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Cells and RFBI

Posted by Mongo on November 5th, 2001 11:40 PM
In reply to Attic Insulation by James Swanson on November 5th, 2001 10:45 PM [Go to top of thread]

First of all, I'm assuming that there is nothing structurally inadequate with your attic framing.

Onward we go:

First, do you want to vent the underside of the sheathing? This is only a player if you have soffit and ridge vents and want to connect the two. If so, install baffles of some sort. You can purchase ones that fit between rafters 24" oc. Run them tight, from the soffit to the ridge, one run in each rafter bay.

Many building codes are recognizing the viability of unvented "hot roofs," and allow for them under the "alernative methods" section of the code. Read up and decide for yourself.

Also, before tackling your attic, ensure that you've done a pre-emptive strike in your basement regarding moisture. Liquid and vapor.

For insulation: I wouldn't mess with batts, not with your uneven rafter spacing. Think about blowing in insulation instead, I'd vote for blown cellulose over blown FG. The cells will pack tightly, air-locking the rafter bays, and will provide better resistance to radiant energy gain in your attic. That'll be especially helpful on summer days.

You can't blow insulation in to open rafter bays, so here's what I'd do to seal the bays:

Install two layers of 2" thick polyisocyanurate rigid foam board insulation. Shorthand? "2-inch polyiso RFBI." It sells at most home centers for around $20 per 4'x8' sheet. It's easy to install, it goes up fast, it'll prevent air infiltration like nothing else you can do, and will prevent radiant energy from being transmitted into your attic from the roof structure, resulting in a hot attic. The ends of the sheets do not have to fall on the rafters.

Here's the deal grand scheme of things:

1) Install one layer of 2" polyiso RFBI attached directly to your rafters. Gap the sheets so they are about 1/4" to 3/8" apart. Attach them to the rafters using nails and caps (metal discs about 3" in diameter that the nail goes through). You'll only need a few nails to hold up each sheet of polyiso. Mark the location of the rafters on the face of the RFBI as you install it. A simple tic mark with a marker will do.

2) Once the first layer is up, use cans of spray insulating foam to fill the gaps between the sheets. that'll seal everything nice and tight.

3) If you have the money and the headroom, repeat steps 1) and 2). You'll end up with a 4" thick layer of polyiso, total R-value of 28.8. stagger the second layer so the seams don't fall on top of the first layer's seams.

4) Attach furring strips, running horizontally, spaced 16" on-center, over the polyiso. The furring strips can be either REAL furring strips (typically 3/4" by 1.5" by 8' long), or you can rip your own 2" wide strips from strips from sheets of 3/4" ply. Run a bead of liquid nails or equivalent on the back side of the furring strip, then use 6" (6" screws for 4" of polyiso, 4" screws for 2" of polyiso) screws to mount them. Screw through the furring strip, through the 4" of polyiso, and into the rafters, using your previous tic marks to find the rafters. The ends of the furring strips don't have to fall on the rafters either, but it will help, given your rafter spacing, if you biscuit or spline the ends of the furring strips to each other for added rigidity.

When attaching the furring strips, if the rafters are out of plane, you can shim the furring strips to bring them into plane.

5) you can now blow insulation into the rafter bays. You can cut a single access hole into the RFBI panels to get access top the bay, then blow, then replace the cut out sections, foaming the gaps to seal.

5) Screw your drywall to the furring strips 12" OC. Where the ends of the drywall panels meet in a "vertical" butt-to-butt joint, simply slide a 13.5" (or so) long length of furring strip behind the butt joint, and screw the drywall butts into that extra furring strip.

I didn't address wiring, but work that out before all fo this. Wiring chases, backer blocks for anything that needs to be mounted to the ceiling, etc.

Total R-value? About R10 for the blown-in cells, R28.8 for the polyiso, R1 for the air gap behind the drywall, and R0.5 for the drywall. Total R-value, about R40.

If that's too much, instead just use a single 2" layer of polyiso, foam the gaps, furr, then blow behind, then seal the blow points, then drywall. you'll be around R-25, but the polyiso is so darn airtight that it'll be even more effective than R-50 worth of FG batts.

Long apologies to all...but for attics, cells and polyiso are FAR superior to FG. FAR superior. Still, don't forget about moisture in the basement.

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