Here’s to beating a not-quite-dead-but-should-be horse…
I’ve just spent the last 2 hours or so reading through archived posts, and I still am not sure about the best practice for my situation…
Cold climate…Minneapolis, MN. My clients house was built in 1905, lath and plaster interior walls, obviously no vapor barrier other than the likely 12 layers of paint. No insulation in exterior walls either I’m guessing, and likely lots of air leaks to the wall cavities from the inside. Part of the project is a full redo of the hot water radiant heating system (inslab in basement, underfloor, some existing radiators to remain), so at least there isn’t a forced air system pressurizing the rooms and really forcing interior air into the walls.
The house is in need of a full siding and trim tearoff to the sheathing (probably 3/4″ shiplapped pine.) There are currently 2 layers of siding on the house. Almost all of the existing window frames (jambs and sills) are solid, so we are planning on retro-fits with Marvin Tilt pacs. Given the quality of the interior trim work and cost, an interior strip is not happening.
The new siding/trim is likely to be 4″ exposure hardi with Miratec trim.
What would you do? Cost is an issue, but not a huge one…this isn’t a “budget” project. We get few periods of consistent rainfall, so I’m not to concerned about rainscreen details. I’ve removed lots of old (70-90 yrs, not backprimed) clap siding off of houses in this area installed directly over tar paper, and unless there was an obviously poor detail, it is always dry and solid.
I like the idea of 3/4″ or 1″ rigid foam over existing sheathing, but am obviously worried about creating a vapor barrier on the wrong side of the sheathing. Cut holes and fill cavities with cellulose? My concerns are sagging insulation and condensation issues due to interior air leaking into the wall cavities. This obviously happens now, but they dry quickly with no insulation present.
Strip the exterior sheathing and spray foam from the outside? Seams unjustified from a cost benefit standpoint, but maybe I’m wrong. Or is the safest bet just to use a thin, vapor permeable foam (1/4″ – 1/2″) on the outside, and then do a very detailed air sealing job with a good housewrap (tyvek or equivelent) and call it good?
Replies
Joe, you have a real project on your hands. You live in a much colder climate than mine (Vancouver, Canada) and you have pinpointed the issue perfectly: If the exterior needs to come off anyway, what gives the biggest R-bang for the buck.
Most of us are looking at green building, and a lot of that "green" is heat/comfort at a lower cost.
In MY house (1931) there is a little insulation from having been blown in during the '50's but little else, and in my case, the original stucco is mostly sound, so I'm tearing out the lath & plaster, and insulating with R14, R22 and R40 in the ceilings.
In your case, I might think about adding exterior foam insulation, taping the joints and re-siding the house.
There was an article in FH recently by (I think Mike Gurlain) about adding exterior insulation, and he's from Rhode Island. It would pay you to find that issue and read it.
Quality repairs for your home.
AaronR Construction
Vancouver, Canada
I'd say either cellulose or foam, blown in holes cut in the sheathing. Done right cellulose will not sag. There are types of foam that can be injected like cellulose.
The paint on the interior is a good VB if you can seal most of the air leaks through outlets, etc. And tight-packed cellulose prevents the air infiltration that is most responsible for condensation.
But of course you can't use blown insulation if there's any K&T wiring in the walls. (Though you may want to take advantage of the situation to rewire the outlets on outside walls anyway.)
Did you see the thread titled !" foam house wrap started by jpeaton on 9/6/2007 under construction techniques. There is a lot of information there.
Yes, I read that one, and I can't remember how many others. Lots of ideas, but nothing definitive. After much other reading, I'm beginning to think that a thinner, more permeable foam combined with a good housewrap job may be the safest bet and still relatively cost effective. I have read that for our climate, at least 2" of foam would be needed to effectively reduce condensation issues at the sheathing plane, and this is just not practical for this house given all of the problems such a thick foam installation presents. 1" foam may be more manageable, but then you may be creating vapor management issues.Have any of you done hardi installations over 1/2" foam? I'm thinking it seems reasonable to nail it directly through the foam to the underlying 3/4" sheathing with ring shanked nails. The 5 1/4" siding is not as heavy per piece and a 1/2" gap doesn't seem so large as to create likely sag issues. Off stud joiners are probably still a good idea.
<<Have any of you done hardi installations over 1/2" foam?>>I did, but i used furring strips over the foam, so i could straighten out my walls. I blew cels while i had the siding off, used the foam to prevent thermal bridging. I used 30# felt paper under the foam, too. My understanding is that the cels will inhibit moisture movement to where condensation isn't a problem. I'm pleased with the result for the price, but of course spraying the interior of the cavities with foam would be even better.
Most moisture moves with air into the wall cavities, cellulose effectively stops that air movement thus it greatly limits the potential for wet insulation. Sealing penetrations, and around doors and windows with gun foam also reduces infiltration. I like to use 1/2 or 3/4" foam with taped seams as a drainage plane and thermal break on the exterior.Sounds like a big project.Garett
The house i did before, i put a layer of foam on the inside instead of the outside. The surface of the walls themselves seemed warmer to me than this house with foam on the outside. I had plaster walls already here so that wasn't an option. Both houses are snug and easy to heat, but the apparent temp. of the wall surface was/is dissimilar. Hmmm...
Plaster will feel colder than drywall or wood because the plaster is more conductive. Your hand doesn't instantly warm up the surface it's touching. But this effect is not related to the insulation effectiveness of the wall "system".
If your view never changes you're following the wrong leader
Why would plaster be more conductive than drywall? I suppose i'm assuming they have similar densities...and i could easily be wrong.
Drywall has a paper coating. Probably about half as conductive as plaster.
If your view never changes you're following the wrong leader
WHEN USEING FOAM THAT IS YOUR VAPOUR BARRIER , YOU DO NOT WANT ONE IN THE INTERIOR THAT WOULD TRAPE MOISTURE BETWEEN WALLS .
installing it that way in that function is the norm here...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
I'm just across the river from you and planning the same detail on my house when we re-side. Cellulose for the bays and 1" foam over the sheathing seemed to make the most sense to me too, but I don't have the weight of the Hardi to deal with. If you can, check back in and let us know how it goes and what unforeseen problems you run into...