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General Attic and Crawlspace Insulation Questions

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I have been going back and forth on hiring someone to insulate my attic and crawlspace and at this point I think I’m going to tackle it myself but I want to make sure I do it right. I’ll separate my questions by attic and CS.

1. Attic. I am planning on blowing in insulation in the attic. House was built in 1973 and it has gable vents, no ridge vent or other venting on the roof and it does not have soffit vents. The recessed lighting is “IC” rated but there are other electrical lines and junction boxed run throughout the attic. It also has a significant amount of plywood flooring. Blowing in insulation seems fairly straightforward but I was wondering if there are any issues with covering the electrical wiring or IC recessed cans or are there any other cautions associated with blown in insulation. Do I need to be concerned about ventilation if I have gable vents and no other venting or should I just evenly fill the attic? Also, should I pull up the (nailed down) plywood flooring and insulate under that? There is also a part of the roofline/attic that goes over the front porch (meaning it is not over living space), should I insulate that too or only the portions of the attic that are directly over living space?

2. Crawlspace. The CS had a moisture issue that I have since addressed with a perimeter drain and sump. At the same time, the contractor sprayed all of the joists with mold killer (there was minor mold growth) and hand wiped the joists. I did notice recently that there was one area that he seemed to miss so I plan to take care of that before insulating. Should I use faced or unfaced insulations? The CS is now dry but should I be concerned with mold growth after I install the insulation? I wasn’t sure if because the area once had mold growth was it more susceptible to future mold growth.

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  1. Expert Member
    Dana Dorsett | | #1

    Before blowing insulation in the attic take pains to air seal EVERYTHING you can. Even flues can be air-sealed with metal sheeting, and wrapped in unfaced rock wool to maintain necessary clearances to other insulation types.

    You can drill a few holes per joist bay for insulating under any plywood flooring, in much the same way that a wall cavity can be insulated. Use a hole-saw and save the plugs- you can usually put them back and foam-seal the edges & holes. Mastic-sealing the plywood seams with duct-mastic makes it a reasonably durable air-barrier too.

    Insulating out to a foot or so over the porch roof doesn't hurt, and if you ever re-side the place putting rigid foam you'd have a more continuous thermal boundary.

    Be sure to keep at least 2" of air space between the insulation and roof deck at the eaves or you may end up with rot there. If there isn't sufficient depth to get a decent R value there, it's worth using closed-cell spray foam or cut'n'cobbled rigid foam stacked on the studwall plates (and foam-sealed to the joists) to get you there.

    In the crawlspace it's almost always better insulate the crawlspace walls rather than the underside of the floor. A 10mil poly (or EPDM roofing membrane) vapor retarder on the crawlspace floor would get mastic sealed to the foundation (held in place with furring through-screwed to the foundation. Air-seal the foundation sill and band joist with spray foam. If you don't have a metal or EPDM capillary break (or at least a foamy sill gasket) between the concrete & foundation sill, if there's any question of ground moisture wicking up the foundation wall to compromise the foundation sill it's better to insulate the walls with an unfaced rigid foam or up to 2" of closed cell spray polyurethane. Alternatively a fiber-faced polyiso can be used, but steer clear of foil facers unless you're sure the bulk water issues are fully under control, and you have a foot or more of exterior foundation for the foundation to dry toward. If you DO have a decent capillary break using a highly impermeable facer is preferable.

    If you insulate between the joists the joist edges will run cold and be subject to greater risk of mold at the edges. If you don't air-seal the sill & band joist and have fiber insulation between the joists you're subject air-leakage into any thermal bypass gaps or into the air-permeable fiber insulation itself.

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