We’ve been experimenting with it since 1981, when we built our house from 6″ concrete blocks and insulated it on the outside with 4″ of blue SM. We used 2 coat cement plaster over stucco mesh on that. A lot of work, also expensive for the mesh. CMHC published an interesting “Building Research Bulletin” on bonding plaster to foam back in the late 60’s or early 70’s.

I built my shop using post and beam, with 4’x8′ infill panels of 2″type 2 (double density) white beadboard (wish I had used 4″). The panels are plastered on both sides with “Surewall” which is premixed surface bonding compound, basically portland, sand, an accelerator, and chopped fiberglass. It’s been great for 15 years now – zero maintenance, zero problems. The shop was fast and cheap to build. It was amazing how rigid the building frame became once the first pair of corner panels was plastered on both sides – a great demo of the “box beam” effect of stress skin panels.

Since then, we’ve put a couple of additions on the house using 4″SM with glass mesh embedded in the homebrew mix described earlier. The 1/8″ plaster on the inside is not up to firecode, however. To make it legal, I think you’d have to increase the plaster thickness to 1/2″. The only problem I have seen has been on our concrete block house, where carpenter ants found a nice habitat in a section of wall where there had been a water leak from a faulty roof flashing.

Proper flashing and detailing for moisture is important for most construction. The big EIFS problems that I first heard about were in slam-bam-thank you-ma’m type tract housing in North Carolina. It is not a system that has really been around for that long, so those types of things are probably to be expected. However, you sure see a lot of it in commercial use.

I’m most impressed with stressed skin panels, however, for a variety of reasons, including great structural capability with a minimum of materials. We made an insulated 8′ x 8’sliding door for our shop, for example, that weighs 150 lbs and is completely rigid. As a mason, I’ve never liked stick frame construction, and how many times you have to go over the walls, and how boring most of those tasks are. And I’m getting lazy in my old age, so regular plaster is way too much work.

No drainage space. The foam was over masonry. It was mechanically fastened, so there is in fact a drainage plane between the foam and the concrete blocks, although you’d need a lot of water back there before it would drain rather than get absorbed by the masonry. There is extruded styrofoam such as the blue SM. Then there is expanded foam, or beadboard, usually white.

Expanded foam is made by mixing two chemicals together and casting giant blocks, and then cutting them into slabs with a hot wire. You can vary the density of the foam. It is expensive to ship because of the bulk, so extruded foam from my local Home Hardware costs about the same. Extruded foam is harder to bond to, and the thickest I can get is 4″ in 2×8 sheets.

Type II foam is “double density”, twice the density of standard beadboard. You can get any thickness slab you want – we used 6″ for the roof of our sunspace. It is used a lot these days for the manufacture of commercial SIPS (structural insulated panels), which are used often to skin over timberframes. The SIPS can also be engineered to provide the structure, so the timberframe itself becomes optional.

>No imbedded mesh?

No, the tensile strength for the stress skin is provide by chopped glass fibers, rather than glass mesh. The glass is already in the pre-mix. You have to use a special alkali resistant glass. The product was popular for surface bonded concrete block construction, where you stack the blocks dry and then skin them with a thin layer of tensile plaster on both sides. A lot stronger than regular unreinforced concrete block construction. We have an old USDA document online that has a recipe for the mix: http://www.mha-net.org/msb/docs/surfbond.PDF

>I’d be very interested in a little more detail on this door.

We put two 4×8 sheets of 2″ double density beadboard on the floor and skinned the exposed side with Surewall. The next day we turned it over and skinned the other side. You also need an edge detail – a 1×2 wood frame works well. We actually just made another one for an addition to our shop, using 2″ extruded foam, glass mesh, and 1:1 white portland:sand with 50% of the mixing water replaced by Weldbond.

I am curious about the possibility of plastering right over the foam (I don’t want to lose headroom in the attic. Any one have any suggestions/experiences?

We have done this. It is a little tricky getting a good bond to extruded foam, so I’d highly recommend doing a test panel first, letting it cure, and then trying to pull the plaster off the foam. We used the glass mesh that is sold for EIFS systems (STO, Thoro, etc.). There are two types, a heavy and a lighter “detail”mesh. We used the lighter. It comes on a 36” roll.

You could buy a commercial acrylic plaster in a 5 gal bucket, and try that. It would be the easiest, and the bond should be pretty good. The commercial systems use a 2 coat system with a separate portland/acrylic base coat to embed the mesh, but that is more involved, and the base coat needs more skill to apply.

We mixed our own – see recipe (in link above)

The other issue will be that you can only plaster up to the edge of your rafters, so there will be a joint there between plaster and wood that you’ll have to consider unless your attic is outside of the house envelope.

If all you’re trying to do is cover the foam, you could also check out Stuccoflex, which is a super rubbery acrylic that stays permanently flexible and is very sticky. You wouldn’t necessarily need mesh. An almost identical version of the product is sold through Home Hardware stores here in Canada as “Perma Chink”, and is used for chinking log houses.

You may also have fire code issues. Ie., you may have to cover foam with 1/2″ of drywall or equivalent. Drywall would be a lot easier than 1/2″ plaster in an overhead application.