Insulate the underside of the roof?


Old 12-03-02, 08:40 PM
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Insulate the underside of the roof?

Howdy folks.

I bought a 1956 Rancher last year, and I need to insulate my attic this season.

Most of the attic is finished, but there is a large portion that is unfinshed that is on top of some living space. This is the area I need to insulate.

I was reading the info on the website, and it led me to believe that I should also be attaching insulation to the underside of the roof (the rafters) in addition to the floor of the unfinished attic. Is this true? It looked like they were saying to also use a buffer in between the wood and the rafter. But I want to make sure that I am considering this correctly.

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Old 12-04-02, 10:52 AM
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As long as you don't plan on finishing that section of your attic , I would put all of your insulation in the the attic floor joists.
If your joists are 2x6's or 2x8's, you'll want to install your first layer of fiberglass between the joists, vapor barrier down, your second (and possibly third layers), should be installed to run perpendicular to the first, and should be unfaced (no vapor barrier). Be careful not to block any soffit vents, and be careful around heat sources (recessed light fixtures, etc.).

If you do have plans of installing floor decking in this area, for storage or whatever, you will need to install the appropriate fiberglass for your joist depth (ex. R-19 for 2x6's), vapor barrier side down, and then install additional insulation between the roof rafters.
When installing the fiberglass in the rafters, first install polystyrene ventilation baffles in each bay. These should run from your soffits, the whole way up to your ridge vent. Insulation is put in place next, vapor barrier down toward the attic. This can be covered with an additional vapor barrier of clear poly if desired. Any gable end walls will have to be insulated also.
Old 12-04-02, 11:07 AM
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Thanks wingnut -- a couple more questions.

Thanks for answering Wingnut, i have a couple other questions that maybe you can help with.

Since I don't plan on making the currently-unused portion of the attic I am insulating a usable space, I am going to forgo insulating the rafters -- correct?

But, the old insulation in the attic space is R-11 (!) with the paper vapor barrier. Can I use this to doule-up the insulation in between some of the joists? Or, should I just use the current one layer of R-11 in between the joists and then run something like R-19 unfaced perpendicular to the first?

Also, what should I use for the Gable ends, R-19?


Old 12-04-02, 01:10 PM
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That's right, I'd forget about insulating between the rafters then.

If that existing R-11 insulation is still in good shape (not too matted), you can probably leave it in place.
If its too compressed, or the paper facing is disintegrating, I'd tear it all out and get new (remember, you're going to be relying on that old paper facing as your only vapor barrier).
Fiberglass insulation really isn't that expensive anyway.

If you do keep the existing insulation in place, add enough unfaced to each bay to bring the insulation up to level with the tops of your joists, then run your perpendicular layers.

If not, buy the appropriate size insulation that will completely fill the bays up to level with the tops of your joists.
R-19 fits 2x6 bays, I think R-25 fits 2x8 bays.
The house my wife and I just bought is about the same age as yours, and it has 2x6 joists, so yours probably does too.
So that would give R-19, I'd run another layer of R-19 perpendicular, and then maybe another layer of R-11 on top of that yet.
That would total an R-49, which is the recomended insulation level for your climate, and mine (we live near Harrisburg).

If you insulate the unfinished part of your attic in this fashion (insulation between the floor joists and none in the rafters), there is no need to insulate the gable end walls.

Now, the finished section of your attic should (hopefully) already have insulated gable end walls, insulation in the rafters and insulation in any walls that separate the finished area from the unfinished area (partition and knee walls).
Insulation in all walls should be R-19, just like the outside walls throughout the rest of your house. Treat that unfinished area just like it were "outside" space, and insulate heated areas from it.
Old 12-04-02, 01:16 PM
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Thanks again

Thanks again wing.

I might just pull the old out and put in the new. Fresh insulation would certainly improve the value of my home, not to mention save $$$ on bills. And I could probably use the R-11 under the floor in the crawlspaces.

Take care.

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