A Good Time to Add Exterior Insulation
I’m in a tough situation. I live in a 1920s house in New Jersey. There’s almost zero insulation. We are due to replace out windows, doors, siding, roofing. In the meantime I’m also trying to add insulation to the exterior walls — from the inside – room by room. We have plaster and lath thats a bit messy — so I’m replacing its with drywall.
My question is — if I’m about to replace all the siding should I just add insulation to the outside? If I do what products should we consider? Most of the people I deal push me down their path of thinking — the siding companies say 1/2inch is great, the general contractors say talk to an architect ….
the goals are simple – I want to improve the comfort of living in my house. Right now there’s part of the house that are too hot, too cold, or have poor air quality – pollen is a problem.
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The air quality and pollen problems are solved with air tightness, not just exterior insulation. Air tightness will also make the biggest impact to your energy efficiency. Consider ways to take the opportunity to get a better air seal.
If you're residing, that's the time to consider exterior rigid foam. Look for the article "Rethinking the rules on minimum foam thickness" by Martin Holladay (https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness). Use AT LEAST the thickness recommended in that article. You can use as much more as you want, although above about 2" exterior details get more complex. That's when you get into what we call "thick" exterior rigid foam.
If you will have access from the exterior when you reside, then that's the time to replace any insulation in the stud bays too. You can go with fiberglass or mineral wool batts. I like mineral wool. If you go from the inside, you'll likely also want to add a smart vapor retarder (and you'll need one if you don't have any exterior rigid foam).
While you're doing these projects, it's a good idea to try to improve your air sealing details at the same time. This means careful use of caulk and canned foam to seal things as you work.
Bill
Thanks. 10 exterior + 13 interior is fairly standard which I can get with 2 inches.
I’ve read your replies elsewhere in the forum. Is foil-faced iso appropriate for this. Can it be a barrier or is it safter to install a self adhered membrane directly to the existing sheathing?
Foil faced polyiso would be fine here, and would be what I would use. If you use 1/2" against the sheathing, and 1.5" over that, you can stagger the seams and still keep a reasonably fat final surface, although it's more labor. Either way you want to tape all the seams. I like to use Nashua 324A for this, in part because it matches the foil facer (324A is a foil tape). Flashing tape will work fine too.
If you want an additional WRB, put that over the polyiso. Some people put a drainage plane between the sheathing and the rigid foam, using something like Tyvek Drainwrap ("wrinkled housewrap"), but it's not usually necassary.
Bill
Many thanks. This link https://www.greenbuildingadvisor.com/article/rethinking-the-rules-bloxd io-on-minimum-foam-thickness
help me so much.