Insulating 2x4 attic ceiling rafters help needed
My friend in Sacramento CA (climate zone 3 has cathedral ceiling recommendations of R22 to R38) is looking to insulation his attic ceiling. The home is in the historic part of Sacramento has was built around 1910. The attic has full 2x4 in. rafters that are as straight and solid as the day they were put on the house (I have looked at them). He is starting a project to finish off the attic and make a very large usable room in it. The house is 40 feet by 40 feet with a mansard roof. The roof has four trapezoid sides that meet at the top at an 8ft by 10ft flat roof (at the very top of the house). The 8ft by 10ft flat roof is 13ft above the attic floor. There is about a 4 foot knee wall extending from the floor of the attic to the roof rafters on the outside edges.
Can anyone suggest a good method to insulate the 2 x 4 rafters? My friend is thinking of purchasing 8ft by 4 ft. foam insulation sheets and nailing them to the underside of the rafters (could he use foam sheets thicker than 2in ?). This leaves a 4 in air space between the roof and the insulation.
He is wondering if he would have to create openings at the bottom and top of the rafters to allow air to circulate up the rafters. His house has a stucco exterior including a stucco overhang.
Does anyone have suggestions as how he can best insulate his attic ceiling?
Thanks for your help. It is much appreciated.
I would suggest a larger than 2" rigid foam. Make that the inside insulation layer and go bigger for better overall R-Value.
2" of foam is at most an R-13 plus the additional 2" in the rafter bays is only an R-25 max (if using Poly Iso foam). That will give you the 2" ventilation space as well.
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