What is the best way to do a cottage reno adding an insulation blanket?
Hello, I am planning to replace the siding and add insulation to turn a 3 season cottage to 4 season. The current wall is drywall with poly vapour barrier, 2x4" framing with fiberglass. The sheathing was 7/16 asphalt sheathing with an exterior stained plywood on top (with batten strips).
We have restained it several times over the past 35 years. (Dad originally built it) I was thinging I could just remove the batten strips and then wrap with WRB (Delta Vent SA or similar) and then add 2" of EPS and then vertically strap for Rain screen and put vinyl or wood siding on top. My question is whether I should be removing the old plywood and asphalt sheathing and possibly the old insulation. and reinsulate and sheath. As well should I be putting the WRB barrier on the sheathing or the outer rigid insulation? Lastly, when adding a rain screen, I assume insulated vinyl siding would be of limited insulation benefit...correct?
Thanks, any input would be appreciated.
Depending on how much insulation you add, and the type, you could leave the exterior walls intact, but I wouldn’t do that with just 2 inches of rigid foam insulation. 2 inches of EPS rigid foam would be a vapor barrier, but it’s not thick enough to protect your wall properly.
Have you ever heard of the 2/3rds rule? That refers to keeping at least 2/3rds of your insulation outside of your vapor barrier, so given that it’s a 2x4 stud wall with fiberglass you are looking at about R15 at best. And 2 inches of foam will be between R8 and R10, so the interior surface of foam would be cold and at risk of developing condensation. Here is our building science video that explains it best –
Building science made easy – video
So my two cents would be – yes on the WRB weather barrier membrane on sheathing as you will then be airtight, and if you add 6 inches of rigid foam insulation you would have R24 and a warmer ‘vapor barrier’ surface of the interior foam. That would be a super insulated, airtight and durable wall. It may be more than you planned, but this way you save all the labor and cost of removing sheathing and insulation.
Alternatively – you could do the same thing (leave the sheathing and add a WRB) then add 2 inches of rigid mineral wool / Rockwool insulation as it doesn’t trap moisture. That’s a much safer way. Mineral wool is sort of a ‘foolproof’ insulation in that way, as moisture moves right through it and causes no harm. 2 inches of Rockwool you could attach with strapping, and if you wanted to go the thicker foam route then |I’d look at Thermalwall exterior wall foam insulation panels. They are easy to attach, are perfect for insulation as well as vapor protection.
And you can read here to find out how to choose the right rigid foam insulation for the right application so your walls can dry.
Great, thanks very much for your input...that makes a lot of sense.