Looking for advice about retro-fitting attic insulation

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Permalink Reply by Lair Tienter on July 3, 2009 at 11:55am

I agree, you do need more insulation but I'm not that sold on Dow styrofoam. It sounds like a whole lot of work. If it were mine I would blow in several inches of fiberglass. No weight and it is fireproof. I'm not an insulation expert but have a whole lot of experience with old homes. Sometimes more work results in more problems.

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Permalink Reply by Italianate in Ionia on July 4, 2009 at 5:36am

I understand your viewpoint about doing the least amount of work.

Fiberglass has R=3.5 per inch, which would give me R=21 if I fill the joist bay (assuming the 2 inches of old insulation has a comparable R-value and is left in place). But I already can see that there is no vapor barrier beneath the old insulation.

Also, the previous owner possibly had the cost/benefit mindset that a whopping 2 inches of insulation (for obtaining R=7) was adequate enough given the heating costs of his day. Nowadays, my neck of the woods is recommended to have R=38 to R=60 attic insulation. (My neck of the woods has about 7100 heating degree days, and about 4100 cooling degree hours.) Any way one chooses to slice the cake, old houses are expensive to bring up to modern expectations.

If I laborously do "a whole lot of work" in removing the old loose-fill insulation, then I have the opportunity to close off some of those numerous air infiltration holes with low-density polyurethane spray (using those little aerosol cans of Dow "Great Stuff") when I slide in the styrofoam sheets (R=5 per inch) to get R=30 insulation. The styrofoam sheets will give me a vapor barrier that I'm lacking now.

If I were to go with professional application of high-density polyurethane spray foam (R=7 per inch), then I could get R=42 insulation in my attic joists. But, my electrical wiring would pretty much be locked in place for all eternity if I install spray foam.

I probably would be disappointing the future owner if I put 12 inches (R=42) of fiberglass insulation into the attic without bothering with a vapor barrier.

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Permalink Reply by Lair Tienter on July 4, 2009 at 10:11am

You have clearly given more thought to this than I did. You have very good points and my experience with Styrofoam is working over my head in very cramped areas. I tend to remember the difficulty and forget the benefits. Maybe the easiest is not the best solution.

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Permalink Reply by Phil on July 4, 2009 at 10:21am

I recently upgraded my attic insulation in the front of my house, including insulating my tower attic that spent its first 112 years with zero insulation, planked shut from the rest of the attic. I just used paper faced fiberglass insulation batts placed paper down for a vapor barrier, with a thickness equal to the joist heights and cut to fit exactly into the joist centers. Then, I added really thick unfaced fiberglass insulation laid perpendicular to the joists to jack up my R value into the 40s. Because the first layer filled up exactly to the top of the joists, most of these didn't even have to be cut to size. I am sure that better insulating strategies can be used, but mine cost very little, and those batts can easily be peeled up and laid right back to access wiring. By hitting a Lowes insulation rebate cycle and the federal tax credit I think that it cost me less than a dollar per square foot to do that.

Be greatful for the open floor joists. In the back of my house, my third floor is unfinished and unheated with the standard victorian 1' pine T&G floors. We use that area for storage (critical given our wet basement). There is insulation in the attic above that, but to improve comfort and heat retention in the second floor back of the house, I should lift those pine T&Gs and insulate between that floor and the second floor ceiling below it. But it seems like a pain, esp. with a back staircase

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