How should I insulate my exterior walls?
I live in zone 4a, northern Virginia. My house is a single story, 2×4 exterior walls, no plastic wrap on the interior, no sheathing on the exterior, no house wrap on the exterior, and the exterior siding is RB&B board. The house is 40 years old and has just 40 year old batting insulation with siding. I want to spray foam, from the exterior, but one person says do not use closed cell because it is too hard to cut smoothly and water could settle in any uneven places, the next person says to only use closed cell. I don’t know if I should use a house wrap, I have been told it is not needed with spray foam. I don’t want to put up sheathing, because of cost, and not wanting to adjust around the windows. Any help would really be appreciated.
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Replies
Michael,
I have never heard of "RB&B board." Does that mean "rough-sawn board and batten (board)?"
(That's one too many "boards" -- but it's the best guess I can come up with.)
First, you don't want to install spray foam directly against your siding.
Second, every wall needs a water-resistive barrier (WRB), and spray foam doesn't work as a WRB. For more information on this topic, see All About Water-Resistive Barriers.
There are a couple of tricks that are used to insulate walls without any sheathing. Here are two possibilities:
1. You insert 3/4" x 3/4" vertical "sticks" in the outside corners of each stud bay, followed by a layer of rigid foam. Then the spray foam is installed on the interior side of the rigid foam. (This creates an air space between the siding and the rigid foam.)
3. You insert rectangles of asphalt felt in each stud bay. Each piece of asphalt felt is wider than the stud bay -- wide enough to create two tabs, one on the left and one on the right. These folded tabs are stapled to the studs. Then the spray foam is installed on the interior side of the asphalt felt. For photos illustrating this technique, see Sticking With Spray Foam for My Renovation.
RB&B is reverse Board and Batten. Just a reverse pattern.
I should have said that my plan is to remove - carefully - the siding. Remove the old batting insulation, and then spray foam directly against the back of the drywall, in between the 2x4 studs. I planned on filling the entire area with open cell spray foam, then thought I would use Tyvec next, and replace the old siding. Though not perfect this would be better than the current situation. I was just not sure if I would have any moisture problems.
The house has a 30 inch overhang, so direct rain is only a problem in a bad storm.
If you pose the question as wanting to improve your insulation without excessive cost, you might find options that are more cost effective than spray foam. Although replacing the fiberglass you have with foam will increase the R-value a little, it won't do all that much, because you will still have thermal bridging of the studs.
But I don't see any moisture problem with your plan.
Thanks for your answers.
Now I am considering replacing the old batting with new batting, from the outside, then using Thermablok 1-1/2" wide encapsulated strips on the outside of the 2x4 studs and any other framing, then tyvec and the old RB&B siding, which is still in perfect condition even after 40 years...
It is in perfect condition because of the overhangs AND because it is it's own rain screen assembly. It keeps dry mostly and when it is wet, it dries fast!
Michael,
Removing and reinstalling all of the siding on your house is a big job. It's going to cost a lot, either in time (if you do the work) or dollars (if you hire someone to do it). Having made that investment of time or dollars, it makes sense to do a good job of insulating this wall. It's an opportunity that may not come around for another 40 or 50 years.
The fiberglass batts aren't really the problem (unless they are infested with rodent nests or installed in a truly haphazard fashion). The main problems with your wall are: (a) it leaks air, and (b) there is no insulation to interrupt thermal bridging through the studs.
If you don't hire a spray foam contractor, you could take that money and use it to pay for a continuous layer of exterior rigid foam. That rigid foam would perform better because it would interrupt thermal bridging. If the rigid foam is installed in an airtight manner -- or if you take the time to install a layer of OSB sheathing with taped seams to act as your wall sheathing and air barrier, along with a layer of rigid foam -- you'll have a much better wall.
And by the way, you might want to consult an engineer to determine whether your walls have adequate bracing.
For more information on this topic, see How to Install Rigid Foam Sheathing.