Remodel – Adding Exterior Insulation
I have a large post and beam home in NH circa 1909. The hone is uninsulated and the wall and ceiling cavities are open. The exterior is painted wood shingles in very good condition. It’s appears that the exterior sheathing (under the shingles) is bead board. I intend to Sheetrock the walls and leave a large section of the ceiling open; so I would insulate the ceiling from the outside.
Everything I’ve read seems to recommend insulating from the outside (which would include insulating the wall cavities). I thought that I might use double studs to increase the R value and reduce the thermal bridging. But I’ve read about potential moisture issues with this approach. What I have read seems to recommend installing the vapor barrier on the exterior wall in this climate. I’m wondering if it is worth removing an exterior facade that is both attractive and in great condition to accomplish this.
I’m learning that wall systems and the issues involved in maximizing the true R value in, and tightness of, a home while reducing moisture problems that can arise is more complex than I thought. I’m wondering if a sub would simply blow foam into the wall cavity and tack on a vapor barrier.
Replies
I would not remove exterior siding thats in good condition. Closed cell foam insualtion will eliminate any air movement through the wall and it will give you great R value plus it locks the framing together nicely giving the structure more rigidity.
Great reply for this topic. It will very helpful for anyone.
We had the same issue with the shingled wall portion in our 1840 house here in Vermont. I opted to leave the outside alone, except for the walls that I needed to re-side, and filled the stud bays with 4” of rock wool with 2” polyiso on the inside, covered with new sheetrock. The polyiso is foil faced, taped and foamed for very good vapor retarding and air sealing. The house breathes to the outside. For the newly sided sections (with fiber cement clapboards) I have a WRB outside of the sheathing, and a 1/2” rainscreen. We don’t get a full thermal break, but the R-13 polyiso does most of it. There are lots of reasons to install exterior insulation, but given the siding’s in good shape, the interior solution works well, if you can give up a few inches on the interior. As for moisture issues with interior vapor retarders, it’s all in the details. The moisture that does make its way into the cavity needs to get out one way or another. I recently tore off the exterior sheathing on my son’s home, that I built in 1983, to add an addition to that wall. I didn’t use polyiso on the interior, I used 6 mil polyethylene. It was 1983! The entire wall cavity, insulation, sheathing, and sheetrock were in like new condition, after 37 years. It’s because I paid special attention to caulking all the sheetrock and penetrations on that exterior wall.
Thanks for the replies. Eddo, how did you attach the Sheetrock to the polyiso?
The key seems to be the air barrier as the bulk of vapor movement is through air movement.
It seems like any interior vapor barrier is going to get trashed a bit in the construction process. But if the sheet rock and any openings were tightly sealed, this could be an effective air seal.
As Sawdust Steve says, the closed cell foam should be an effective air seal from the outside. If there is an effective interior (sealed Sheetrock) and exterior (foam) air seal and a hopefully intact interior vapor seal, there shouldn’t be too much moisture getting into the walls in the winter and what does should dissipate out through the exterior wall in the summer.
I’m inclined to go double stud. Building Science Corporation has an article describing an expectation of high moisture content with this method but with findings that rebutted this expectation.
3 1/2" sheetrock screws through it to studs. The polyiso was attached to studs with just a few roofing nails, then foil tape over the nail heads. The sheetrock screws held the polyiso tight. Yes, the key is air sealing. The foil of the polyiso is the same as any vapor retarder, inside or out, it's going to be pierced by whatever fasteners you use for attaching insulation, siding, or sheetrock. Foil has the best low perm rating, and foil tape on the seams helps. The air sealing of the rigid insulation, foaming them to each other, the floor, walls, and ceiling, and caulking the sheetrock where necessary makes a pretty tight envelope. As for the double stud wall, I like that method. The vapor retarder can go within the insulation (carefully placed at the right depth) so you eliminate many of the punctures, you get a thermal break, and wiring can be done efficiently. Good luck!
I never thought about burying the air barrier like that to avoid penetrations. Clever!
I have one method to increase the insulation level of the walls in existing homes is to remove the exterior cladding and install rigid foam insulation.
Sounds very expensive and time consuming. I guess it depends on whether or not you can afford to give up interior space for more insulation.
I take an idea from here when I thought about remodeling by adding exterior insulation.