Tiny weekend cabin cathedral ceiling insulation
Hello,
I have a tiny cabin (12′ x 12′ with 13.5 ft ceiling). It is used once every two weeks for a weekend.. I have 2×4 walls and 2×6 rafters. My roof is osb with steel panels. My wall cover will be wood stripes.
I did some work to insulate my roof but I think some mistakes where done. I filled every nook with spray foam (from can) including my roof Ridge which had this gap that I am not sure if was vent or not… Also, I filled the eaves with foam around the corners.. My logic was air tightness.
I then installed pink Batts (faced) between rafters. They are meant for 2×6 walls…
Reading that article about cathedral ceiling insulation by Martin Holladay – I might have done some mistakes… I can take down the insulation I put but of course I rather not – if possible.
Questions –
1. If I understand the logic of the rot issue – if I install poly sheets (thick 6mil ones) over these Batts then the wood cover over it – I will reduce if not eliminate the passage of moist air into the roof cavities.. My cabin is small enough to do it in one sheet – eliminating cuts. Also, this cabin isn’t used much. Then will this be possible to do? If not – next question
2. Following the article – if I want to do a non vented ceiling I can install rigid foam sheets and then Batts.. But if I am right my zone is 5 (upstate NY – near port jervis). So I will need about 4″ of rigid foam just for that.. Then in a 2×6 it won’t be possible / needed to have a Batts as well but an only rigid foam insulation scheme. Right?
3. Are there any other options for me?
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Replies
John,
You have created an unvented roof assembly. According to the building code and building science principles, you need to install either spray polyurethane foam or rigid insulation between your rafters. (The only other option is to remove your roofing and install your rigid foam above your roof sheathing.)
Fluffy air-permeable insulation plus polyethylene won't work, and it violates the building code.
In your climate zone, you need a minimum of R-38 ceiling insulation. That means about 6.5 inches of polyisocyanurate (some between the rafters, and some as a continuous layer on the underside of the rafters) or about 6 inches of closed-cell spray foam. If you use polyiso, you have to install the rigid foam sheets in an airtight manner, by sealing the perimeter of each rectangle of foam with canned spray foam or caulk.
If you want to install a combination of rigid foam under the sheathing boards followed by an air-permeable insulation like fiberglass batts, you'll need a minimum of R-20 of rigid foam (about 3.5 inches of polyisocyanurate) plus enough fluffy insulation to achieve the code minimum requirement of R-38. That means that you will probably have to build down your rafters by sistering some additional lumber or by thickening up your ceiling assembly in some way.
If you live in a rural area without any building code requirements, you could choose to install less insulation -- but I don't recommend it. If you use fiberglass batts in this type of roof, you risk moisture accumulation that could eventually rot your roof.
Thank you Martin.
Like I mentioned - this is a recreational cabin. I will use it from Friday night until Sunday after noon every other weekend.... Hardly a regular house. How likely am I to risk rot with just leaving it as it is?
John,
I'll admit that the risk is low. The reason we have building codes, however, is that building use changes over the years. Many weekend cabins eventually become year-round homes.