I’m building an off the grid house with double wall construction and I’m concerned about condensation on the sheathing
My plan right now is to use double wall construction and have a catherdral roof. I’m looking for a R-60 on the walls and a R-100 for the roof. A little overkill but when you’re off the grid in zone 6 too much is never enough. The wall construction I’m looking to use is from outside to in Betflex siding, 2×4 batten to create an air space, house wrap, 1/2″ ply sheathing, 2×4 stud, 2″ of spray-on closed cell foam, 15″ of dense packed celulose, 2×4 stud, gyps (no vapour barrier). The cathedral roof would be from outside in standing seam metal roof, 2×4 batten, 1/2″ ply sheathing, 2″ of closed cell spray-on, 27″ truss, 27″ of dense pack celulose, 1×4 batten, gyps, again no vapour barrier. The idea about the closed cell is that it would create a very airtight house but it means that the vapour barrier component would be on the cold side. Am I asking for trouble with condensation with this set-up and can anyone tell me how to make it better?
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If you want to combine closed-cell spray foam on the exterior side of the wall assembly with an air-permeable insulation like cellulose, you really don't want your total framed wall thickness to be more than 6 inches. The reason is that you are eliminating drying to the exterior, and that approach only works if you maintain the proper ratio between the foam layer and the fluffy later. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
The standard advice for double-stud walls is that they should be able to dry to the exterior. If you really want to build a double-stud wall that is unable to dry to the exterior, you would need to install a very thick layer of spray foam to make the assembly work -- and that will get quite expensive.
If you really want to delve into the necessary calculations, this web site explains what you need to know: Are Dew-Point Calculations Really Necessary?
Here's a link to two other blogs that discusses double-stud wall concerns and details:
How Risky Is Cold OSB Wall Sheathing?
Monitoring Moisture Levels in Double-Stud Walls
Just a clarification. If I read your answer correctly I think you misunderstood what I was proposing. I was looking to put the closed cell on the *inside* of the sheathing. The idea is that the sheathing can dry to the outside and the cellulose and studs can dry to the inside. Having said that, if I replace the closed cell with open cell and add the appropriate thickness of rigid foam to the outside, will I still get my air sealing? Can the sheathing dry to the inside through the open cell?Also doesn't the rigid foam prevent the sheathing from drying to the outside.
You are correct that your sheathing will be able to dry to the exterior. But your layer of closed-cell spray foam isn't thick enough to prevent condensation from occurring on the interior face of the cured foam. (What will actually happen is the outer layer of cellulose will gain moisture over the winter.) Since your wall assembly will only be able to dry in one direction -- toward the interior -- it won't be as robust as a double-stud wall that can dry to the exterior.
Q. "If I replace the closed cell with open cell and add the appropriate thickness of rigid foam to the outside, will I still get my air sealing?"
A. Air sealing details do not depend on spray foam or rigid foam. It's possible to have a wall with spray foam that is well air-sealed -- but it is also possible to have a wall with spray foam that leaks like crazy. Similarly, plenty of walls without any spray foam are airtight (or nearly airtight). If you care about air sealing, pay attention to seams and penetrations -- and you can make your assemblies quite tight.
Q. "Can the sheathing dry to the inside through the open cell?"
A. Yes. As long as the drying rates (on an annual basis) exceed the wetting rates, the assembly will probably work.
Q. "Doesn't the rigid foam prevent the sheathing from drying to the outside?"
A. Yes. That's why the rigid foam needs to be thick enough to keep the interior side of the rigid foam (or the wall sheathing) above the dew point -- so that the sheathing is warm and dry, not damp.
"Betflex siding, 2x4 batten to create an air space, house wrap, 1/2" ply sheathing, 2x4 stud, 2" of spray-on closed cell foam, 15" of dense packed celulose, 2x4 stud, gyps (no vapour barrier)."
As noted by Martin, the low vapor retardency of the foam + plywood on the exterior side of the cellulose is a problem for high-R cellulose in a cold climate, since the colder layers of cellulose will be well below the dew point of the interior air, and will take on a lot of moisture over the winter due to the very low drying rate through the foam + plywood. It would be far better to put the 2" of foam on the exterior side of your interior studwall, since at 2" it's a class-II vapor retarder. The cold layers cellulose then take on a much lower moisture burden, and can dry into the air space through the plywood, which is 4-10x as vapor permeable as the 2" foam.
It would take an insane amount of exterior foam to reach the appropriate R-ratio in a nominal- R100 roof. If you want to dense pack the 27" trusses in a vented manner, it's best to give at least 2" of air space under the roof deck. This can be achieved by cutting some spacers out of 2" rigid foam to support cut'n'cobbled unfaced 1" Type-II EPS as the exterior side air-barrier to dense-pack against it can work. It needs to be meticuloulsly sealed at the edges with can-foam to prevent cellulose from getting into your vent cavity. That would give you 25" of EPS + cellulose for a nominal R90. At 1" unfaced type-II EPS is about as vapor permeable as interior latex paint- don't go thicker than and don't substitute XPS or polyiso, or a higher density EPS. Even though it's a flow-through stackup into a vented roof deck, it would be prudent to use vapor barrier latex or a smart vapor retarder (Certainteed MemBrain, Intello Plus, etc.) between the gypsum & cellulose.
