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Some house context: apx. 1300sq.ft. built in 1960, block basement foundation and stick frame, asphalt roof in a relatively rural setting with high winds in plant hardiness zone 6. We usually cool by opening screened windows & doors and running box fans in them once daytime temps cool off, then try to close the house up and keep the cool in during the day, running ceiling fans for circulation. We also have a heat pump as A/C when needed. Heating is with the heat pump, electric baseboards, and mainly with a wood stove insert.

Currently, the attic is (poorly) insulated on its floor to partition it from living space, and it has connectivity with an unconditioned garage (not used for cars) and its attic. There are soffit vents and a small gable vent.

We had an energy audit and want to reduce our electric use, both to save $ and make it easier to cover our needs with on-site generation we plan in the future. Attic insulation is a weak spot and seems to be priority. The auditor's proposal includes:

  • Air sealing accessible attic using one-part foam insulation, caulk, and half-inch R board
  • Rafter baffles for air flow on the roof deck (we have soffit vents)
  • Pull up attic flooring and blow in apx. 12" of new fiberglass insulation to get the attic to an R-value around 50
  • Pre-fab stair cover/thermal barrier over the pull-down steps

This mostly sounds good to me. What I'm unsure of is: will all this attic insulation preclude installing a whole-house fan (or attic fan? not sure which is right) to help with summer cooling? Is it better to install that kind of fan at the same time, or make room for it at least, before/during the insulating work?

In case it helps answer this question or spur other insights, here's photos of the attic.

Gable vent on outdoor end of attic gable vent on one end of attic

Gable vent up close gable vent up close

Half of attic facing outdoors half of attic facing outdoors

Soffit vents go around both sides of house, and this pic also shows the bathroom fan exhausting out a soffit vent soffit vents

Other end of attic facing attached garage other end of attic facing garage

Close up of gap into garage close up of gap into garage

View into garage attic showing chimney view into garage attic

I will confirm this company's "air sealing accessible attic" will include better separating the garage attic from the attic we're insulating above the living space.

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  • Your question is a bit odd. Attic insulation doesn't impact either type of ventilation. Whole-house fans extract air from the conditioned space, and attic fans ventilate the unconditioned attic space. You can still do either with upgraded attic insulation, and what order you do things in is up to you, based on the nuances of the project. Please revise to clarify your concern.
    – isherwood
    Mar 13, 2023 at 18:11
  • I think you answered my question. I'm just starting to learn about whole-house fans as an option, and I wasn't sure if covering my attic in 1ft+ of insulation would be a bad fit alongside a project that would blow air up from the house and into the attic most nights of the summer.
    – cr0
    Mar 13, 2023 at 18:47
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    The bathroom fan with flex vinyl duct is a hack job you should improve upon before burying it in insulation - but if not, you can shovel the insulation aside when you get to fixing it, finally.
    – Ecnerwal
    Mar 13, 2023 at 19:26
  • Looks like you have about 6" of insulation between the floor joists. Is that correct?
    – SteveSh
    Mar 13, 2023 at 20:17
  • @SteveSh yes that's correct
    – cr0
    Mar 14, 2023 at 14:15

2 Answers 2

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Simply box out the area you want to put the whole house fan to several inches above the planned insulation depth. Or shovel the insulation aside and do that when you prepare to install it, if you are not sure that you are installing it. Ideally, box it out in a way that makes dropping a rigid insulation box cover over it for the winter easy, so you can actually do that, rather than have a large R 0.5 hole in the attic insulation all winter from a summer-use fan installation.

Opinion: based on my research, use cellulose, not fiberglass, for the blown in. It offers moisture management and far less air movement in the insulation itself. Also better fire resistance. You'll have to weigh competing propaganda from the makers of each kind to figure out who you want to believe, unfortunately.

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    It may also be easier to install the wiring before adding insulation. Add an electrical box with a blank coverplate in an appropriate location in the living space to accommodate a switch or timer and pull wires for power to the switch box, then onward to a box in a suitable location in the attic. You may want to provide an extra conductor from the switch to the attic if you might install a two-speed fan. High for quicker cooling in the evening, low for the rest of the night for quieter sleeping. And a disconnect in the attic to allow safe work around the fan.
    – HABO
    Mar 13, 2023 at 20:16
  • @Ecnerwal thanks for your helpful answers here. As for blown-in insulation type, my overall goal is environmental impact which relates to safety, efficiency, longevity, and manufacturing. In that context do you still think cellulose is the best bet? From the little research I did on it, it seems cellulose gets significantly more petrochemical-based fireproofing treatment. But I hear you, there are many trade-offs and room for spin.
    – cr0
    Mar 14, 2023 at 20:43
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Ecnerwal covered the major points adequately. Just want to add that if you want to keep the flooring in the long run, remove it, lay down 2" of XPS foam board (this adds R-10 to what you already have), then just lay the flooring back down on the foam board.

Added this 3/14

Your attic looks almost the same as mine, except I have a steeper pitch on the roof. I have 6" unfaced fiberglass batts (rolls) laid perpendicular to the joists. That gives me ~R-36 across the whole attic. The nice part about the fiberglass batts is that it's a relatively easy DIY project, and you can do it a bit at a time. You should be able to push the batts to the far end of the joists with a broom, stick, or board, and so don't have to crawl out along the joists.

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