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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:

Note: Spray foam insulation cost can be estimated here

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I know that they sell rigid foam insulation in 6" thickness, which is what I intend to use, so that I will not have to attach them in layers. No idea if they sell them in 8" thickness or not. –  user20335 Mar 8 at 23:04
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3 Answers 3

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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
  • vapor barrier (e.g. poly sheeting)
  • 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: Exterior foam image

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!

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+1: Yes, we've used XPS foam sheets to insulate a concrete slab with stone pavers glued on the outside (pavers are also sitting on a sill for support and everything properly mortared in and flashed above). –  Conor Boyd Feb 10 '13 at 21:52
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I want to add that putting a vapor barrier right behind the drywall is a terrible idea in pretty much every climate. By doing so, you are preventing the wall sheathing from drying to the interior. So now it can only dry to the exterior. But if you add several thick layers of foam, especially faced foam, you're creating another vapor barrier. Now, I'm glad you added the recommendation for a drainage plane behind the cladding, but the fact remains with with such a wall, if any water does manage to touch the sheathing, it has no way to dry. That's very dangerous. Omit the interior vapor barrier! –  iLikeDirt Apr 22 at 17:13
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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?

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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.

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Do you know if they use it in the thicknesses (8") the OP was asking about? –  Niall C. Aug 18 '13 at 14:33
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