Dana, I had thought about venting the roof in the manner you mentioned, but doesn't that leave me with the same problem of having my dewpoint in the middle of my dense pack?
I quess the question I should be asking is "IS there a reasonable way to achieve a r100 cathedral roof?" I should mention that I'm north of Montreal so about a zone 7.
Cold cellulose insulation won't cause any problems if there is a ventilation space on the exterior side of the insulation. If the cellulose picks up a little moisture during the winter, the insulation dries readily when temperatures warm up.
So if I understand you correctly. I could use the same technique to solve my wall problem. I could put in a ventlation space on the inside of my outside wall, perhaps even leave a 1/4" gap between the sheets of sheathing. Would the following work , unconventional as it may seem? Betflex siding, 2x4 batten to create an air space, house wrap, 1/2" ply sheathing,another layer of house wrap (to contain the the celulose) 2x4 stud, 17" of dense packed celulose, poly vapour barrier, 2x4 stud, gyps. I like the idea of putting the vapour barrier back in if for no other reason than to keep the building inspectors off my back.
Also for the cathedral roof could I do the same thing except no house wrap and using rafter vents? Could I still use a membrane on the exterior of the sheathing, allowing the entire assembly to dry to the rafter vent?
You don't need or want ventilation through the wall, just vapor permeability toward the cold side. For a very high-R wall in a cold climate, it's better to move the plywood sheathing to the outer face of the inner 2x4 frame. This makes two cavities, with most of the insulation at the exterior. Tape all seams and edges in the sheathing layer for air sealing. Enclose the outer frame with a permeable material like solid wood or glass-mat faced gypsum board (e.g. DensGlass). Make the windows and doors "innie", installed near the center of the assembly. Fill the inner 2x4 cavity with batts.
What this does is give you a well protected sheathing / air barrier layer and the ability to dry the assembly both ways. The exterior cavity could be any depth and any amount of cellulose insulation should stay dry.
You wrote, "I could put in a ventlation space on the inside of my outside wall..."
Your suggestion is not a new idea. Including a ventilation channel on the exterior side of your wall (along with vapor-permeable sheathing or, in some cases, housewrap without any sheathing) has been a standard approach for a long time. That's why builders include rainscreen gaps. The approach you suggest was mentioned in the article I linked to, How Risky Is Cold OSB Wall Sheathing?
For a further discussion of these issues, see:
All About Rainscreens
The Klingenberg Wall
"...doesn't that leave me with the same problem of having my dewpoint in the middle of my dense pack?"
Dew point is a temperature (the temperature at which the moisture is at 100% saturation for the moisture content for the given temperature and pressure conditions), and not a location within an assembly. With 3-5 perm latex paint between the cellulose and the conditioned space, and 3-5 perm foam (1" of unfaced Type-II EPS) between the cold side of the cellulose and the ventilation gap, the entrained air inside the cellulose layer in winter will have a dew point no higher than the interior surface of your exterior foam layer.
That will approximately split the difference between to the outdoor air's dew point and the interior air's dew point. The outdoor air's dew point will be strictly less than or equal to the outdoor air temperature which (in winter) is always colder than the temperature of the foam/cellulose interface. While there may be hours when some of the cellulose is cold enough to take on moisture, those numbers are few, and the hollow structure of cellulose fiber allows it to adsorb on the order of ~20% of it's weight as moisture without damage or loss of function. While there is a migration of moisture through the interior latex paint via vapor diffusion, it doesn't overwhelm the buffering capacity of the cellulose, since there is also migration of moisture through the EPS into the ventilation cavity, which is always at about the same absolute moisture level of the outdoor air. If you used a smart vapor retarder on the interior side the interior side would be less than 1-perm most of the season, and with 3-5 perms of drying capacity at the EPS the flow-through would result in even lower moisture cycling of the cellulose. But at 3.5lbs density the cellulose can easily handle the moisture loading without settling. (At open blow densities the material would settle a bit over years/decades due to the mechanical creepage due to the subtle dimensional changes of the fiber as it gains then loses moisture as adsorb.)
DensGlass (fiberglass faced reinforced gypsum) or fiberboard sheathing has even higher permeance than 1" EPS, and would stay well ahead of the moisture game, relying less on the buffering capacity of the cellulose. But these materials run about ~R1 at standard thicknesses, to R4 for the EPS.
This is an add-on to the original question. My basement is a walk out with three sides poured foundation. same double wall construction. What should be my vapour barrier strategy. The foundation guy is proposing to tar the exterior of the foundation, and I'm thinking to membrane it as well as its on a slope, we're putting in three levels of drains. Should I let the basement walls dry to the inside?
If you install an asphaltic compound on the exterior side of your foundation wall, that qualifies as dampproofing.
If you install a full peel-and-stick membrane and dimple mat, the qualifies as waterproofing.
Waterproofing is more effective than dampproofing, but it is more expensive.
Concrete walls don't have to dry inward. They don't have to dry in any direction. They can stay damp forever. For more information on this topic, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.
I understand the concrete not needing to dry, I'm more concerned with the dense pack celulose between it and the interior double wall. It will need to dry to somewhere, won't it?
If you want to insulate the interior side of a below-grade concrete wall, you certainly don't want to use cellulose insulation. The only acceptable types of insulation for this location are closed-cell spray polyurethane foam or rigid foam.
Here is an article with more information: How to Insulate a Basement Wall.