How much does it cost to insulate a 1500 sq ft house? when you are calculating square foot prices for spray foam, you will need to know the total length and height of all your exterior walls. a 1500 sq ft house with 10 foot walls will generally have about 1500 sq feet of walls. most spray foam contractors will deduct the windows and doors. a standard door is usually about 20 sq ft and windows will be about 15 sq ft. so if you have 4 doors and 10 windows, you would deduct 80 sq ft of doors and 150 sq feet of windows from your 1500 sq foot wall area. this would leave you with 1270 sq ft of walls needing spray foam. spray foam prices vary depending on the contractor and the product chosen- open cell vs. closed cell. open cell prices will range from $1.25-$1.50 a sq foot for a 3" average thickness. closed cell prices will range from $1.75-$2.15 a sq ft for a 2" average thickness. so open cell in this example should cost on average $1750.00 for the walls. the roofline of a house will vary depending on how steep of a roofline you have. in this example we will use a 1500 sq foot house with a 8/12 pitch. 1500 sq ft times the pitch factor of 1.2 would be 1800 sq feet of roof decking needing spray foamed. most homes use open cell at 5-6" thick. and your average prices per sq ft will range between $1.45-$1.80 a sq ft. roofline cost for this example would be on average $2925.00. subfloors should be sprayed with closed cell and avg. cost is $1.80-$2.10 a sq foot.
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