I have spent the past year debating on how to move forward insulating my attic. I looked at every possible type of insulation in my area and the associated cost. In my Climate zone 5 (Find your Climate Zone) the energy star recommendation for roof insulation is 49-60 this is impossible with my cape as it stands as I only have 5 1/2 inches of space for the insulation and don’t want to lose space on the interior. I could add insulation on the outside but the cost is too high. This sent me on a journey to find the best possible solution that I can afford.
I looked into having a company out to spray foam the underside of the roof but the estimate was around $10,000 USD to insulate the house. That was also out of my price range. After hours of searching the web I stumbled upon an article on Green Building Advisor titled “cut-and-cobble-insulation” I was intrigued by this article and nervous about trying to do spray foam myself. It was time to do some real world testing.
200 board foot foam kit for initial testing
Last October I bought a 200 board feet spray foam kit from my local box store to give the diy spray foam a try. I wanted to get a better cost estimate of the whole project based on the actual yield of the kit. I found that the kit yield was almost exactly what was advertised. 200 square feet at 1 in thickness and was easier than expected to apply. This made the decision for me when buying in bulk you can expect a price of 1 dollar per square foot at 1 inch thickness. So a 100 square foot roof section at 5 1/2 inches deep would cost approximately $550 USD yielding an R35.75 roof (R6.5/inch).
“Closed-cell spray foam provides a higher R–value per inch (6.5) than less expensive insulation types like cellulose and fiberglass (3.5 to 3.7). Most spray polyurethane foam is called “two-component” foam. (Green Building Advisor)”
This was still a little out of my Price range so I decided to use a combination of cut and cobble and spray foam. The article called this method “peanut brittle” method.
Here is what I did I bought 20 sheets of 2 inch xps foam R10 and cut and secured this to the underside of my roof decking then added approximately 3 1/2 inches of closed cell spray foam yielding a R32.75 roof. This is a huge improvement from the fiberglass insulation that was there and for a fraction of the cost of using all spray foam. The xps foam was approximately 44 cents per board foot. So the same 100 square foot section would cost $388 USD. This equates to 30% reduction in cost and only a 10% reduction in the R value. This was the best R value per dollar I could get that fit into my budget and available options.
I’ll be posting more photos and a video of some of the Foam installation later this month.
Above is a photo of the first coat spray foam over the XPS
Cut and cobbled 2 in XPS on the roof deck and walls
Below are some related links for additional information