Home>Discussions>INSULATION & HVAC>Attic insulation & Venting ?
3 posts / 0 new
Last post
Attic insulation & Venting ?

I own a 34 year old single story ranch with gable roof (just shy of 4 in 12 pitch). Location is in a hot & humid zone where it doesn't snow. The original attic insulation was 3.5" FG batts laid between ceiling joists, now compressed to about 2". Attic venting is accomplished by soffit vents (24" overhang) and near ridge ventilator caps.

Having lived through this summer's searing heat with the A/C running most of the time, energy costs are literally through the roof. It's my intention to do this job right the first time so I don't have to do again, ever. But I have a few questions.

The current plan is to remove and discard the FG batts from the accessible attic spaces (3), then: seal all air leaks from the ceiling & walls below, insulate both hot & cold water pipes, and re- tape all A/C ducts and insulate them since they run in the uninsulated area.

When those tasks are completed, the re-insulation will take place in 2 phases. Phase 1 will be to blow in FG loose insulation.

Phase 2 will occur during a later re-roof and will add both new decking that has radiant barrier and at least 2 if not 3 inches of rigid foam sheeting above the new decking.

My questions are:

1) If I make a 2 inch roof vent channel (probably 1/4" sheet plywood) underneath the rafters that are holding up the new radiant barrier decking, will it be a problem if I fill the entire remaining attic space up to that vent channel barrier? (It also means burying the A/C ducts & water pipes and capping any lighting protruding through the ceiling.)

2) If I'm going to use polyiso rigid foam sheets(in order to get the highest R value) above the new decking should I also use a vapor barrier on top of the polyiso sheets to avoid a water problem with the sheets? if so, could anyone recommend which one? I would, of course, still use roofing felt directly on top of new decking. My guess is that I will be using either a 30 or 50 year composition shingle during the re-roof and hurricane nailing will be used too.

Thanks for any help,


Re: Attic insulation & Venting ?

A couple comments:
Insulating the pipes and ducts is a good idea.
I would probably skip removing the old insulation & sealing, but it won't hurt.
If you blow in insulation be sure to add baffles at the eaves to allow airflow and make sure they don't get clogged when the insul. is blown in.
Usually the insulation is all together at one location. Separating it with an unconditioned space isn't the most efficient way to get r value. adding an inch of rigid insulation with a vented nailboard above could possible help temper the attic space a bit.
Not sure how you are using the radient barrier, but I believe they are usually next to worthless.
No vapor barrier above any rigid insul., just felt and ice & water shield as required.
Some shingles manuf. will not provide a warranty if applied directly to a deck over insulation without a vented space. Too much heat buildup is my guess.
Use a ridge vent along with the soffit vents.

Re: Attic insulation & Venting ?


Thanks for the reply.

I agree with you that putting in ridge venting is better than leaving the existing near ridge passive ventilators (more opening space).

Also agree with the baffles at eaves, that's part of what I refer to as the "vent channel".

As far as Radiant barrier goes, have a peek at this video test of it by Louisiana- Pacific, if seeing is believing than I'm a believer! http://www.youtube.com/watch?v=qH9FD9rvbDw

There have been a number of real scientific studies done that show it does work if used correctly.

South Florida tested and concluded it's the real deal. They actually built 2 identical houses side by side and measured significant energy savings in the one with radiant barrier over the one without it.http://repository.tamu.edu/bitstream/handle/1969.1/6496/ESL-HH-87-09-26.pdf?sequence=3

I'm aware that some of the shingle folks won't warranty over a "hot roof" that is not vented, that's why I mentioned the 2" deep vent channel (between rafters). Sorry I didn't explain it better, should have drawn a picture.

Thanks again & have a look see at those links.


Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.