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Installing foam board insulation over existing plaster walls.

strangebrew | Posted in General Questions on

Greetings all.

I’m doing a small remodel on a 100 year old home to use as rental property. There is zero insulation in the walls. The walls have the original lath and plaster with 1/2″ drywall over that. I really don’t want to tear all that out and open the walls. I’m not a big fan of blowing in insulation from the outside. For one, it’s too expensive for this project and I know there can be issues with putting insulation in the walls of old houses because there’s no vapor barrier behind the plaster. I know there are many opinions on the subject. I’ve opened the walls of old houses before to find the insulation wet and moldy due to the lack of a moisture barrier on the inside. I thought perhaps I could install 1″ foil faced foam board over the existing walls, fur it out and install 3/8″ drywall over that. I know the R-value would be minimal, but it’s better than zero R-value and should help at least some to slow down the heat transfer. My concern is, will this also create a moisture problem. Will moisture be trapped between the foam board and the new sheetrock? Any and all opinions would be appreciated. Thanks

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  1. Expert Member
    Dana Dorsett | | #1

    Depending on local climate and stackup you might not WANT a vapor barrier on the interior when retrofitting an antique.

    The problems that arise with cavity insulation in antiques varies with stackup (and roof overhang). Clapboards nailed directly to studs is a show stopper unless you can gut it from the interior and build in at least 1/4" of air gap between the clapboards and insulation. Most plank-sheathed houses would be fine with blown cellulose though. Without sufficient air gap to back ventilate the siding the paint will fail sooner, and with no air gap the clapboards themselves can fail. But when the siding is on the outside of a plank sheathing there is usually at least SOME space between the planks & siding sufficient for a capillary break over at least half the surface area.

    Wet insulation is usually a bulk water or air-leak issue and (almost) NEVER a vapor diffusion issue. In most cased putting an interior side vapor barrier will make those conditions worse(!), since it limits drying toward the interior. In 100 old buildings there was typically NO flashing around windows directing the rain/dew outward, which allows quite a bit of water into the wall cavities. If there is no felt, rosin or kraft paper between the siding and plank sheathing, wind-driven rain or splash-back under the drip line of the roof can wet the insulation too. If the house has deep roof overhangs (say, 1-foot per story of height), and isn't in some high-wind area, those bulk water incursions are infrequent enough that it isn't usually a problem for a cellulose-retrofitted house. Air leaks from the interior side into the cavity can cause mold issues, but rarely does the insulation actually get wet unless it's HUGE air leak.

    It's fine to put rigid foam on the interior side of your assembly, but without knowing the stackup it's safer to use something other than foil-faced goods. At 1" XPS is almost a class-II vapor retarder, and may be too vapor tight, depending on just how much exterior side moisture drive there is. Unfaced Type 1 EPS is 4.5-5 perms, and is about as vapor retardent than latex paint. An inch of Type-II EPS comes in at about 2.8-3 perms, which is also pretty safe. If there are any foil or vinyl wallpapers in the stackup, that would create a moisture trap with foil-facers, but not with semi-permeable foam.

    A balloon-framed 2x4 empty cavity wall with 3/4" of plaster & lath + 1/2" drywall, and an inch of plank sheathing w/ rosin-paper under wide board clapboard comes in at a "whole wall" R just shy or R4 after thermal bridging, with credit for the multiple air-films. An inch of EPS would double the performance, cutting the heat loss in half. Depending on just how air-leaky that wall is, it could do even better than that, since it would lower the air infiltration rate. And an inch of EPS would not be so vapor-tight as to cause problems if you decide to insulate the cavities at some later point, or put foam on the exterior under some new siding, etc.. Use only UNFACED EPS. A lot of Type-I EPS (1.0 lbs per cubic foot nominal density) is sold with facers when it's only 1" thick, since the stuff is otherwise breaks too easily, and the facers are usually very low permeance. Seek out Type-II goods (1.5lb density, R4.2 @ 1"). If you can't find it, 3/4" XPS (~R3.8 & ~1.5perms) would still be OK.

    Where this house located? (Which US climate zone, if in the US?)

  2. strangebrew | | #2

    Thank you so much! That was a wealth of information. The house is in central Illinois. The stackup is vinyl siding directly over clapboard siding (nailed directly to wall studs) lath and plaster and finally 1/2" sheetrock. I cut out a very small 3" X 3" section of one wall to peek inside and noticed that there was tar paper between the plaster wall and the sheetrock. I don't know if this is true for all the exterior walls or not, or whether this would make any difference in the equation.

  3. Expert Member
    Dana Dorsett | | #3

    If you promise to always put back ventilated siding such as vinyl over the old clapboards, it's fine to blow cellulose into those cavities (any density) with or without using interior side vapor retarders. The place where you have to be careful is around windows. If there isn't flashing there can be drying issues from bulk-moisture getting in. Roof overhangs mitigate most of that, so there are some judgment calls to be made there. If you have no flashing and only 6" overhangs it may be better to leave those stud bays above/below the windows uninsulated until you can do something about the window flashing.

    The asphalted felt you found in some sections is something of a "smart" vapor retarder, 1per when dry, 5 perms when wet. It doesn't affect the basic recommendation. Central IL is the warm edge of zone 5, cool edge of zone 4. With back ventilated siding (like you have), you don't need any thing more vapor retardent than standard latex paint on the interior for a vapor retarder for wood sheathed houses. (In your case, the antique clapboards under the vinyl siding behave as the sheathing.) While it would be better if there was a layer housewrap or felt between the old clapboards & vinyl siding, this stackup will still work just fine, assuming there are still layers of paint clinging to most of the clapboard(?).

    The vinyl siding over clapboards-as-sheathing is effectively the "Vented cladding over wood structural panels" allowance built into the IRC 2012 R702.7.1prescriptives for zones 4 & 5:

    Any rosin-paper or tar paper between the clapboards & studs?

  4. strangebrew | | #4

    Nothing between the clapboards and studs. I started investigated the rest of the exterior walls to see if the stackup was the same throughout. I got a big surprise, it's not. The satckup on the remaining few walls is very strange. It goes - vinyl siding over clapboards, but now on the inside it has 3/4" x 6" horizontal board sheeting, THEN the lath and plaster and finally 1/2" drywall. I've never seen a wall sheeted like that on the inside before, and I've torn out a lot of lath and plaster walls. I've seen plenty of old house with horizontal board sheeting on the OUTSIDE, behind the clapboard siding. I don't even have a guess on why this setup.

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