Vapor barrier needed if interior walls are horse-hair plaster?
I plan to put new, pre-stained cedar shingles on the exterior of my 1920s bungalow this spring. This is the only side of the house that hasn’t been renovated. Earlier renovations to other parts of the house revealed tar paper under the shingles, so I assume we’ll find the same thing under these shingles. The interior wall is horsehair plaster and there is no insulation (this is the only exterior-facing wall in the house that still has horse-hair plaster). I’ve read some confusing posts about Tyvek/Typar and moisture barriers. Do they create moisture problems with horsehair plaster? If so, what should I put under the shingles?
Thanks for your help!
Bonnie
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Tyvek and other housewraps are quite permeable to water vapor (most are more vapor permeable than traditional #15 asphalted felt, aka "tar paper") and will not trap moisture in the walls. But it's also fine to use #15 felt.
Is there an insulation in that wall? If not, prior to or in conjunction with re-siding is the right time to improve the air tightness and thermal performance of the walls. It may even be worth installing some insulating sheathing over the existing (probably plank) sheathing, and detailing it as an air barrier. How you go about it matters, as does the R-value of any insulting sheathing. The details are climate dependent- where are you located?
Bonnie,
It sounds like you are confusing the idea of a "vapor barrier" -- generally a layer installed near the interior (for example, polyethylene) -- with the idea of a "water-resistive barrier" (WRB) -- generally a layer installed near the exterior (for example, Tyvek, Typar, or asphalt felt).
A WRB is required by building codes and recommended by building experts as a good idea. As Dana correctly noted, either a plastic housewrap (Tyvek or Typar, for example) or asphalt felt will work. None of these products will trap moisture in your wall.
For more information on WRBs, see All About Water-Resistive Barriers.
Thanks, Dana and Martin. I live in New Hampshire and there is no insulation in that wall according to an energy efficiency test I had done on the house. So improving the air tightness and thermal performance of the walls will definitely be a good idea. Any advice on the best way to do this in a cold, snowy climate will be greatly appreciate. Thanks!
Bonnie
Find a contractor that does blown cellulose or blown fiberglass, get an assessment on whether the walls are in good enough shape to be dense-packed or not and some quotes (either dense-packed or low density.) Most of the time horsehair plastered walls can be dense packed, but if the lath or lath nails are in lousy shape there is some risk of cracks forming (or in worst cases, a blow-out) if the insulation is installed at dense-pack pressures. If it can't be dense packed, low density cellulose is far preferable to low density fiberglass, since it's far more air retardent than low density fiberglass. If it CAN be dense packed there is a slight (not huge) performance advantage to going with 1.8lb fiberglass (a standard dense-packed density for fiberglass) over 3.5lb cellulose (a standard dense-packed density for cellulose.)
The window flashing details need to be verified and possibly updated prior to insulating too, though that becomes somewhat less important if you have deep roof overhangs to mitigate the possibility & amount of rain penetration around the window framing.
Most of NH is US climate zone 6, but the very southern-most counties (including Strafford ) are all zone 5. Oil based (often leaded) 1920s vintage wall paints tend to be very vapor retardent, and plank sheathing is more vapor open than OSB or plywood, which makes insulating the walls reasonably safe in a zone 6 climate. But if you're adding insulating sheathing under the new siding the capacity for drying to the exterior is severely restricted. To reasonably protect the assembly from building up and retaining wintertime moisture the R value of the exterior insulation needs to be at least R5 for 2 x 4 framing in zone 5, or R7.5 in zone 6. There are many things to consider when going that route, so check back if you think you're going there.
If the exterior insulation needs to be thin, there is still some benefit to be had using 3/8" PERFORATED fan-fold XPS (the perforations which still allows some drying toward the exterior.) It's labeled R will be something like ~R1.9, but long term (after several decades) it'll be more like R1.6. That doesn't sound like much, but without it the "whole wall-R" of an insulated 2x4 wall with cedar shingles comes in at about R10 after accounting for the thermal bridging of the framing and the R-valvue of all other layers. Adding R1.6 to that would be about a ~14% reduction in heat flow through that wall, adding R1.9 would be about a 16% reduction, so call it an average of ~15% performance improvement over the full life cycle of new shingles, and it only adds 3/8" to the thickness of the wall.
Without the insulation or exterior sheathing you're looking at about R4- R5 "whole wall" best case, depending on the thickness of the sheathing & shingles, and the lath & plaster.
FWIW: I live in an 1920s bungalow in MA with the original horsehair plaster that was successfully dense-packed with cellulose, which made a world of difference!
Thanks so much for your detailed answer, Dana. I do know a contractor that does blown cellulose, so I'll give him a call and see what he thinks. Our house is in Grafton County and was apparently constructed from wood salvaged from an old barn around the corner. The lath I've seen in other parts of the house appeared to be in good shape, so hopefully that's the case in this living room wall. And that's a good point about possibly updating the window flashing details, since the three windows along this wall are the only windows that are original to the house (we like the wavy glass). Thanks again, and I'm glad to hear that you've also renovated a 1920s bungalow!
If the antique windows are reasonably air tight (or can be made so with retrofit weatherstripping, etc) installing an air tight low-E storm window over them gets you within arguments-sake of a U0.32 IRC 2015 code minimum performance level for zone 6A. That also makes the window a daily net energy gainer (even on north side windows). With clear glass storms it's more like U0.5-U0.6, and a net energy loser.
Harvey Tru-Channels are the tightest storm windows in the business, and they have hard coat low-E glazing as an option. Larson 's Premium Series low-E storm windows (distributed through box store chains) are also pretty tight (tighter & nicer than their Performance Series), and will do the trick, usually for less installed cost than a code-min vinyl replacement window.
https://harveybp.scdn1.secure.raxcdn.com/wp-content/uploads/2016/08/harvey-andersen-storm-doors-and-windows.pdf?x19811
https://www.larsondoors.com/windows