Condensation Cathedral Ceiling
We have a house under construction and a conditioned attic space and cathedral ceilings that need to be insulated. We need to achieve a minimum of R35. We are located in Montana Zone 5
We have Smart Baffles installed 1.5” from the roof. We have 6 inches left to insulate the cathedral ceiling.
Our two options:
- Closed cell Spray Foam
- Polyiso. A total of 3 – 2” boards with an r value of R 39.3 non each truss cavity. In a few cavities we may have to use 2-2” Polyiso with 1- 1.5” of Polyiso due to limited space with baffle install slightly off with greater than a 1.5” vented space. This would equal an R value of R 35.8
In order to save some dollars and some of the issues that may arise with spray foam (off gassing, poor install) we decided to install the polyiso ourselves. We installed a couple of truss cavities and on one of the cavities we installed of the three boards (total of 2”) in the truss cavity before calling it a day. The temperature overnight got to 15 degrees. When we go to the house to install more polyiso boards one of the boards fell out of the truss cavity as we had not spray foamed the sides yet and there was condensation on the smart baffle. There is no heat in the house.
We tore out another cavity where we had spray foamed and had all three boards installed and there was minimal condensation. Not as much as the one with one board.
Now we are rethinking this entire install. Did we have condensation based on the cavity not having all the insulation completed and sealed? Will the smart baffle produce condensation or will this type of insulation method cause condensation issues? Our plan is to spray foam the polyiso perimeter and even tape all the sides after spray foaming if that helps with air sealing. I have been reading about dew point and I am not sure if that may be the issue.
Help! We welcome your feedback and expertise.
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Replies
You have a baffle, so it sounds like you intend for the assembly to vent.
1. Is the baffle continuous from soffit to ridge?
2. Do you have a vented soffit?
3. Do you have a vented ridge?
What assembly do you intend when completed? ie please name each layer from outside to inside including your structural members and their depth.
We have vented soffits and a ridge vent at the top of the roof. Vents are not continous. we have a smart baffle that we installed at the soffit to protect the soffit from attic insulation.
Small attic is open and then we have smart baffles running up to the top of the roof with a ridge vent.
The cathedral ceiling is 24 OC with 7.5" depth. smart baffles have a 1.5" vented airspace and the remaining space will be either spray foam or approx 6" of polyiso providing a R value of 39.3
pal2,
You a much better off having the insulation follow the roof slope all the way to the eaves, and bringing those areas behind the knee walls into the conditioned space.
https://www.greenbuildingadvisor.com/article/insulating-behind-kneewalls
The depth of the top chords in the trusses doesn't have to dictate your insulation choices. You can fur them down as much as you want, or add continuous rigid insulation on the interior.
pal2,
If you want to continue with your plan to cut & cobble all the bays, I'd suggest you take out the smart baffles and use your first layer of foam as one instead. That both gives you more room for insulation, and also eliminates the void between the foam and baffle where condensation could occur - although I think what you found was almost entirely due to not having the foam air-sealed, and once that's done I doubt the void matters much.
As a general rule of thumb foam on roofs is best placed on one or the other side of the rafters where it can be continuous and therefore more effective. Cut & Cobble (and spray foam) is something you usually see when there aren't any other good options - and if it is new construction there should be lots.