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Insulating cathedral ceiling in cold climate

0 votes
Tim says:
October 4, 2011 at 8:57 pm

Hi Dariusz,

I am working on my sons Minnesota house with him and are approaching the time to insulate the cathedral ceiling. It is constructed with 20' trusses on each roof side of the 30' span master BR. We completed installation of OSB baffles about 6? from the underside of the plywood roof decking to provide air flow from continous soffet vents to the continous ridge vent.

Question: What would be the appropriate way to insulate?

1. Sprayfoam the underside of the OSB with R7 and then blow in cellulose OR fiberglass between the sprayfoam and the finished ceiling (fastened to the underside of the trusses), the finish will be T&G pine? Would a 6 mill vapor barrier be used under the T&G?

2. Apply 4'X8' sheets of 1? foam blueboard/pinkboard and then T&G to the underside of the trusses and the blow in cellulose OR fiberglass in the cavity between the foam sheets and the OSB? The foam sheets would provide a vapor barrier.

3. Do both, apply sprayfoam AND apply 1? 4'X8' sheets of foam blueboard/pinkboard under the T&G and then blow in fiberglass OR cellulose? This would provide 2 vapor barriers, would this be acceptable because the sprayfoam will have no condensation because of its R value?

4. The BEST plan would be to sprayfoam the underside of the OSB to R40 and then apply the T&G BUT for the fact that the cost would be very high, like $5,000. We can use 4'X8' sheets blue/pink board for a lot less with blown-in fiberglass OR cellulose. Of course we will make sure there is a good vapor barrier at the foam, tape all joints and maybe glue joints as well. The fastners for the T&G will of course poke holes in the barrier (blue/pink foam 4X8 board).

We are leaning towards blowing in packed cellulose instead of fiberglass for blowing bcause of better R value and MAYBE less health risk with blowing cellulose over fiberglass, thoughts?

Any insight you can provide will be much appreciated.


asked in Attic Area by darekrudy (21,730 points)
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1 Answer

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Hi Tim,

A few questions first:

  1. I’m not sure what you mean by “We completed installation of OSB baffles about 6? from the underside of the plywood roof decking”. Did you create a 6” deep ventilation space between the roof decking and the baffle all the way from the soffit to the ridge vent?
  2. What size of material are you using for the roof rafters, is it 2”x10” framing?
  3. Is it OK in your area to install wood paneling / T&G without a fire-resistant backing of drywall?
  4. Check with your local building code / fire officials and insurers for specific information on foam panel board installation requirements / what type of fire blocking surface it requires in your area.

Since you’re in Minnesota I believe the minimum required insulation value for a new construction attic is R-38. However, federal standards recommendation oscillates around R-50 for your climate. To lower the cost and still use closed cell insulation you could spray roof decking underside to get at least 3” thickness (no baffles / directly on OSB).

That should minimize possibility of condensation and give close to R-20 value. You can fill remaining space (assuming it is 2”x10” rafter) with 6 inches of fiberglass batt (or cellulose blown-in) insulation which would give you additional R-20.

You could also lower spray foam thickness to 2” (less isn’t wise because you might end up in condensation buildup on the OSB), staple 6 mill plastic foil to the rafters (not under T&G) and blow-in cellulose insulation into the remaining space.

Additional solid foam boards (even better if they are laminated with reflective aluminum foil for vapor blocking and heat reflection) would give you extra “R’s”. You can use aluminum duct tape on all seams to provide even better air tight envelope. Again, check if they can be installed on the ceiling with no drywall layer on top of them, just T&G

You can replace fiberglass and cellulose with cotton butt insulation which has very good R values as well – http://www.greenbuildingsupply.com/utility/showProduct/?objectID=663

Any combination of insulation that will give you over R-40 value with a good vapor retarder on the warm side will do (you can additionally put a coat of a moisture vapor barrier primer / paint).

Also, without insulating spray foam applied directly to OSB you definitely need those baffles and they should be as wide as the rafter space. During my inspections I’ve seen mold developing on both sides of the baffle if there was any space left between the baffle and the rafter wall.

One thing to remember when applying spray foam to the decking is that roof leaks will be extremely difficult to detect and often result in extensive roof decking damage before you notice any problems.

answered by darekrudy (21,730 points)
Hi Dariusz,

1. Yes, we created a continous 5? cavity from soffet to the ridge vent. The roof is constructed with trusses and the trusses are built with 2X6s and 1/2 inch plywood decking. We nailed 2X2s to the insides of the trusses at the lower edge of the 2X6s away from the plywood decking and nailed OSB to the 2X2s. The trusses are about 1.5 feet at the narrow end and 4' at the roof peak. The ventalation space is full width and continuous, there are no parts of the plywood roof decking without venting.

2. Trusses are used for the roof span.

3. We will check on a drywall barrier under the T&G wood based on what insulation we will use in the cavity behind the T&G wood. Thinking some types of insulation might be fire resistant enough to not need a drywall barrier. Example: Blown in cellulose is allowed behind T&G. Extruded foam may require a drywall covering.

4. We intend to fill the truss created cavity with cost effective cellulose and the final R value will be R40 at the wall and probably R120 at the peak with a 3' cavity to fill. The fire risk with extruded foam is understood and maybe the best approach would be to apply a 6 mill poly vapor barrier under the T&G to keep moisture out of the cellulose and eliminate the fire risk from foamboard.

The local code will be checked, thanks for your feedback.