We need help figuring how to insulate our old house
House was built in 1919. It has a 500 sq. ft. footprint with 2 floors + full walkout basement (1,500 sq ft total). Located in Climate Zone 4.
Because of serious structural issues (failing foundation and significant racking), the house was completely gutted. When opened up it was discovered that there is no sheathing on the house. Lapboard siding is nailed directly to studs (5″).
We are in a historic district and the original lapboards and windows/doors have to stay. 1/2 of the house has aluminum siding on it. We are hoping to keep the aluminum siding on until we can afford to do the outside (we’re working on the inside to get moved in). So insulation has to go in before we replace any lapboard and/or exterior trim.
What is the best way to insulate that takes into consideration:
-installation cost
-performance value
-health (VOC and inhalants) ?
It is my understanding that we need a vapor and moisture barrier in addition to insulation. How can we get all of that from the inside? Our GC recommended foam and batt. Is that our best option? If we do foam and batt now, will we be able to replace any of the siding later?
Thanks so much!
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Replies
Full dimension 4" studs or some other depth? Is it 16" o.c. regular spacing or something else?
You can't use spray foam directly on the interior side of the siding, or the paint will fail pronto. Since you have it all open, take the time to give it a coat of latex or acrylic latex primer on the interior side, which will limit the potential for paint peeling on the exterior, and limit the propensity of the siding to split and peel.
Install some 3/8" thick 1.5" wide strips of OSB or XPS foam on the interior side of the siding on both sides of each stud and stud plate as a spacer to guarantee a minimum air gap for the next layer. Then install 1/2-1" foil-faced polyiso sealed to the studs and stud plates with 1-part can-foam to make the sheet foam an air tight weather resistant barrier. In zone 4C it takes at least R2.5 of foam for dew point control on 3.5" of fiber insulation, and it's not a bad idea even in zones 4A & 4B. A cavity of 3/8" is sufficient ventilation gap for a 2 story wall, which has a very good convective drive to aid in keeping it dry. (Even in rainy Vancouver B.C. a 10mm rainscreen is deemed sufficient even on 1-story structures. 3/8" = 9.5mm, which is good enough.)
The rest of the cavity can be filled with damp-sprayed fiberglass (JM Spider can be damp sprayed) or damp sprayed cellulose, either of which will fill to near perfection, much better than batts, since batts are designed for the dimensions of modern milled lumber on standardized spacing. Cellulose is lower environmental impact than fiberglass, fiberglass is slightly higher performance from an R-value point of view, but has less favorable moisture buffering characteristics and usually higher cost. The alternative would be dry-blowing cellulose or fiberglass in mesh, which adds a step, and more expensive than damp spraying.
You do not need or want an interior side vapor barrier in zone 4- standard latex paint on wallboard or plaster is sufficiently vapor retardent, and with R3-R6 foam on the exterior side it just doesn't matter. If you prefer to have a broad-sheet material as a continuous air barrier, 2-mil nylon sheeting is a "smart" vapor retarder (eg Certainteed MemBrain), that limits the rate of wintertime moisture uptake without appreciably limiting the drying rate. (Material cost is about 10-12 cents per square foot.)
Then finish out the interior with a paper faced gypsum wall board finished with standard latex paints, no oil paint, no vinyl or foil wallpapers. Latex paint on wallboard runs 3-5 perms, and is perfectly adequate as a vapor retarder in any zone 4 climate, even zone 4C, if there's at least R2.5 on the exterior side of the fiber insulation.
Wow, thank you so much! I don't understand much of that, but I am hoping if I show this to my GC he will understand.
The studs are a full 5" and I don't know the spacing. I am in NC, so that's 4A.
Is there anything else that you can recommend for the windows (and the sash, casings, etc) and how to make the best of the originals wood ones. They are all coming out to be completely refurbished by a local restoration specialist.
Thanks again, much appreciated.
Full 5"! That's great--you can put in a 1/2" gap, 1" foam, and still have 3.5" cavity left for blown in or batt insulation. If the spacing happens to match a modern standard, you might go with mineral wool batts, if you have confidence they'll be installed carefully. But more likely it's non-standard spacing and so you'll want blown in along the lines Dana suggested.