Is super insulating exterior walls with rigid foam board insulation possible?

Question

2" rigid foam-board insulation can have an R-value of up to R-10, layering 4 boards would create a R-40 value, right? However that creates an 8" foam wall above the actual wall. What would be a recommended way to attach all the boards to the exterior wall? Also how would you attach the finishing wall surface to the foam?

Reason to use rigid foam board insulation, is that its cheap?

Cost to insulate 16 Square feet to an R-40 value, $70 USD for rigid foam, $238 (not including installation) for closed cell spray foam.

I am thinking plywood base, vapor barrier, foam board base, screwed and glued down to base. Then can I glue??? the remaining insulation foam boards in place on top of one another? Then lastly how about attaching the exterior weathering wall, maybe something like exterior cement fiberboard? Can that be glued to the insulation foam-board.

Products in mind are:

• FOAMULAR® 250 rigid XPS foam insulation for exterior walls

• Natura Pro exterior fiber cement panel

Note: Spray foam insulation cost can be estimated here

Answer 1

Yes, Rigid foam (XPS and ISO) boards are frequently used in exterior applications, and can be a great option for improving efficiency. There are a couple of issues with your proposed approach, though:

1. Vapor barrier location. The vapor barrier should be on the warm side (probably inside, unless you live in a very hot & humid climate). Ideally, this is immediately behind your interior drywall. You also do not usually need an extra vapor barrier layer adjacent to rigid foam sheets, they are typically highly resistant to moisture.
2. Attachment. You cannot rely on glue to support foam and siding panels. These need to be mechanically fastened.

On a wood frame house, you'd want to use this order:

• interior drywall
• possibly a vapor barrier (e.g. poly sheeting), though the wisdom of this is debatable (see the comment from iLikeDirt below)
• wall studs (optionally with batt or cellulose insulation in between)
• exterior sheathing
• rigid foam
• wooden strapping for attaching siding, screwed to studs through foam. This provides support for the siding and also allows air circulation behind the siding, so any moisture that gets behind the siding can dry out.
• siding panels, nailed to strapping

Here's a visual from Fine Homebuilding, which has a couple of good articles on this:

Note that adding foam on the exterior of your house complicates some things. You will likely need to redo the trim on your windows and doors, which will become more in-set. These inset areas will need to be flashed properly or you will have leaks, which have more potential to do damage when you've got multi-layered, thick walls that could trap moisture. If done well, you can get very good insulation. But make sure to work with someone who knows what they're doing!

Answer 2

If you do this, make sure to stagger the joints and it would be best if you tape each layer's joints with a high-quality long lasting tape to prevent any air movement. UL181 rated tape would be good.

I would be pretty dubious about gluing the weather layer. It seems that spikes (long nails) are more commonly used. Or perhaps long screws would have better holding power.

You're basically building a SIP. Is it cheaper than buying one pre-made?

Answer 3

I was actually looking for information on how to attach the rigid foam insulation to the outside of a brick house, when I ran into the question, above.

I do not have a great deal of technical expertise or anything, but, I just wanted to say that in Southern Arizona the rigid foam insulation is often used on the outside of buildings. What I do know about using it in this climate is that there is no vapor barrier used, no fiberboard or plywood, either. The insulation is attached directly to the walls (but, I don't know how to do it) and then it is covered over with stucco or adobe. It's very simple and very effective.

I know that some people do glue it; but, I can't imagine that would be effective for long term, as glue dries out very quickly in the heat.

Answer 4

1. Don't ever use a vapor barrier under the drywall, only use under a concrete basement floor. Or if you enjoy trapping moisture then go ahead and use it.
2. 8" of XPS? Do you live in the North Pole? Firstly, you should use two layers (one overlapping the other). Depending on your location, use either 1", 1.5", or 2". So that gives you up to R20 on the outside and covers all the wood which is better than any system that does not use exterior insulation.
3. The 2nd layer of XPS nails to the first with those nails that have plastic washers on them. You only need enough to hold temporarily until #6 below.
4. You don't need 2x6 walls unless you're building really tall. Use 2x4's which is cheaper and then throw cheap R11 fiberglass in it. With 4" of foam, that system will give you a total R value of 29.5 (assuming your studs make up 20% of the surface area of your walls....your R11 fiberglass becomes 9.5).
5. After your tywrap you need something to create a 1/8" gap before the XPS....and don't count on "crinkle" wrap to do this.
6. Don't put your exterior boards right on the XPS (good lord....you've got nothing to nail into). Use 1x4 vertical wood boards (3/4" thick) screwed into each stud. This is what you will use to nail your exterior to. This also gives the wonderful benefit of a natural air flow of summer heat behind the exterior which will vent out the top into the soffit.