Proper insulation of Cathedral Ceiling

Good afternoon. Chicago, zone 5, metal roof, cathedral ceiling, TJI14" rafters, new construction. I have done lots of research - it seems that financially, there are a few options available to me: 1) for an air gap, put in R38 HD batt that leaves 3" gap at the sheathing and call it done. 2) put in R38 HD batt leaving 3" vent at sheathing, and put 1/2" polyiso on interior with drywall over that. 3) spray foam layer against sheathing, fill the rest with R38 batt for no vent install For 1 & 2, I am concerned about the 24" OC spacing of the TJIs that have 3/8" web thickness. There isn't much compression on the sides of the batt - will I get a good seal? For 3, how much spray foam is needed for Z5 and is closed or open better? Open seems to collect moisture eventually. Also, isn't there a dew point issue with this type of installation that eventually will cause problems? The TJI wood will have less R value than the spray so won't dew happen deeper in the cavity through the TJI web?

I am a little concerned that this could cause condensation issues. Because standing seam roofing has a lot of direct contact with the substrate beneath it, cold is transferred downward. This could drop the temperature enoygh that condensation occurs where it dod not occur before (and honestly I wonder if it hasn't occurred some in the past). My suggestion would be to add some more insulation...that will help stop the transfer of cold and reduce the possibility of condensation. All Best.

I just purchased a house inear the beach on the Washington Coast. It is a contemporary, cabin style building (built 1976) -(28' x 28' footprint) with a cathedral style ceiling (double 2x10 built up beams under 2x6 car decking). Currently it has three layers of composition roofing over what appears to be an inch of foam board, which lays on the 1.5" car decking. It's old and in poor condition, but is not leaking, but has leaked (and stained the interior ceiling) in the past at the propane chimney perforation and possibly in other locations possibly due to condensation. The roof is an offset rake and pitch, with a 28' rake on the longer side (10/12) and 16' on the opposite rake (6/12). I got a bid from a licensed, bonded contractor in our area who says he will perform the following: 'Remove existing multiple layers of composition roofing Install Deck Defense Synthetic moisture barrier underlayment Install drip edge metal and gable end cap metal Install 1" insulation board Install 1/2" CDX plywood sheeting Install 12" Snap Loc Standing Seam Metal roof panels Install new pipe flashings Install new ridge cap with closure strip 10 yr Labor guarantee 130mph wind uplift protection ================================= So now, my questions: Is the contractor's proposed installation adequate to prevent leaks and/or condensation problems? Also, I have read on some building forums that TWO layers of iso board, at 1.5" each, for a total of 3" of insulation is recommended. It has and inch of foam board now, (contractor says it will be too damaged to save after removing the multiple layers of compostion roof.) Is there anything being left out of this installation and finally, will the R-value of that single layer be enough? This is a vacation home, used primarily in the summer (temps rarely exceed 75 degrees nor go below freezing). Thanks, Scotty

Michael, If the condensation is to the exterior, there is nothing to be done about that. In some cases, it is a good indication of an insulation systems performance because it is keeping the exterior surface colder. We field this concern from customers who see condensation that was not there previously.

http://www.finehomebuilding.com/how-to/articles/a-crash-course-in-roof-venting.aspx Google - A crash course in roof venting. Great read.