While at the Greater Montgomery Expo last Saturday a spray foam company suggested the following:
Drywall
2x4 stud wall
Open or closed cell foam between the studs
OSB sheathing
2 inches of closed cell polyurethane foam (instead of DOW Chemical Styrofoam) to insulate the studs and OSB
1 inch air space
Brick

They said I should install the brick ties before they sprayed the closed cell foam.  The brick ties would have to be longer than normal to go through the 2 inches of spray foam to reach the brick.  I was assured that they could spray the foam smooth and consistent enough that the 1 inch gap between the foam and the brick would be maintained.

My preference would be to install regular brick ties with longer nails after the spray foam has been applied.

Question:  Does anyone foresee any problems with this system being used in the Southeast?  Should a vapor barrier be installed between the closed cell spray foam and the brick to keep the foam dry?

Posted By Alton on 28 Apr 2010 08:16 PM
While at the Greater Montgomery Expo last Saturday a spray foam company suggested the following:
Drywall
2x4 stud wall
Open or closed cell foam between the studs
OSB sheathing
2 inches of closed cell polyurethane foam (instead of DOW Chemical Styrofoam) to insulate the studs and OSB
1 inch air space
Brick

They said I should install the brick ties before they sprayed the closed cell foam.  The brick ties would have to be longer than normal to go through the 2 inches of spray foam to reach the brick.  I was assured that they could spray the foam smooth and consistent enough that the 1 inch gap between the foam and the brick would be maintained.

My preference would be to install regular brick ties with longer nails after the spray foam has been applied.

Question:  Does anyone foresee any problems with this system being used in the Southeast?  Should a vapor barrier be installed between the closed cell spray foam and the brick to keep the foam dry?

At 2" thickness,  exterior closed cell foam IS a vapor retarder (at ~1.0 perms), and will be sufficient to protect the OSB as long as there is still a 1" cavity between the brick & foam.  Sun on rain or dew-wetted brick (or even capillary ground moisture wicking into the brick) results in a very high inward vapor drive, but that gets interrupted by the cavity, and any subsequent condensation runs to the bottom of the cavity & out the weep holes rather than stagnating in the wall structure.  A 2" layer of closed cell will make the wall perfectly air-tight, so no high-moisture-content air from those vapor drive spikes can enter the wall, and the permeance is low enough that there's very little vapor transport into the wall either- it's the right approach.

Avoid using more than an inch of cc foam in the stud cavity, or you may run into issues of moisture being trapped in the OSB. If you went with a full 3.5" of cavity fill the inward-drying capacity would be dramatically lower, but at an only  inch you have ~2.0 perms (about the same permeance as standard interior latex paints.) 

If doing a full cavity fill, go with open cell foam, spray fiberglass, or wet-spray cellulose, all of which will keep the OSB from accumulating moisture via vapor diffusion, and in the cellulose case, capillary draw.  (Cellulose would give also give it both higher hygric buffering capacity, protecting the wood from rot  by wicking moisture away and dispersing it to dry slowly, and thermal mass to delay and lower the air-conditioning peak loads.) Fiberglass & open cell foam dry more quickly than cellulose in a flooding/plumbing leak situation, but allow more moisture that finds it's way into the cavity to end up in the wood under normal conditions.  A full cavity fill will give you a total exterior-foam + cavity fill R value of ~ R25 (center cavity), with a clear-wall R  (thermal short-circuits of studs & framing  factored in) of over R22, which would be great from a comfort and cooling/heating cost point of view.  That's more than twice the R value of traditional 2x4 + batt  construction.  An all-foam solution of 2" on the outside and either 1" of closed cell or 2" of open cell in the cavity probably meets code minimum.  A hybrid of 2" cc on the exterior + wet-spray cellulose on the interior as best bang/buck.  Once you have a full foam exterior it's already air-tight, and the rationale for the foam on the cavity-fill goes away, and it's usually more expensive. 

Batts work too, but not nearly as well as sprayed insulation due to the inevitable gaps & compressions that occur even with the most fastidious installers. If you go with batts, high density "cathedral ceiling" batts work measurably better than standard-density stuff.  If you go with spray fiberglass, wet-spray super-fine filiament versions with water-activate adhesive (eg. JM Spider) are won't settle or sag. (Wet spray cellulose also has water-activated adhesives to prevent  sagging/settling over the decades.)

Thanks Dana1.  I was hoping you would respond.  Your answer makes sense to me.

Using polyurethane spray foam on the exterior as part of a ventilated rain screen wall is relatively common in southern Europe.  The difference is that they use masonry instead of sticks and OSB. 

It is sprayed directly to the wall and no other barries are used.

Bruce

Dana1,

Would you recommend this setup in hot/humid climates over other methods for conventional framing?  Up to this point I was considering 1-2" of styrofoam on the exterior with spray foam in the stud cavities as Alton discussed.

If I go with cc foam on the exterior plus cellulose on the inside, how would you recommend insulating the attic?

Thanks!

I'm glad Dana is on here....she is verrrrry smart person! But for the OP.....why would you want to go thru all of  that expense with more labor involved and using verryyyyy exp foam when you could just use ICF instead? What do you think Dana?  IMO......it seems the foam folks are just trying to make something like an ICF with a higher price/profit for them......

