Tiny attic / flat roof insulation options
Hi GBA – I’m seeking some advice on best options for insulating my small 1100 sq ft single level home. We have a modified bitumen flat roof that is over 30 years old. It has a parapet wall with a height varying between 2-8″ (most places are 3-6″) depending on its distance to one of two side house drains.
We would like to have the new roof be SPF. I’ve read that the old roof does not need to be removed, and I could consider polyiso or SIP as an underlayer to the SPF for even better R-value. But I’m not sure that would work with my parapet wall height unless I build a taller parapet or somehow make arrangements for rain water to continue towards the gutter, and not over the side of the house onto neighboring property.
The “attic” space is about 20 inches in height with 2×6 joists 16 OC above the ceiling, and 2×6 32 OC below the roof – both running in the same direction. There is existing blown-in cellulose that does not cover the ceiling joists, so I’d guess 3-4″ deep.
It’s a 1955 home in the SF Bay Area. If we add an SPF roof, what should we do regarding the attic insulation?
– Leave the existing 4″ blown-in.
– Add additional blown-in on top of the existing.
– Remove existing blown-in and leave attic without insulation.
– Remove existing blown-in and add spray foam insulation to the underside of the roof.
The first option is of course the easiest/cheapest, but I don’t know what happens if I have 2 distinct building envelopes
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