Adding Roof Insulation to a Summer Cottage
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I am about to close on a 50 year old cottage on a tiny island off the MA coast. The house has no roof insulation and has historically been closed up in the winter. My wife and I would like to be able to use it year round and I was planing to add some insulation right away. The asphalt roof shingles need to be replaced, so I thought I would use that opportunity to add some rigid foam board to the assembly. The existing roof assembly is comprised of some thickness (likely 3/4″ or 2 1/4″) of x6 T&G pine planks on top of 2×6 rafters, all exposed on the interior side. I thought I would strip the roof, add ripped down 2×6 KD (to 4″) around the perimeter on edge. In the field, I would lay two layers of 2″ foil faced Polyisocyanurate board, with staggered and taped seams. On top of that I thought I would add a layer of 3/4″ roof sheathing and then ice and water and shingles. Thoughts? Thank you for your help! I’m also thinking this will help beat back some summer sun as well…
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Replies
I would use nail base panels.
Eric, you should check with your code enforcement officer (CEO) to see if your scope of work will require you to meet modern codes, R-49 in MA ceilings, or R-38 if the insulation extends fully across the top plate.
If they will allow you to proceed with your plan, a couple of years ago I did almost exactly as you describe on a client's house, except I used three layers of 1 1/2" polyiso and set the top layer between 2x sleepers. We had insulation in the framing cavities for about R-50 total. The downsides of not using enough insulation are that you will pay more to heat and cool it, you won't be as comfortable and you increase the risk of ice dams.
I looked into nailbase panels but they do not come in thick enough panels for what I needed, and the edge details are complicated. I also looked into SIPs but they were overkill for what we needed.
On another project that I designed but did not build, the last of the photos attached, we had an assembly that sounds exactly like yours except it was 3" of polyiso. We framed over the existing roof with 2x6s on 2x battens and placed 6" Rockwool in the framing cavities. That allowed us to vent the roof sheathing and get R- 42 total.
For lower carbon emissions you could use wood fiber insulation (Gutex or Steico), or reclaimed foam.