Raising attic floor to install new insulation
Greetings. I need some advice about a plan for attic insulation.
I have a colonial house in Massachusetts with attic dimensions of 45×25′ and 2×8 joists on 16″ centers that run perpendicular to the rafters with the longest span being 15′. House was built in the early 60s and the wood looks like good quality pine or fir.
I would like to add 4″ to about 60% of the attic floor to allow an increased depth of blown in cellulose, which will then be covered with 3/4″ ply for light storage. I have a lot of sound used interior 3/4 ply available so that part is a given.
The questions are:
1. Should I run the 2x4s perpendicular to the existing joists and tie them in with hurricane ties or run them over the top of the existing joists and tie them in with plywood gussets?
2. If the recommendation is perpendicular, should the 2x4s be at 16″ centers or 24″ centers?
3. Are there any other variables I should consider?
Thank you.
Replies
Run them perpendicular, unless there's some good reason not to. You'll get a flatter, more stable floor, eliminating any tendency for the joists to turn or the new lumber to "jump" off the old.
With 2x4s you can drive a 4.5" GRK screw down through the 2x4. With 2x6s the screws become impractical, but with either size lumber you can either use hurricane ties or toenail/toescrew.
Assuming this isn't a finished floor, the spacing is up to you, based on how stiff your flooring will be and how much load you'll apply. 24" is adequate for most purposes and should be fine with 3/4" plywood, so long as no one will be throwing a dance party up there.
Generally one needs to give a warning about not storing too much stuff in an attic, but the construction of your attic floor sounds sufficient to carry a fair amount of weight.
Dan
Thank you very much for the excellent advice.
Simon
Several years ago I was asking myself the same question. I chose to run them on top. Here's a link to a short description I found back then:
http://www.thisoldhouse.com/toh/asktoh/question/0,,845054,00.html
I also made it a point to add solid blocking between joists (now heightened) mid span.
I think either method would help stiffen up the new subfloor so the determining factor likely would be what is easiest in your attic.
You might consider air sealing the ceiling around penetrations with foam cans while you're pulling away exisitng insulation. All of my neighbors had serious ice dam issues this past winter, I didn't.
Regards,
Tim