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Attic Air Sealing and insulation questions

kidcharlemange | Posted in General Questions on

We’ve owned our house in Cincinnati (zone 4A) for 3 years. It’s a 2300 sq ft, two story garrison colonial built late 1970s. The roof has gable vents only; short roof overhangs (5”) have no soffit venting (which I might add this year). The attic has R-19 of fiberglass batts with Kraft paper facing the conditioned space. 


We noticed massive icicles this winter on the north facing roof; we did get much more snow than usual. It gets hot upstairs in summer (much warmer than the 1st floor), and the AC doesn’t cycle much on the very hottest days, leading to an occasional freeze up). We have a gas forced air furnace and AC, installed within the past 5 years. Our electrical and gas bills are a fairly reasonable (approximately $250 max), but with the poor performance on the hottest days (and obvious heat loss in the winter), I’m planning to air seal and add insulation to get more consistent temperatures, prevent those icicles, and hopefully save a little money. 


Here’s my plan:

Seal gaps in the attic with expanding foam (top plate / drywall gaps, holes for electrical wires, electrical boxes). 

Box off the drop soffits with foam board.

Add soffit baffles to allow later venting. 

Replace builder grade bathroom vent fans with nice Panasonic units; run new 6” diameter insulated vent pipe to the existing gable end vent caps. 

Lay R-30 non faced fiberglass batts perpendicular to the existing batts. 


The second floor AC supply ducts are in the attic. This is insulated with R4 fiberglass with foil facing outwards. This ducting sits on the bottom chord of the roof trusses and runs to the registers. The insulation is stapled but not taped; I’m fairly certain the ducting is not mastic’d or taped since the exposed ducting in the basement is not. 


My plan is to use foil tape to tape the seams on the insulation jacket to tidy things up. Not planning to open up the insulation jacket to seal the ducts since it seems like a great way to make more work and get in over my head. 


I know that I should give some clearance between these ducts and the new insulation since water could condense on the ducts (since they’re only R4) and sweat, but how much clearance?  I was thinking 12”. 


Can I cover the plumbing vent pipes with insulation? Can I also cover the exhaust fan duct assuming that duct is insulated to R8?


Any other feedback on this plan or pieces of advice?  Do you think this will move me towards my goals I outlined?  I’m planning to bring some 4’ pieces of 3/4” plywood up to the attic for working platforms. Any other tips for working near the eaves and other right spots? I’m also planning on wearing a bike helmet and a respirator. First time on the forum – thanks in advance for the help!

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  1. Expert Member
    BILL WICHERS | | #1

    You could tape the insulating jacket, but you'll want to check that it's airtight and not damaged. It's common for those jackets to have pinholes or tears after installation. For insulating purposes, small holes don't matter much, but if you want to keep it from leaking they can be a big deal.

    I'd use foil tape on holes in electrical boxes before foaming over the top. This will prevent the foam from expaning to the interior of the boxes which is a problem.

    You will be able to do a better job of air sealing if you remove the existing insulation first. Old insulation tends to hide things that you would otherwise be able to see and deal with. I'd price out the cost of removing the old insulation and replacing it with blown insulation. If the costs are similar to the new batts, I'd remove the old insulation, do a good job of air sealing, then blow in new loose fill insulation. If there isn't much cost difference, it's an issue of how much time/work it will be to do the project if you're doing it all DIY.

    You can insulate plumbing vents but that's not commonly done. My guess is your main issue with the ice damming is air leaks, but you probably also loose a good amount of heat with only R19 insulation. Batts are rarely installed perfectly, so you'll likely have thin spots and squished areas with something less than R19 worth of actual performance.

    Sometimes 2x10 or 2x12 boards are easier to work with than plywood. If you do go with big sheets, it's often easier to use some 2 foot strips and a few 3 or 4 foot squares since they're easier to move around than full sheets are. Remember that you'll be constantly moving stuff around, so how easy it is to move your platforms around will be a big deal while you're up there.

    Where a full tyvek suit if you're working with fiberglass. This will help a lot with itching. Take a long shower IMMEDIATELY after leaving the attic too. Do everything you can to avoid itching. Working in a fiberglass filled attic all day is miserable work.

    BTW, I highly recommend a good cordless LED work light. I really like the big one Ryobi makes that looks like a security light. They are light weight, last a long time on battery power, and do an excellent job of lighting your work area and can be MOVED as you move around. I like to use a few plug-in lights (unless there are good installed lights), and a cordless that I use as a task light.


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