Installing foam on exterior foundation walls
I want to install 6″ of GPS on the exterior of an existing concrete basement wall.
One question that has come up is whether we have to install single pieces of 6″ thick foam, or if we can layer them (3x 2″ thick pieces) in a similar way to above grade wood walls.
Above grade finishing is likely to be parging, and the assumption was to use adhesive (PL300) to attach pieces to the wall and each other.
A single piece of unfaced 6″ thick foam would be the easiest option, however it’s a custom order thickness with longer lead time. Faced 2″ pieces are available off the shelf (and cheapest) and unfaced 2″ pieces should be easier to get than the 6″ pieces.
Alternatively, instead of adhesive, are there any other suggestions for attaching the foam to the wall (for above ground) and finishing details if using multiple sheets?
Thanks
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Either way should be fine. A single layer would be less work, but thick sheets of foam are surprisingly heavy and you'll have the potential for thermal bridging if there are gaps between sheets. Foam shrinks a little as it ages, so even if the sheets are tight on day 1, they may leave gaps later on. But taping or parging over the joints would maintain airtightness so the gaps probably wouldn't have a big effect.
Hilti IDP fasteners are like plastic nails for attaching rigid insulation to concrete walls. Unfortunately they have limited lengths and are only good for up to 3" foam: https://www.hilti.com/c/CLS_FASTENER_7135/CLS_INSULATION_FASTENERS_7135/CLS_BASEMENT_PERIMETER_INSULATION_7135/r217. There may be competitive products that are longer.
Hi Andy 2022, this appears to be a very common issue. If someone invents a new good product, there is a market out there.
To start, here is a link with a Q&A with the same question and a few options and some comments. This is dated 2019.
My thoughts.
You mention: A single piece of unfaced 6″ thick foam would be the easiest option
Comment:
I agree with the post above that you may very well be better off with the 2" sheets to avoid thermal bridging. While this is on the exterior and below grade, I think of the issues with structural SIPs panels that fail. The gaps between these thick sheets of "foam" insulation are difficult to seal. Applying the sheets in layers allows you to overlap the sheets and cover the seams of the first layer with the next layer of XPS board.
Taping the seams with a tape designed for XPS would help reduce airflow at seams above grade and would help keep the sheets in place as you install them. If it is a DYI job and you have the time and a little extra $$, a person could also apply glue/sealant at the seams on the portion that is above grade.
You asked: "instead of adhesive, are there any other suggestions for attaching the foam to the wall (for above ground)"
Comment: It is glue or some mechanical fastening mechanism. Finding 7" or 7-1/2" concrete fasterners might be tough. If they are out there, they must be expensive per unit. I suggest using a different fastening technique for above ground. (see attached images)
1. Install the XPS sheets vertically on the last layer (never heard that this against code).
2. When you backfill, the weight/pressure from the backfill will hold the lower part of the XPS firmly in place.
3. At the top of the XPS sheets, install a heavier gauge (maybe 22 gauge) metal flashing that would come over the 6" (3 sheets of XPS). The vertical lip at the front of the flashing can come down as much as you want in front of the insulation boards to protect the top of the XPS board from an force that might want to pull them off the wall.
4. You can size the flashing - after - you finish installing the XPS to ensure that the vertical front lip on the flashing is as tight as possible to the XPS.
You asked: " finishing details if using multiple sheets?"
Comment: I am not at all against parging but you asked about alternatives. This response focusses on alternatives. I may get heat for this suggestion but I have two suggestions.
1. Place a galvanized corrugated sheet of metal in front of the XPS.
> Corrugated sheets could be vertical or horizontal. Placing horizontal would allow you to use really long lengths. One shop in my area produces sheets up to 40' long and 30" wide.
> You would have the metal flashing extend over top of the 3 sheets of XPS, as well as the in front of the corrugated metal. The corrugated metal would be held in place at the top by the flashing.
> At the bottom, you could have the galvanized sheet extend below grade a few inches and again have the pressure from the backfill keep the sheet of steel in place against the wall.
> These will rust eventually... but parging does not last forever either.
2. Place PVC sheet material in front of the XPS.
> Just know that it is easier to dent and scuff the PVC but it will never rust or rot (what are doing to our planet?)
> It is easy to wash with soap and water for the most part. But I believe it only comes in white.
> Link for Lowes in Canada - https://www.lowes.ca/product/pvc-trim-boards/royal-mouldings-limited-38-in-x-4-ft-x-8-ft-pvc-sheet-board-2878-677203
My response is open for peer review :)