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BikerAggie 05-02-2013 01:01 PM

Fixing 20 yr Old Attic Insulation Job
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I recently bought a 20 year old house in Houston, Texas. Over that time insulation in the attic space has settled and shifted and I have some work to do to improve the comfort level in my home. Right now I am getting ready to air seal, install sofit vent baffles and replace damaged batt insulation before I blow in cellulose insulation over the top of everything.

Question #1 The batt insulation on the top of my stairwell (at a 45 deg angle) was slumped down and no longer covering where it should. What can I do to make sure the replacement insulation stays put? Is it ok to tack it to the back of the drywall with a bit of construction adhesive?

Question #2 I have a house with 3 ceiling heights. In the picture below I am standing on the ceiling of the first floor over the living room, taking a picture of the ceiling of my master bedroom, also on the ground floor. The insulation between the rafter bays has fallen down and needs to be replaced and secured. I am planning to blow a bunch of insulation in place, and I also have idea. Can I build out those rafters with some furring strips and then come back over the back of the batts with some rigid foam board? I thought I could have the furring strips and foam board stick up above the existing master bedroom ceiling height 12" or so. This would give me a dam to hold more blown in insulation up to the edge of the master bedroom ceiling, and make sure the batts never fell out of the rafter bays again. The potential problem I see is I could be setting up a moisture trap, but if I don't seal the insulation board to the bottom of the wall, is this a problem?

Gary in WA 05-11-2013 10:11 PM

Make sure the new insulation is larger than the stud space to friction fit. The foam board may work, ask your local AHJ if it needs an ignition barrier covering. Blow-in in a closed cavity may create moisture problems, be sure to leave an air space for soffit/ridge venting.


DaveWolfOC 05-24-2013 11:42 AM

Hi BikerAggie, I’m Dave Wolf – senior research and development program leader at Owens Corning. I’m passionate about building science, and I’ve worked extensively on air sealing research, so I thought I could help a bit.

In terms of your question about keeping the insulation in place (Question #1), an adhesive may work, but I’d recommend installing several three-inch long screws instead. Just install a couple of the screws at the bottom and a couple at the top of where the insulation will rest, but sure to leave them sticking out a bit in order to impale the insulation unit. This will hold it firmly in place and keep it from sliding down.

Regarding keeping the insulation in place on the short walls (Question #2), I would recommend furring out the cavity, as you’ve indicated, but then apply some scrap pieces of drywall to back it, rather than the foam insulation. Most home centers sell 3/8”-thick drywall, which would be easy to cut and carry into the attic. The advantage of drywall is that it won’t trap moisture and it would also constitute an acceptable material to be left exposed from a building code standpoint. For fire safety reasons, some building codes prohibit foams to be left exposed in the attic.

While you’re doing this, I would also look for places to air seal along the drywall, including all terminations and penetrations. As drywall is a good air barrier, applying a sealant around all the places where it is terminated (e.g. at the top plate of partition and exterior walls) and penetrated (e.g. around light fixtures) is needed to make sure that the whole attic is properly air sealed. (We actually offer a product called EnergyComplete that can help you accomplish this.) After you seal everything, you can insulate the ceiling plane.

Finally, I wouldn’t recommend adding blown-in cellulose insulation over the top of everything because you will compress the insulation on the bottom (cellulose is heavy). Doing so will diminish the insulation’s effectiveness and reduce its R-value. Using blown-in fiberglass loosefill will add to the R-value and effectiveness.

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