exterior foam  you could pretty much skip the interior paint and still do OK, even with bare sheet-rock. Poly would be a mistake, not a requirement.

The ~1 perm of exterior foam vapor retardency of the foam in assembly # 8 also makes it resiliant to the intermittent but very high vapor drives of sun on water-saturated stucco, but were 0.08 perm poly used on the interior it might not work out so well with that type of cladding, particularly since #8 had no rainscreen ventilation.  An R13+5 (1 perm foam) with rainscreen can  work with only interior latex for a vapor retarder in US climate zones 5 or less, independent of cladding type. (But higher-R is still cost-effective.)

Without the exterior foam the sheathing runs colder, and is more susceptible to winter interior moisture drives The only assembly considered a failure in that test was the unvented (no rainscreen) stucco-clad 2x4 wall with only latex for an interior vapor retarder.  Even though it's highly permeable to water vapor, stucco is a "reservoir" cladding (as is most masonry), retaining a lot of rain/dew moisture, which gets released in intense bursts when warmed by the sun.  Without a rainscreen to vent these bursts, the sheathing ends up absorbing moisture.  With 1-perm/R5 foam over the sheathing, even unvented those short, intense drives are severely blunted, yet 1-perm still allows the sheathing to dry toward the exterior through the stucco.  With 1-perm foam + rainscreen it would do even better.

Paint and a smart vapor retarder aren't at all the same - besides the significant variations in perm rating, the latter serves as a air barrier (a significant factor in moisture movement, even if you have another exterior air barrier).

But in temperate coastal PNW climates MemBrain willl have an average winter perm rating of greater than 1, possibly greater than 2. Interior RH rarely drops below 35% there, even in not-so-tight houses.

MemBrain probably works best in cooling dominated or mixed climates with high exterior moisture drives, so when the cavity air RH rises from exterior moisture it lets the moisture escape into the the interior.

In heating dominated climates it goes sub-1 perm only if the interior air humidity is low. In a MN or ND bathroom it would open up during a shower, allowing MORE moisture into the wall to condense/frost in winter. In homes where people are actively adding humidity to over 35%+ it also increases the wintertime moisture drive. In zone 6 stackups with no exterior foam and no rainscreen need an interior vapor retardency reliably well below 0.5 perms, not just sub-1. But with sufficient exterior foam relative to the cavity-fill R the interior permeance can be relaxed to latex-paint levels.

And like anything else, MemBrain not an air barrier unless it's actually detailed as an air barrier. It's not particularly easier to detail MemBrain as an air-barrier than poly, sheet foam, or air-tight wallboard.

BTW: The Washington State University Natural Exposure Test Facility is in Puyallup, WA west of the Cascades, (and not cooler-drier Pullman) with Dec-Jan mean temp of ~ 40F, but a mean dew point of ~34F (yes, it rains all winter in Puyallup- I lived there in 1979-80.) The wintertime moisture drives there are often from the exterior, not the interior- you can't dry toward the exterior through wet siding without a rainscreen, but the moisture drives from the interior are low, on average.

It's no surprise that the stucco reservoir-cladding proved most problematic in that environment. The authors speculated that it was permeance of OSB vs. plywood that made stackups 1 & 5 perform somewhat differently, despite many similarites but they didn't spec the perm rating of the oil paint used. Oil paint are typically low- perm- as low or lower than the kraft facers on the insulation in wall 5, but not as low as the poly used in wall 1. But they also noted that the poly slowed the drying time in spring too.

They didn't test any lap-sided structure without poly vapor retarders, only the stucco. Conclusions about the necessity or effectiveness of low-perm vapor retarders on buildings other than stucco clad buildings are supported by the test data. Only one assembly tested had just latex paint, and it was stucco clad, without rainscreen.

MemBrain would be of advantage in the damp parts of the PNW, since it would enhance drying toward the interior when the humidity rises inside the studwall cavities from exterior vapor drives. But even 2-3perm latex would cut into it's drying capacity a bit. The WDU paper doesn't really demonstrate the risk from interior moisture drives in that climate. It does demonstrate that rainscreen will pretty much neutralize the extra moisture drive from the exterior in a stucco wall though. It also demonstrates the effectiveness of 1-perm foam (that they didn't specify) protecting from those same high exterior drives of stucco, even with semi-open higher-perm MemBrain interior vapor retarder.

Posted By jonr on 01 Jun 2011 04:37 PM
Paint and a smart vapor retarder aren't at all the same - besides the significant variations in perm rating, the latter serves as a air barrier (a significant factor in moisture movement, even if you have another exterior air barrier).

Can i know as to what is a perm rating.......... ?

permeability

the ability to allow vapour or liquids through

Posted By JohnyH on 05 Jun 2011 05:59 AM
permeability

the ability to allow vapour or liquids through

Permeability is strictly about water vapor, not liquids.  Many materials will not pass liquid water, but will pass water vapor (such as most closed cell foams.)

The perm rating is the amount of moisture that will pass through the layer at a given difference in water vapor pressure (not to be confused with air-pressure.)

US perms are denominated in grains of moisture per hour per square foot for an inch of mercury delta in vapor pressure.

Metric perms are in grams of water per DAY per square meter for every MILLIMETER of vapor pressure difference.

Here we're usually talking US-perms unless specified otherwise, but one metric perm is very very close to 1.5 US perms (to within the manufactured permeance of actual products.)

Then there are SI perms (nanograms per second per ... who cares? )

Posted By JohnyH on 05 Jun 2011 05:59 AM
permeability

the ability to allow vapour or liquids through

Thanks a lot for answering my query.....