Expert: Ted Barnhill (Principal, Design45 LLC) - 10/27/2008

Question
Ted,
I am building a 28 x 40 gambrel barn style cabin (house) in West Central Wisconsin.  The interior is completely open to the Cathedral ceiling.  R-38 insulation will meet my building code requirements but it has been suggested that I go as high as I can and still leave a one inch gap between the roof and the insulation.  There is plenty of room in the majority of the truss area to have both an R-38 batt and an R-11 batt for a total of R-49.  Is this practice recommended or is there a better way of achieving a higher R-Value.  The building was built as a recreational cabin and will only have a wood-burning stove for a heat source.  The potential exists (8-10 years down the road) for it to become a retirement home.  Keeping the project inexpensive is a factor.  Thanks in advance for your help

Answer
Gary,

There is no problem whatsoever with using two layers of batt insulation as you describe, as long as you:
1. Don't compress the insulation - this reduces its R-value.
2. Maintain proper ventilation.  Foam vent chutes work well to maintain the 1" airspace.  Be sure to provide adequate soffit and ridge venting.
3. Be sure you have a good air barrier, and a vapor retarder as required.

That said, I recommend that you use spray foam insulation if you can afford it.  It will cost significantly more up front, but will provide much more insulation, as well as serving as your required air and vapor barriers, and will thus help ensure a warm, durable, mold-free home.  Of course, when used as a cabin, your energy costs will be low, and you won't have a big problem with in-wall condensation in the wintertime.  However, should you convert the cabin to a year-round home, the benefits of foam insulation will be well worth it, and the foam will likely pay for itself over a few years.

I hope this helps; enjoy your new cabin!  -Ted