We decided to use what's known as the "Cut-n-Cobble" method for insulating our attic ceiling. The Cut-n-Cobble method is an insulation technique where you cut strips of rigid foam boards to fit in-between your roof rafters or wall studs and fills the gaps between the boards and the rafters/studs with spray can insulation. Ideally, there would be a 1/2" - 1" gap between the boards and the wood to fit a spray nozzle to fill the gaps. There a few reasons why we chose to insulate our roof / attic ceiling this way. Perhaps the most important reason is that it was less than half the cost of doing spray foam. We spent some time reaching calculating the cost of doing spray foam vs. cut-and-cobble method, ultimately we decided to save the money and do the work ourselves (with spray foam there was no way we could do this ourselves--although i definitely looked into it!) Be forewarned though: this method is very time consuming. Contractor's don't provide this method as an option for a reason! It was especially time consuming for us because each and every rafter was spaced at a different dimension, so each rafter spacing had to be measured and each insulation board had to be custom cut to fit the opening. This by far was probably the longest stage of this renovation. I want to say it lasted 3-4 months!! Needless to say, if it's in your budget, go with hiring a spray foam contractor to do your insulation. Otherwise, be prepared to put in some sweat equity!
You can see from the above photo that we used 3/4" wood furring strips to bring the dimension from the underside of the roof sheathing to the face of the furring to 6-1/4". This allowed for approximately 6" of Rigid Insulation. We used three layers of 2" rigid pink Owens Corning Foamular 150 rigid insulation. I can't even tell you how many boards i bought.... 500, 1000? A lot! I had home depot make two separate deliveries to our house with two pallets full both times. In addition to about a half dozen trips to home depot for a full truck load. It was crazy! Also, for the spray foam, we bought the spray gun and the industrial size cans. It was quite the endeavor.
We used small strips of wood fastened with temporary screws to keep the boards in place until we spray foamed with the spray gun. The foam took about a day to dry completely before we could remove the temporary strips.
You can see the major thermal bridging occurring in the rafters / gap from the above photo. This is a thermal scanner we borrowed to test the thermal transmittance. The 6" of rigid foam gave us an insulation value of about R-30. In addition, we were going to provide 5/8" drywall and 3/4" wood below the insulation, so we assume the total R-vale would be a little higher to meet code requirements in Northern Ohio.
This is in the midst of adding all the canned spray foam. We found that we had to do the spray foam over the course of multiple days... We would fill the gap with 2"-3" of foam, allow it to expand and dry, and then come back and do the remaining 3" or so of foam. If we did it all at once it would either expand too much or not enough and would be a huge waste of expensive material.
Once the spray foam was dry we used a foam saw to cut the foam so that it was flush with the furring strips. You can buy these at your local big box hardware store for about $10.
We also had to fur-out the bathroom ceiling with additional furring because there was a vent stack running along the ceiling. We used salvaged lumber from the old walls that once framed the attic partitions!
Here is a photo of the front exterior wall, with new cut-n-cobbled insulation in place between the wall studs. We added additional 2x4 framing to the existing wall framing to improve the energy performance of this wall. I also added a new header and window sill.
Another advantage of using rigid foam and spray foam is it has a better energy performance than batt insulation. The above photo shows new plumbing along the exterior wall. The likelihood that there would be freezing pipes would be much greater if batt insulation was installed. I hate batt insulation in exterior walls. There's always going to be thermal bridging with batt insulation. The small gaps between the insulation and the studs allows hot/cold air to transfer.
Another reason why we chose to do this method is it allowed us to do the framing and insulation simultaneously. If we were framing a wall, and had to bridge between rafters, we could do so without having to cut away dense spray foam.
One additional point I would like to make: if possible, the best option for insulating your ceiling / roof is to provide continuous insulation above your roof deck. This is because your roof rafters are a major thermal "bridge" for hot/cold air. However, in our case, we had a 120 year old slate roof that we are planning to keep for the next 120 years. Replacing the roof was just not an option.
One additional thing to note, I lot of people choose to vent their roofs. There's a lot of debate about doing a vented roof vs. unvented roof that I don't want to get into. Obviously, we chose to do an unvented / "hot" roof assembly. For a new construction home, that is basically "air-tight", I would not recommend this approach.
Ultimately, this method was very time consuming and labor intensive but I feel very comfortable now knowing that there is a full 6" of closed cell insulation in our ceiling and ceiling with a perm rating of zero (basically). After the drywall was installed, which ill get into next... we made sure to seal all the gaps with tape to create a true air barrier. I'm confident that the R-value combined with the sealing will leave us moisture-free and comfortable in this attic space for a long time (knock on wood!).
Brandon E. Young