Insulating an old home
Jac195465
| Posted in Energy Efficiency and Durability on
working on an old house, removed lathe down to the studs and the outside clapboard siding. There was no sheathing so what is exposed is the horizontal clap board siding. What is your recommendation for insulating the outside walls. Is it possible to use foam insulation (closed cell or open cell foam) directly against the siding or do I need some kind of air space between? If an air space, would15# black felt paper against siding be sufficient? the back side of the siding has never been painted. Siding is pine and 140 years old. Would appreciate any input.
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John,
You need to establish an air gap (even if the air gap is relatively small) between the back of your siding and your insulation. There are two methods:
1. Install vertical strips of asphalt felt in each stud bay. These strips of asphalt felt should be cut a little bit wider than the width of the bay, so that you can create stapling flanges on the sides. These strips of felt are stapled so that they are against the back of the siding. (You'll get enough of an air gap because the felt tends to wrinkle.) These strips of asphalt felt won't be airtight, so this method shouldn't be used with fluffy insulation like fiberglass batts. This method makes sense if you plan to install spray foam. For more information on this method, see Sticking With Spray Foam for My Renovation.
2. Install vertical "sticks" measuring 1"x1" or 1.5"x1.5" in the corners of each stud bay, up against the back of the siding. Then install 14.5-inch-wide or 22.5-inch-wide pieces of rigid foam in each stud bay, up against the sticks. Install the rigid foam in an airtight manner; the seams between adjacent pieces of foam should be taped, and the perimeter of each piece of foam should be sealed with caulk, tape, or canned spray foam. This layer is your air barrier and your WRB. Then you can fill the rest of the stud bay with the insulation of your choice. Ideally, this layer of rigid foam should either be somewhat vapor-permeable (for example, unfaced EPS), or should be thick enough to keep the interior side of the rigid foam above the dew point during the winter. For more information on dew points, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Hi Martin, I just saw this question/answer in the latest Fine Homebuilding edition. I have a follow-up question regarding existing stucco. I am currently going through renovations on my kitchen and will be moving through the house and have walls open. Since stucco absorbs moisture I would assume that the air gap behind the sheathing at the interior is of great importance. I was thinking of using the strips as you suggest. However, in zone 5 / Minneapolis do you have recommendations as per insulation. 2-in of XPS is ~R10 and will allow a very small amount of vapor through or should I fill the remaining with Polyiso (2 inches right at limit).
Of note: my house has newspaper wrapped in wax paper between the studs and sheathing.
As long as it's vented both top and bottom even 1/2" is sufficient air-gap for the stucco to dry into. If you use 1" XPS as the exterior side air barrier sealed in place with can foam it would be fine to do the rest with fiber insulation, which would be a lot cheaper.
Air leaks are a bigger problem than vapor permeance, which is negligible through even 1" of XPS (about 1-1.5 perms). Air sealing cut'n'cobbled foam board between studs is pretty questionable for the long term and a full cavity fill of cut'n'cobble is a laborious waste of good foam.
Using blown or compressed-batt fiber insulation (or open cell foam) in the rest of the cavity and ~1-2 perm 3/8" fan-fold siding underlayment under the new wallboard (seams taped with housewrap tape) gives you a modest thermal break over the studs cutting the framing losses by almost half, but also provides a fairly robust air barrier / vapor retarder layer. Fan fold XPS has thin laminated facers to lower the vapor permeance, so it's important to pull the specs on anything you install there. Something in the 1-perm range is preferable in this stackup than 2 perms, but 1.5-ish would still be fine. The additional R1.9 of thermal break over the framing with rock wool, cellulose or fiberglass in the cavities yields a comparable or higher whole-wall performance to R5-R6 foam board thermally broken by studs. It's cheaper, less work, and more resilient than a full-fill cut'n'cobble approach.
Possible alternative to foam on the U.S. market for option 2: WRB wood fiberboard. https://foursevenfive.com/product/gutex-multitherm-40-pallet-54-boards-512-sf/ Air barrier from this outfit can be added if appropriate.
Gregg... Gutex is so cool, but it is also so hard for me not to imagine it as crumbly oatmeal like a MDF ikea dresser left out in the yard for a winter.
Ethan: Fiberboard sheathing has a long and largely succesful history of surviving the moisture extremes. You can even leave this stuff out in the rain, uncovered on the job site without concerns about degradation.
The bad rap for fiberboard sheathing in the US is due to it's high vapor permeance, which can create high moisture content conditions in the finish interior walls of air conditioned buildings in higher humidity climates. It also becomes an issue for masonry-clad buildings that are air conditioned- the sheathing does fine, but the finish wall paint can fail, or the paper facers on the wallboard can become moldy. Even 3/4" asphalted 2-sides fiberboard runs ~15perms under high humidity conditions, and Gutex, SonoClimat ECO4 et all are north of 20 perms. In comparison, half inch OSB is a "smart" vapor retarder, under 1-perm when dry (class-II vapor retardency) , and ~ 5 perms (a class-III vapor retarder) even at fairly high humidity conditions.