parging for slab edge insulation
My plan calls for rigid foam insulation on the slab edge, covered by 1/4″ of parging. In order to get the right thickness I’m using courses of two different insulation materials. One is unfaced EPS, the other is fibreglass faced polyurethane. The facing is very smooth, so the actual surface is pretty much plastic resin. This seems like a pretty durable substance, but even if I thought it could stand up to UV exposure, it would look wrong if left unfinished (it’s got graphics and text on it…I guess I could try painting it).
My preference is to have the fibreglass skin as the outer layer, with some kind of parging. But will parging stick to it, even if I roughen the surface? My other option is to use the EPS as the outer layer. I know from experience that parging will stick to that pretty well.
The other question is what product to use for parging? I used Quikwall last time, but in looking at the product description it actually seems intended to stick to masonry, not foam or plastic. I used it on EPS about 7 years ago and it’s held up fine. Not sure if that would translate to plastic resin adhesion. I guess I may have to try a sample. Any other suggested products?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I don't think iso foam is approved for ground contact is it?
I'm not sure, but I didn't say iso foam.
polyurethane foam, iso foam usually the same thing
Usually not intended to get wet
Perhaps people mistakenly interchange but they are not the same, having different physical properties like flame spread. Also they can be used for below grade.
The resin components of spray polyurethane foam and polyiso are similar, but the blowing agents are different--usually hydrofluoroolefin these days for spray vs. a pentane compound for polyiso, which results in different characteristics. Some polyiso has glass fiber included in the mix which adds fire resistance.
Reply to #7
Yes the components are similar but the concentration are different. With polyurethane having a 1-1 ratio of polyols to iso and polyiso have a 1-3 ratio, meaning that the actual chemical structure is different as the isocyanate reacts with itself as well. This makes the base material more fire resistant, and it is not solely due to the glass fibers added.
I would look at parging products from the ICF folks. This can be installed right over the EPS, if you add an impact resistant EIFS mesh, it should take a fair bit of abuse.
Apparently there are also some ground contact rated cement boards you can use as well.
If the foam edge is not too tall, I would price out something like 1/8" steel strips. Keep at bare and let it rust over time.
I plan to use Styro Industries FlexCoat on my EPS. Steps are: attach it to the concrete using foam glue, then spray EPS with 3m 90 spray, attach EIFS mesh, spray again (maybe), then coat with FlexCoat. Time will tell if this is a good plan.
Styro IndustriesTuff II might work better, but it's more expensive.
You probably considered alternatives, but I used 12” Boral. I “painted” it with Semcoworks liquid membrane primarily to color match the siding.
The mention of flammability has me questioning whether to use this product for slab edge or skirt insulation (by skirt I mean the 3-4' wide insulation around the building for frost protection). It's only been approved for under slab use. Is there a legitimate fire risk if it's 6-8" underground? I'm trying to picture spilling a bunch of gasoline on the ground, having it ignite somehow and that igniting the insulation. Seems highly unlikely to me, but what do I know? Similarly, the insulation on the slab edge, if covered by a layer of cement parging, seems unlikely to ignite. How flammable is the stuff? Maybe my next test should be to try setting the sample piece on fire.
I must be overthinking this. This can't be worse that closed cell spray foam, which people install in their walls and under their roofs, right?
This is the specific product in question:
https://instapanels.ca/products/smooth-fiberglass-skin/
I would not worry about flammability of buried foam unless you were in a wildland fire area, even then I don't think I would be super worried about the buried stuff. The slab edge I would worry more about, especially in a wildfire area. If you are not in a wildfire area an approved ignition barrier should be more than sufficient.
there are a bunch of youtube videos of people burning the different foams in there backyards. I would check these out and not burn it yourself. It is pretty incredible how flamable XPS insulation is. Polyiso does the best, and polyurethan burns pretty readily, but with its added retarders seems to put itself out if nothing is feeding it.
If I were you, I would use rockwool. It is non combustible, Termite and ant resistant (or proof?), has a claimed R value wet almost equal to dry, and acts as a drainage plane. It is a bit more expensive though, and kind of hard to source right now for some reason. One option for the parging is metal, you could have it bent in an L shape that matches up with the lower drip edge metal you should install, and rivet them together, so the long leg covers the insulation and sticks down into the earth a few inches. Then you can paint this.
freyr_design,
+1 on rockwool if he can afford it and using flashing as a protective cover.
I’ve used a paint on product from Styroindustries. Very simple to put on and gives a stucco look to some extent