Is continuous polyiso over plywood and concrete wall OK?
I have a 200-year-old timber-frame Cape in western Massachusetts. Last year we had Mass Save insulate the roof and three side walls with dense-packed cellulose. Unfortunately the fourth wall was an addition (70’s or 80’s) and already had fiberglass so the program couldn’t do anything to improve it.
We are ready to tackle this final wall. Since the inside is finished we won’t be removing the fiberglass but will be adding rigid foam to the outside.
From outside in our details will be:
- 1 x 10 Ship Lap Siding (Vertical)
- 1 x 4 Horizontal Strapping (with saw kerfs for drainage)
- 2 Staggered Layers of 2″ Felt Faced Polyiso (Seconds)
- Innie Windows (Flashed with details from Installing Exterior Insulation in Cold Climates by Thorsten Chlupp)
- WRB
- Original Plywood Sheathing
- Unfaced Fiberglass batts
- Drywall (ADA as best could be done with an old house)
The main issue is that the bottom half of the wall is concrete and part of a walk out basement. In addition it isn’t a nicely poured wall. Rather it looks like a home owner formed it roughly with planks and has some random rock protrusions.
My questions are:
- Is there any reason not to make this a continuous installation or should I stop at the transition and insulate the basement wall differently?
- Should I use a layer of XPS against the basement wall or some other moisture barrier for the Polyiso?
- How to space the strapping kerfs and what depth?
Some additional info:
There is NO interior vapor barrier. I am using ship lap siding because I had it available and to match the historic siding of the rest of the house at this time in not possible. Eventually I hope to get lucky and find a good siding source or rive my own shakes with a froe and lots of trial and error.
Thanks
-Jamie
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Replies
Jamie, will your proposed new wall have the ship lap all the way down over the concrete wall part?
Yes. The boards are long enough to go the entire 14' length.
Jamie,
As long as the concrete wall is above grade and relatively dry, it's OK to install polyiso over the exterior of the wall.
However, if the wall is uneven and bumpy -- with "rock protrusions" as you describe -- I think that it will be very hard to get your wall co-planar. You may want to come up with a system of exterior 2x2 studs on stand-offs, so that you can insulate this portion of your wall with closed-cell spray foam.
I wouldn't worry too much about the kerfs in your horizontal furring strips. The rainscreen gap is there mostly to allow evaporation of any moisture that gets past your siding; there really shouldn't be any need for drainage of liquid water.
I vote sledge hammer to even the wall surface out, parge with stucco mix right onto the exterior of the concrete wall (this is remarkably easy and quick to d) as to get an even smooth surface, insulate as one plane with the wall above with the exterior foam, add all the other bits and enjoy life.
Thanks Martin!
I was planning on some type of framing system for the basement 'bumpy' wall to create a straight line. Is closed-cell spray foam my only option? It is too small of a job to get an insulation company to the house and I haven't heard good things about the DIY kits. The wall is above grade and dry. The concrete wall framing (and any insulation between studs) will be behind the WRB. Could I use some type of other material there?
I am glad I don't have to worry about the kerfs. That will save some time and keep the full strength of the 1x4's.
Thanks for being such a great resource for everyone trying to improve the comfort and durability of their houses while diminishing their carbon footprints. There is a lot of misinformation out there. Your work and site make it easier to figure out.
Jamie,
If you have a method to keep the wall co-planar, there is no reason not to use the polyiso. Spray foam is not necessary.
It would be co-planar for the Polyiso but the details for the concrete wall would be:
Ship Lap Siding
Strapping
2 Layers of Polyiso
WRB
New 2x Framing
Closed-Cell Spray Foam, Air Space, or Other Insulating material?
Bumpy Concrete Wall
I gather the spray foam would be best but if that isn't feasible what other insulation could go against the basement wall?
With polyiso you want to be sure that the cut bottom edge is not in contact with the soil, and doesn't get chronic splash-back from the roof drip-edge line, since it's somewhat hygroscopic. If you don't have deep roof overhangs (rarely found with Cape style archetecture) you might be better off using EPS on that portion.