I have a plain gable roof built in the early 50’s with 2×6 douglas fir rafters. 2×4 stud walls first floor. No dormers, no skylights. I want to insulate it, which means putting up 1/4″ plywood as baffles in rafter bays, building out 2″ on the underside with ripped 2-by’s perpendicular to the rafters at 16″ intervals, and filling the space with mineral wool. Is the added weight of all of this on the existing structure something I should be concerned about?
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No, not at all but have you looked at foam? Much faster and easier and will make your house stiffer. But, is there a reason you're insulating the roof instead of over the joists? Typically insulation goes on the ceiling.
no.
Thanks. Forgot to mention, roofers added 1/2" plywood over the original 1-by roof sheathing so it seemed to be adding up.
I looked at foam but I want a vented roof and there's a lot of carpenter ants around here. The attic is finished, otherwise, yes, I'd be stuffing the joist rafters (and I am where I can). I have more detail in an energy/insulation/heating post, this question seemed a better fit for this category.
I bet if you looked up the weights of your building materials you want to add and figure out the weight of a square foot of it combined it wouldn't be much.
If you plan to make it a finished ceiling after insulation you might want to consider the total weight and check it under dead load on a span table. Also consider snow load, depending on your climate. Insulating the roof will mean more snow building up.
Had a question about using span tables and snow load but moving that into its own post under code.