Hi I am helping my son build a total icf home in western New York and are to the point of insulating the ceilings. Our plan is to spray 4" of closed cell foam on the flat ceiling drywall and covering the 2x4 bottom cord of the roof truss. We were also considering installing 2" rigid foam board prior to the drywall to help with thermal bridging from the truss. Would this work or could there be issues with condensation etc. thanks for any input

Assuming the use of trusses precludes use of the attic space for anything, perhaps a less expensive and better insulated option would be just to blow in 18" of cellulose above the sheetrock. You wouldn't have any significant thermal bridging through 2x4 bottom chords buried that deeply, and you'd have an R60 lid, vs much less using the thickness of foam you indicate. Of course you would detail the ceiling sheetrock properly wrt air ceiling, and use vapor retarder primer paint, and you'd want soffit/ridge venting, with insulation baffles of some sort at the periphery to keep the cellulose off the roof deck. Ah, those roof trusses do have a raised ("energy") heel to permit the full depth of insulation right over the outer edge of the wall, right?

(see edited reply above)

Keep in mind that covering up the structural members in the attice will make it very difficult to make roof inspections from within the attic.  However, the bottom chords of the trusses will need to be covered with insulation if you want a well insulated attic.  One way around this problems is to build a raised cat walk from one end of the attic to the other so that inspections can be made from within the attic.  Storage in the attic can also be had by expanding the catwalk.  Raising the cat walk will allow for insulating under it.  The height of the cat walk will determine how thick the insulation can be.  All of this assumes that the attic height is enough to allow access.

Posted By lajuul on 25 Dec 2013 08:19 PM
Hi I am helping my son build a total icf home in western New York and are to the point of insulating the ceilings. Our plan is to spray 4" of closed cell foam on the flat ceiling drywall and covering the 2x4 bottom cord of the roof truss. We were also considering installing 2" rigid foam board prior to the drywall to help with thermal bridging from the truss. Would this work or could there be issues with condensation etc. thanks for any input

Spraying 4" of closed cell foam in this application is the opposite of "green", due to the EXTREMELY high global warming potential of the blowing agent used (almost always HFC245fa), at about 1000x CO2.   If the purpose is to air seal, you can get better air sealing with 3-4" of high expansion half-pound density open cell foam, which are almost always blown with water.  To avoid self-ignition or adhesion issues the closed cell would have to be installed with two lifts of 2" with a cooling period in between, and will cost a bit over $4/sq.ft, compared to about $1.20/sq.ft. for 3.5" of open cell foam sprayed in one lift.

Dick Russell has it dead-right that burying the truss chords in blown cellulose is a better and cheaper thermal break than 2" of rigid foam.  But if you take the rigid foam route, use either EPS (the beaded stuff of disposable coffee cups, R8.4 @ 2"), or polyisocyanurate (R12-13 @ 2"), both of which are blown with pentane (which has a GWP of about 7x CO2), and NOT XPS (pink, blue, green board), which is blown with a mixture of HFCs, the major portion of which is HFC134a (about 1400x CO2). 

But since you'll need something like a 8-12" fastener spacing long-screwing/nailing the gypsum through that foam you will get significant thermal bridging due to the high thermal conductivity of steel, which is about 300x that of the foam it is penetrating through.  That can be mitigated if instead of long-nailing the gypsum through the foam you screw 1x furring 24" o.c. perpendicular to the truss chords between the foam and gypsum, and use short ring-shank nails to hang the gypsum on the furring.  That cuts the thermal bridging through the foam by half or more.

With a vented attic design you don't need a vapor barrier at the ceiling-  orders of magnitued more moisture gets into the attic via air leaks than by vapor diffusion through standard latex paint, and simply by air-sealing the ceiling plane (verified & remediated tight via blower door testing) you have taken care of the most likely moisture problems.

From a raw $/R point of view, cellulose is generally the cheapest way to go high-R.  In an open blow situation you're talking 3-4 cents per square foot per R, so hitting R50 is on the order of a couple bucks per foot.  Closed cell foam is about 5x as expensive per R, open cell foam about 3x as expensive per R.  By far the greenest approach would be to use tapes caulk & can-foam/FrothPak for the air sealing, and cellulose for the actual insulation.  If the trusses are 24" o.c. you may want to put up half-inch OSB on the under side of the trusses to manage the weight of high-R cellulose without sag, air sealing the seams with duct-mastic, and go hog wild on the cellulose. Even R75 plus an OSB support deck would be cheaper than just the 4" of closed cell foam, and the same or less labor installing the gypsum + rigid foam.

***With a vented attic design you don't need a vapor barrier at the ceiling- ***

Seems to me, a 6 mil poly vapour barrier is about the easiest way to air seal an attic and as the attic is vented it can do no harm.

It makes it easy to seal the vapour hats over the electrical outlets and you can let it run down a foot over the ICF walls. Add a bead of caulk between the poly and the foam and about all thats left is the plumbing stacks and the attic access.

Posted By FBBP on 29 Dec 2013 11:52 PM
***With a vented attic design you don't need a vapor barrier at the ceiling- ***

Seems to me, a 6 mil poly vapour barrier is about the easiest way to air seal an attic and as the attic is vented it can do no harm.

It makes it easy to seal the vapour hats over the electrical outlets and you can let it run down a foot over the ICF walls. Add a bead of caulk between the poly and the foam and about all thats left is the plumbing stacks and the attic access.

6 mil poly is easy to tear or poke holes in, shrinks/melts/burns when subjected to sufficiently high temps, which creates issues for air-sealing around flues or over lighting fixtures, etc.  Most poly vapor barriers I see in sealings are NOT detailed as air barriers, and have a gazillion electrical & plumbing penetrations, nail nicks and holes.  It's easy enough to detail the gypsum or an OSB high-R support deck without resorting to sheet goods that would also have to be detailed properly to become truly air tight.  OSB plus duct mastic is pretty rugged stuff by comparison, and doesn't take any more time to detail for air tightness as it would to detail poly or housewrap.  It's still a matter of fixing every penetration & seam.

Any ceiling that is designed with an open chase layer for the electrical is comparatively easy to air-seal on the deck above, and sure, flexible sheet goods may make it somewhat easier in some situations, not so much in others.