Thanks Dana1 for your response, I just keep reading and learning!

John

Posted By awhite on 01 May 2010 09:05 AM

Dana1,

Would you recommend this setup in hot/humid climates over other methods for conventional framing?  Up to this point I was considering 1-2" of styrofoam on the exterior with spray foam in the stud cavities as Alton discussed.

If I go with cc foam on the exterior plus cellulose on the inside, how would you recommend insulating the attic?

Thanks!

Styrofoam (EPS- expanded polystyrene bead board) has fairly modest vapor retardency, and in truly hot-humid climates may not be sufficient, depending on the type & installation methods of the siding.  Extruded polystyrene (XPS, pink/blue from the big-time vendors) is semi-permeable at 1", and semi-impermeable at 2" or more.  Poly or foil facers sometimes used on EPS (or iso) is highly impermeable.   

In hot humid climates you want the less-permeable materials to be outside of the the structural wood including the sheathing.  It needn't be highly impermeable though, just significantly less permeable than the cavity-fill and interior wall finish (no vinyl or foil wallpapers, please.)

If you used closed cell foam in the cavity fill you'll need to use highly-impermeable stuff on the sheathing.  If you go open-cell or fiber in the cavity, 1.5-2" of XPS and building in a 1/4-3/4 rainscreen-gap back-ventilation behind the siding would be enough.  Using foil-faced goods in combination with a 3/4" rainscreen gap may be useful in some areas as a means of rejecting radiated heat as well as vapor permeation.

Any exterior foam sheathing needs to be made air-tight to work well.  Tape/caulk/mastic or foam-seal the seams & edges.  If using multiple layers, lap the joints by ~12".  A square inch of air leak is worth a whole wall of vapor permeation through 1" of XPS.

In attics, if it can take the weight between the, cellulose has a performance advantage over other fibers at high temperature differences, but rafter mounted radiant barrier will measurably enhance cooling performance if you're below ~R35 on other insulation (unless you're using "cool roof" materials, which lessen the effect.)  Getting at least 3" of cellulose (or 6" of fiberglass) over the joist-tops provides a decent thermal break over the framing, but at 1.6-2.0lbs/ft^3 cellulose starts adding up when going thick. Fiberglass is roughly half the density, but loses significant R-value at high temperature differences, and is slightly translucent to radiated heat (making radiant barrier even more useful).  But It'll have more performance per-pound of material on an attic floor, when loading issues are in play.

Air sealing between the attic and conditioned space is job-1, before the insulation goes in.  Sometimes there's a rationale for going all-foam here, but usually just spot sealing or a thin sealer layer over the whole thing is sufficient.  Other times (such as homes with ducts & air handlers in the attic it's easier/better to make the roof deck both the pressure & thermal boundary going with a conditioned or semi-conditioned attic/cathedralized ceiling approach, sealing & insulating the roof deck.

If you're going with a sealed-attic with an insulated/cathedralized roof deck various combinations can be used, but either wet-sprayed cellulose (blown-in-bag) or open-cell foam with a few inches of spray-fiberglass for the code-required ignition barrier may be the easiest best bang/buck for making it into a "conditioned attic" works well in hot-humid climates. Read this.

If you have fairly simple rooflines, thick iso or EPS panel goods with pre-laminated nailer decks placing the insulation above the structural roof deck can be an even better/cheaper way to create conditioned attics.  The performance is quite good since it provides 100% of thermal break over the rafters/trusses. You have to do the math on cost, but it's typically cheaper than spray foam.  (Detailing the air sealing the interior still has to be done on the attic going this route.)  Both Hunter & Atlas (and others) have vented-nailer versions of high-R iso panels.  A 3.5" iso+ OSB panel typically has ~R20 of insulation factory-applied to the OSB nailer, and they come as thick as 5.5-6".  If you need to keep the thickness down for construction detail or labor-cost reasons (long-screwing a 6" panel is a bit of a pain compared to a 3-4" panel.)  Combining a thinner panel  above the roof deck with spray insulation on the interior works.

Dana1,

Thank you for your detailed response.  I agree with your recommendations and explanations.  However, I would like to mention that "Styrofoam" is a trademark of The Dow Chemical Company and refers to XPS (Extruded) instead of EPS (Expanded) polystyrene.  See:  http://building.dow.com/media/trademark.htm

Posted By Alton on 03 May 2010 03:30 PM

Dana1,

Thank you for your detailed response.  I agree with your recommendations and explanations.  However, I would like to mention that "Styrofoam" is a trademark of The Dow Chemical Company and refers to XPS (Extruded) instead of EPS (Expanded) polystyrene.  See:  http://building.dow.com/media/trademark.htm

Thanks for sussing out the trademark issue! (And my apologies to Dow Chemical.)

 I was just making sure as XPS rather than beadboard, since like small-k "kleenex" , etc the trade name is often mis-used even in context.    In common-parlance styrofoam (small-s) is EPS bead, and many folks (even in the trades) use the term to mean any number of things, including non-styrene insulating board, despite relevant differences regarding vapor transmission.  Blue like Styrofoam(tm) or pink or any other color, XPS is the right stuff for the app.