Modified Perfect Wall, What happens?
jdclassen
| Posted in Green Building Techniques on
When looking at Joseph Lstiburek’s Perfect wall system; What happens when we change the order of some of the layers. I am looking at the Zip-R Sheathing produce which would place the Water and Air control layers outside the Thermal Control Layer. Are there any concerns with this. The only downside I can see is that we no longer get the opportunity to use multiple layers of Insulated Sheathing with staggered seams.
Would this change depending on the climate region?
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Replies
Jeff,
We need more information to answer your question.
Please describe your proposed wall stack-up, from inside to outside, describing every layer -- and please tell us your geographical location or climate zone.
-- Martin Holladay
"When looking at Joseph Lstiburek's Perfect wall system; What happens when we change the order of some of the layers. I am looking at the Zip-R Sheathing produce which would place the Water and Air control layers outside the Thermal Control Layer. Are there any concerns with this."
-No.
"The only downside I can see is that we no longer get the opportunity to use multiple layers of Insulated Sheathing with staggered seams.
Would this change depending on the climate region."
- One reason for multiple layers of foam with staggered seams has to do with tape failure resulting from foam shrinkage. ZIP-R panels don't shrink and the tape is apparently very durable.
-Colder climates and high wind load areas that require thicker ZIP-R panels will require additional bracing for the framing.
-ZIP-R requires a bunch of nails.
Climate is Zone 4 (Midwest) Stack up is as follows: Latex Paint, Gyp Board, 2x6 Framing (possibly 2x4 if thicker insulated sheathing is used), Zip-R sheathing (deciding on Thickness), Rain Screen, Cement board siding.
Jeff,
Unless you plan to leave the stud wall uninsulated, and place all of the insulation on the exterior side of the sheathing, this isn't a "perfect wall." Here at GBA, we use the term PERSIST to describe the wall that Joe Lstiburek calls a "perfect wall." For more on the PERSIST approach, see this article: Getting Insulation Out of Your Walls and Ceilings.
In Climate Zone 4, there are no restrictions on the thickness of the exterior rigid foam layer for the type of wall you are describing. The R-value of the Zip-R sheathing can be any R-value you want. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
My biggest question is about the control layers and how this might effect primarily moisture in the wall. There is no vapor barrier and the air and water barrier are outside the thermal layer. Would there be any adverse effect of layering this way as opposed to the thermal control being outside these layers?
I realize this is NOT the PERSIST approach, It is a modification that is easier (for me) to build. I do not want to have any issues down the road and want to ensure quality and performance.
Jeff,
Q. "My biggest question is about the control layers and how this might effect primarily moisture in the wall."
A. I don't know what you mean by "this," but the reason that Joe Lstiburek calls a wall with exterior rigid foam a "perfect wall" is that researchers have found that exterior rigid foam, properly specified, results in OSB or plywood wall sheathing that has a lower moisture content (is dryer) than is the case in a wall without properly specified exterior rigid foam.
Q. "There is no vapor barrier."
A. First of all, building codes do not require, nor do building scientists recommend, that a wall include a vapor barrier. Second, building codes require that buildings in cold climates have a vapor retarder -- a less stringent layer than a vapor barrier. Third, the exterior rigid foam is a vapor retarder. Fourth, the building code allows walls with properly specified exterior rigid foam to get by with minimal vapor control on the interior of the wall, as explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
For more information on this topic, see these two articles:
Do I Need a Vapor Retarder?
Vapor Retarders and Vapor Barriers
Q. "The air and water barrier are outside the thermal layer."
A. It's totally normal for the air barrier to be on the exterior side of the thermal layer. For more information on this topic, see One Air Barrier or Two?
That said, many builders who install rigid foam on the exterior side of the wall sheathing end up taping the seams of the sheathing. In this way, the air barrier ends up being on the interior side of the rigid foam insulation , which is perfectly acceptable.
Finally, the water barrier is always on the exterior side of the thermal layer -- so your worries on that issue aren't clear to me.
-- Martin Holladay
Martin, just to confuse the discussion a little, aren't there cases where the water barrier is inside the thermal layer? For example, when using mineral wool as continuous insulation, the water barrier can be (and often is) behind the mineral wool.
Jeff, in terms of where the control layers are, Zip-R is conceptually similar to using rigid insulation as sheathing (with no structural sheathing), and taping the seams at the outside. Here's a BSC article about that practice: https://buildingscience.com/documents/insights/bsi-030-advanced-framing
The potential worry I guess is that moisture from the inside could travel through the seams in the insulation and reach the OSB facing of the Zip-R. At a certain point, you either have to trust Huber (and the experiences of people using the product) that this is not an issue in practice, or not!
Very few walls are BSC's "Perfect Walls". PERSIST walls are. Brick and block cavity walls with continuous cavity insulation are. Most walls are variations and compromises, and that's just fine. Heck, Joe's "Perfect Roof" is even more rare. And the standard commercial roof even has a double vapor barrier (The concrete on steel fluted deck underneath and the roof membrane above), and it works just fine! So everything is usually a little bit more complicated than the idealized designs ;)
You might find skimming through this article where Joe explains how a whole bunch of different types of walls work from a moisture standpoint illuminating: https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
Brendan,
Q. "Martin, just to confuse the discussion a little, aren't there cases where the water barrier is inside the thermal layer? For example, when using mineral wool as continuous insulation, the water barrier can be (and often is) behind the mineral wool."
A. Good point. You're right. That said, a wall with exterior mineral wool usually has two types of insulation: one layer on the exterior side of the water barrier, and one layer on the interior side of the water barrier (I'm thinking of the insulation between the studs).
The moral of the story: Generalizations are tricky.
-- Martin Holladay
The thing that worries me about the thought of Zip-R by itself is that the air control layer is outside the insulation. How much air gets between the seams to the sheathing at a different temperature (and thus condensation). I would guess that there's enough drying to the exterior at those points so that it wouldn't be a problem, but I don't know that myself.
I will note, from BSC's Perfect Wall article "BSI-001: The Perfect Wall": "And once again the best place to control this air thing is on the outside of the structure—but under the insulation layer so the air does not change temperature."
"The potential worry I guess is that moisture from the inside could travel through the seams in the insulation and reach the OSB facing of the Zip-R. At a certain point, you either have to trust Huber (and the experiences of people using the product) that this is not an issue in practice, or not!"
- ZIP-R panels have a gapping feature (oversized foam along one-4ft and one-8ft edge of each panel). This feature is in place so that foam seams do not line up with the edges of the panels themselves and thereby provide a continuous uninterrupted layer of foam.