I am in the process of designing a passive solar home and am looking for a good example of a roof truss design at the wall interface (unvented). Do you know of any detailed construction drawings that provide a good idea of how to apply insulation and air sealing at this area of the house thermal envelope? I live in a ~8300 HDD climate and would like to get ~R-75. I plan on a raised heel scissor truss design with blown in cellulose. I would like to include Zip roof sheathing with taping on top of the truss to achieve a vapor/water barrier. Regarding air sealing, do I need an additional layer of air sealing (spray foam) on the Zip panel/truss interface (underneath)? Is it worth the extra cost of spray foam, or can my air sealing be achieved solely with the Zip and tape? Thanks.

the best way to handle this is very easy: order the trusses without overhangs. Run your wall sheathing up the outside face of the truss and tape the wall/roof joint (and all the other wall joints and roof joints). then build your overhang and screw it onto the trusses. Works very well. I know of several builders who have done this other than myself, so its not a wild idea.

In answer to the 2nd question, tape is a better air seal than spray foam, so get a good tape like Vana (German, available through www.475buildingsupply.com) or Siga (Swiss, available through www.mainegreenbuilding.com) Neither is cheap, but they work. They'll stick to about anything; they are vapor permeable and use non toxic acrylic sealants. My best find this year!

Thanks Bob. Yes, I have been trying to understand how to deal with the truss extension beyond the walls. That sounds like a good idea but questioning the labor costs associated with this. I am imagining it would be less expensive to have this done at the time of truss building? Regarding tape: how does the Zip Systems tape stack up against Siga for example. Siga's claim to fame is its stretch and bonding characteristics. Considering ZIp tape is 1/4 the cost of a roll of Siga, does the performance differ significantly. Maybe, if I don't use spray foam money savings) I can invest it into more expensive/better performing tape. Thoughts?

Zip tapes sticks very well to ZIP sheathing as it's designed to do, but I've found it doesn't stick well to other surfaces. I use ZIP wall sheathing with ZIP tape, but use Advantech on the roofs as it is far more resistant to water than Zip, and use SIGA to tape those roof seams. We were taping the interior of the windows to the studs the other day & decided to use Dow Weathermate tape since it's a lot cheaper. But when we found that it didn't stick well, we went back to Siga. Cheaper doesn't help if it doesn't work. realistically you're probably only going to use a few hundred dollars of tape at most. And its twice the cost, not 4X - $25 vs $50 for the 3" (60mm). the 6" is $100/roll, but you'll only need a little of that if any.

On Siga's website there are numerous types of tape. Does it matter what application to what type of tape? Can you provide me with an example of where the 6" would be used (peak of roof)? Back to the trusses; would it be best to use a 2" closed cell foam board on the inside of the raised heel edge of the truss and caulk the joints? Thanks for the help.

We've used the Wigluv. Use the 6" anywhere you can't get a good seal with the 3", and the 3" will work in most places esp if the plywood joints are tight (as tight as the manufacturer recommends). We've used the 6" to tape the exterior of the windows where we want the tape to extend under the wood window casing. With factory window casings you'd use the 3".

you don't need any foamboard in a raised heel truss; simply fill it with cellulose. You do need a good air barrier between the living space and the attic - this can be airtight drywall, or (taped) plywood on the ceiling or a (taped) vapor permeable membrane product like Tyvek.

"questioning the labor costs associated with this." Thought I addressed this but doesn't seem to have made it onto the page. You are probably already building & attching the gable end overhangs - its the same deal - not that much material or labor: one 2x4 "rafter" & one 2x4 horizontal return both at 24" OC at each truss, one 2x4 or 6 along the front, two 2x4 ledgers - screw the top ledger into the truss framing with 4" GRK screws or equal, nail the bottom. Build it in sections & attach, or build in the air.

"you don't need any foamboard in a raised heel truss; simply fill it with cellulose. You do need a good air barrier between the living space and the attic - this can be airtight drywall, or (taped) plywood on the ceiling or a (taped) vapor permeable membrane product like Tyvek." Can the air barrier start from the roof rather than the ceiling? I am assuming that the roof is part of the thermal envelope (starting from the Advantec or Zip board). Is it more effective to air seal from there or from the ceiling of the livable space (bottom chord of the truss)?

If you are using blown cellulose on top of the ceiling as your insulation, then your roof is not part of the thermal envelope.  In this case, the air barrier needs to be at the ceiling, not the roof deck. Otherwise, air leaks through the ceiling will exchange air with the unconditioned air above the insulation. Plus, if you are going to install a vapor barrier, it needs to be on the warm-in-winter side of the insulation, i.e. at the ceiling.

what if (in theory) the roof deck is airtight?

if the roof deck is airtight, then you have some extra protection against infiltration and your insulation will work better. the air barrier goes on one side of the insulation or the other,so that doesn't change.

I guess I'm still a bit confused. Sounds like you are saying (my understanding) I should place the air barrier (as taped zip panels or advantec would provide) on the outside of the cellulose insulation. Would my ceiling below my truss then simply be sheetrock +/- OSB supporting the cellulose weight with no barrier of any sort?

Posted By Myrtleboone on 09 Dec 2012 04:53 PM
what if (in theory) the roof deck is airtight?

Doesn't matter.  If there is no insulation at the roof deck, then the air temperature between the the roof deck and the insulation will be essentially outside ambient temperature in winter and even hotter than ambient in summer.

So layer by layer, what would my roof cross section look like from exterior to interior? This is my thought-roofing, zip system roof panels (or advantech), truss frame, cellulose insulation within the space, vapor barrier, ceiling drywall. WIth this, am I setting myself up for condensation anywhere within the space? That's my concern.