Insulating Cathedral Ceiling with Foam Board
Written by Todd Fratzel.
Insulating Cathedral Ceilings
Insulating Cathedral Ceilings can be quite challenging. I’m in the early stages of building an amazing workshop to showcase tools over at Tool Box Buzz and shop projects here at HCI. The new workshop will be located over my garage in an area that was framed using attic trusses when we built the new house. Attic trusses typically have a sloping ceiling (cathedral) that poses a different insulating solution.
As you can see in the photo above and the 3D model below the attic trusses create a section of sloped ceiling and flat ceiling much like you’d get in a house with a cathedral ceiling. For my situation I wanted to insulate most of the “roof” surface so that I could take advantage of storage space in the area to the sides of the walls.
Obviously on a project like this there are quite a few options with varying costs and benefits. Feasible insulating options include:
- Spray Foam – Using spray foam would be an excellent option. In that situation the foam would be sprayed directly to the backside of the roof sheathing resulting in an un-vented roof. This type of un-vented roof has proven reliable when properly installed. This is by far the most expensive option.
- Blown-In-Insulation – Another un-vented option is using cellulose or fiberglass blown into the rafter cavities. For this option to work you have to install a layer of sheathing or drywall to contain the insulation. Just this past year we insulated some 4:12 pitch rafters using the BIBS System. Again this is a pretty pricey option.
- Fiberglass Batts – One of the most widely used approaches would be installing proper vent in each bay between the rafters then installing fiberglass batts. I really don’t like this approach for several reasons including the fact that it doesn’t do a good job at all of air sealing which leads to really poor fiberglass performance.
- Foam Board – The last option that I considered and the one I’m using is foil faced polyiso insulation board. I’m using 2″ foam board (2″ foil faced polyiso R value is approximately R14) fastened to the bottom of the 2×6 truss chord which leaves a nice 2-1/2″ air space. The air space will be continuous down to the soffit vents and up to the ridge cap vent. In areas where the finished sloping ceiling and flat ceiling are located I’ll also install a layer of R13 fiberglass insulation. This should bring my total insulating R value to around R27.
While R27 isn’t super but it sure beats an R19 that I might otherwise get using fiberglass. Also by installing the foam board in the manner described below I’ve created a nice air seal which should make a HUGE difference.
As you can see in the photos I’m cutting the 2″ foam board to fit snugly between rafters. I’m then taping the seams with foil duct tape (the foil tape has amazing sticking properties). By taping all the seams I’m ensuring a really great air tight detail which will cut out drafts. This also creates a really great “plenum” for air from the soffit vents to reach the ridge cap vent.
As you can see in the adjacent photo I’m also creating a “flat” ceiling of sorts with the insulation. This is located just above the truss ceiling rafters and about 12″ below the ridge vent. This should allow plenty of good ventilation from the soffits below up to the ridge vent.
I’ll do something similar down at the base of the roof where the trusses meet the wall. At that location I’ll cut pieces about 12″ tall and seal them to the wall top plate and the sloping insulation. The important step there is not cutting off the flow of air from the soffit vents.
Once I get all the insulation in place I’ll be sure to report back on it’s performance especially in the hot sun this summer. The 2″ foil faced polyiso is costing about $1 per sq. ft and I figure the whole job will take about 30 man hours.
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