ventilating a cathedral roof system
Hello, I am new to this site, a builder in Santa Barbara, Ca. for 27 years. An architect spec’d 1/2″ sound channel UNDER the rafters of a cathedral roof/ceiling to “vent” the roof. The rafters would then be filled with Batt insul., and holes drilled in frieze blocks every other bay. My concern is that the house will lose it’s heat out these vents, and I’m certain that the air space/dew point needs to be above the insulation. Some options I’ve come up with/heard of are: 1″ rigid insul on top of the roof sheathing, and venting not needed in insulation stuffed rafter bays; eliminate the batt insulation, and add 3″ of rigid on top of the roof; or, is the architect correct in his spec? The architect thinks it is MY JOB to build correctly, but isnt it the architects job to do the scientific evaluation and tell me what to do?
If you need more info, ask and I will reply. If you know of where I can get info on this, I will appreciate it
Jed Hirsch Jedhirsch.com
Replies
It is the architect's job to be sure that the plans are up to the current building code in SB. It will have to go through the planning department and have the plans checked to be passed. They may ask for energy calculations and a number of other things. It is his job to draw plans that will pass the city codes, etc. It is the job of the architect to make sure that all engineering is done and the building will be built and designed correctly. It is your job to build what he has put on paper that the city has passed. The architect is the designer, you are the builder. Remember it is his design which must be approved first before you can dig a shovel of dirt.
I lived in San Luis Obispo for a total of 14 years and got my license in 1982 and my in laws lived in Goleta for about 40 years. I never heard of any such thing as what that architect wants. In 2004 in SLO I finished a home that had a cathedral ceiling and we did not encounter such a thing. What we did was to have a larger gap between the roof and ceiling so that we could put insulation in there and allow for ventilation on top of the insulation. We vented the eaves and placed metal vents on the roof put in as a large tile to vent the ridge area. In our case the ceiling joists were not the rafters.
Probably the best thing to do is to ask one of the plan checkers at the planning dept. for Santa Barbara.
Welcome Jed,
It sounds like the archi is trying to sidestep his responsibilities.
If I understand you correctly you would be venting conditioned air from the building and bypassing the insulation altogether.
I personally like dense - pack and spray foam hot roof systems designed to control air movement and add additional insulation. BUT, I am in Michigan not California!
There are spacers/vents designed to be installed on top of rafters in hips and valleys to allow sideways air movement rather than drilling holes.
When venting a rafter system is necessary, we use 2lb EPS foam and build continuos vents from the eave to the ridge. Two rips of foam stuck in place with caulk or DW nails(3 if 24" o.c.) then a full width (- 1/4" to 3/8") foam board toenailed to rafters, gun foam in corners and any joints. There you have is full venting.
Now the cheaper option, cardboard baffles stapled to rafters to create airspace.
Dense - Pack with cellulose and you have a tight high performance rafter system. Carful with the packing if you use the cardboard baffles, you can tear them out if your not careful.
Good luck,
Garett
If you have access to them, ask the Archy to provide a detail/section showing the roof/ceiling as specified. Maybe they made a mistake or maybe you misunderstood the specifications. Or maybe not. Whatever the case, clarification requested and hopefully provided in a professional manner between the professionals involved will often resolve discrepancies and/or misuderstood designs and/or miscommunicated designs and/or mistakes.
My cathedral ceiling/roof from the outside in, is constructed as follows: laminated shingles, 5/8 plywood sheathing, 2x12 rafters, 16" o.c., 1" air space, 2" rigid foam (EPS) insulation fit tight between rafters, 7-1/2" r-19 batts, 1" EPS perpendicular to rafters with taped seams, and 5/8 gwb/calcoat, works out to an r-value of 35, with continuous soffit and ridge venting.
I'm hoping this goes out to all who responded, though I'm not sure. In regards to my question about venting a cathedral roof, with 2x8 rafters and 5/8 roof sheathing above, even if I could get 1" air space above my insulation, I would not get venting to ridge at hips and valleys. Also, the plans call for holes drilled in frieze blocks every other bay. This only vents half my roof. Would this work: standing seam copper roof, over 5/8 roof sheathing, over 1" rigid insul., over 5/8" roof sheathing, over 2x8 rafters stuffed with fibreglas batt insul.
Does this put my condensation/dew point in the rigid where it does no harm?
Thank you to all who are answering.
Jed Hirsch
what does your local building code require the insulation value to be in your roof in this situation ?
ours in PA is R-38.
carpenter in transition
Tim, Title 24 and the plans call for r 19 which I can get in the 2x8 rafters, even if I leave an inch and a half above insul. But I still can't vent all bays. Will 1" insul on top move my dew point to safe spot above sheathing?