How to insulate my unfinished attic ceiling?

Questions, answers and advice for people who own or work on houses built during the 20th century.

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Re: How to insulate my unfinished attic ceiling?

Postby avjudge » Fri Apr 11, 2008 11:04 pm

If you're putting rigid insulation between the rafters and the drywall (or panel or planks), do you need strapping, or can the interior surfacing material fasten straight through the insulation into the rafters? OK, yeah I can see with 2", but what if the rigid is thinner? Curious for our own sake as we're starting to think of what we'll use for the room above our garage (having now fixed it so the 110-y.o. garage won't fall down!)

Anne
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Re: How to insulate my unfinished attic ceiling?

Postby tevita » Fri Apr 11, 2008 11:51 pm

What are the baffles made of? The reason I ask, is that traditionally the purpose of attic ventilation
in an unconditioned space was to prevent the attic from warming up enough to melt snow on roof, and forming ice dams in roofing, or to keep temp of attic low in summer heat. If you insulate the roof, the only purpose I can think for baffle vents is to dry out any condensation that might form on the cold roof. If the baffle is vapor impermeable (plastic), and installed so you get no air movement, you should have minimum to no condensation, and thus not much reason for the ventilation.
I think you are correct, that venting an insulated attic doesn't make sense. Instead of all the expense and time installing baffles, and then a variety of insulation types, I would just pay for the closed cell spray foam at 6 inch depth. Add solid insulation to the attic ceiling (underside of the roof), and that will get you to around R40. I think the most recent version of the IRC allows unvented roofs, as long as the insulation is vapor impermeable. Pay attention to the humidity of your insulated attic, you may need to ventilate the attic if humidity gets to high.
I think adding rigid insulation between the rafters is a mistake. It's going to be difficult to get a tight seal without caulking our spray foaming the edges. You will have a good chance of significant heat loss through air leakage.
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Re: How to insulate my unfinished attic ceiling?

Postby avjudge » Sat Apr 12, 2008 10:29 pm

I think adding rigid insulation between the rafters is a mistake.

I don't know if anyone was talking about rigid insulation between the rafters - the question is about rigid insulation across the face of the rafters. (Though in the case of the OP it would be between strapping, so that would be breaks in the rigid insulation that would have to be spray-foamed to seal. I was asking if the strapping was necessary or if the rigid insulation could be a solid uninterrupted sheet between rafters & surface finish material.) - Anne
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Re: How to insulate my unfinished attic ceiling?

Postby Nancy W » Sat Apr 12, 2008 11:57 pm

[quote="tevita"]What are the baffles made of? The reason I ask, is that traditionally the purpose of attic ventilation
in an unconditioned space was to prevent the attic from warming up enough to melt snow on roof, and forming ice dams in roofing, or to keep temp of attic low in summer heat. If you insulate the roof, the only purpose I can think for baffle vents is to dry out any condensation that might form on the cold roof. If the baffle is vapor impermeable (plastic), and installed so you get no air movement, you should have minimum to no condensation, and thus not much reason for the ventilation.
I think you are correct, that venting an insulated attic doesn't make sense. quote]

Another reason for the venting is that it extends the life of the shingles. Who wants to pay for new shingles (and labor) 5-10 years before they need to. Perhaps you should add drilling holes for the round soffit vents to the "to-do list."

Presumeably there will be some space (probably triangular) behind a knee wall, and a small amount above the ceiling, where there will be no insulation against the roof, just on the horizontal ceiling or the vertical wall. Those spaces should be vented and it should go from the soffit all the way to the ridge vent.

Nancy
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Re: How to insulate my unfinished attic ceiling?

Postby coalminecanary » Sun Apr 13, 2008 5:11 pm

The baffles that can be purchased are some sort of plastic or foam that is flexible and squeezes in between the rafters.

So, I can see division forming already here ;-) -- I think I am going to have the poly foam guys in for an estimate and listen to what they have to say.

Right now, there are definitely no soffit vents. The loose fill is jammed all the way to the edges. Each corner of the attic is open to the air so presumably, air can get into the attic through those few spots and exit out the roof vents. Once it's finished, this won't be the case.

So if I install baffles, if I want true airflow, it's going to involve a bunch of soffit vent installation, plus baffles, plus extending the rafters in some way to bump up the R -- I am really starting to lean toward foam here!

Another thing regarding my roof.. it's asphalt now, probably 15 years old. Probably has 5 years of good life in it, and another 5 of deteriorating life. When roof replacement time comes along, I will probably install some sort of solar setup on the south slope (either water heat or cells) and the rest will be a metal roof. I will not be putting any more asphalt on this house, so if the longevity of roof argument only applies to asphalt, then I'm not going to put too much weight into that part of things.

I have some pictures in case anyone is interested. Sorry for the quality... crappy camera and bad lighting today :-(

http://photos.neenerneet.net/attic/

Image
Image
Image
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Re: How to insulate my unfinished attic ceiling?

Postby wletson » Mon Apr 14, 2008 8:08 am

Just a thought, if you need to replace your roof in five years or so anyway, have you considered adding insulation value on the outside instead of completely on the inside?
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Re: How to insulate my unfinished attic ceiling?

Postby PaulG » Mon Apr 14, 2008 11:12 am

I attended an enlightening seminar at which roof ventilation for cold climates was discussed. Ice dams were the focus, which is probably your greatest concern over shingle life or other energy concerns.

Watch out for recessed lights and heating ducts in the ceiling, they will make hot spots.
Cant tell from pictures if you have dormers or any valleys that might become problems, watch out for ice dam conditions there.

The vapor barrier you mention also sounds like its unnecessary, and if your cooling with AC its only a source of trouble in the summer. Besides, if you go rigid foam insulation or even spray in, your getting a vapor retarder built into those materials.
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Re: How to insulate my unfinished attic ceiling?

Postby PaulG » Mon Apr 14, 2008 11:14 am

wletson wrote:Just a thought, if you need to replace your roof in five years or so anyway, have you considered adding insulation value on the outside instead of completely on the inside?


Thats kind of easy to do with a metal roof, and it will help kill noise of raindrops.
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Re: How to insulate my unfinished attic ceiling?

Postby Danno » Mon Apr 14, 2008 11:40 am

I wouldn't worry too much about ice damns on that roof. I'm assuming it's the same house as seen in this picture:
Image

in spite of that, I think you can get by with soffit and/or what I call "peak vents" and something like this:
Image
Image
If you put a nice hip wall up there and insulate your wall , the ceiling from the wall to the peak, and the floor to the wall from the soffit, then that should be adequate. let me see if I can find a picture online. i have a good picture in a book at home, but that doesn't do us any good here...
here ya go, read this:
http://www.hometips.com/articles/sunset ... ic075.html
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Re: How to insulate my unfinished attic ceiling?

Postby PaulG » Mon Apr 14, 2008 12:03 pm

Your defense dollars at work:
http://www.crrel.usace.army.mil/techpub ... 2-5778.pdf

Gives a method to designing for attic ventilation including vent opening sizes and corrections for pitch, etc.

On the other side of the vent/no vent argument: If you dont have ice dams, you dont have a need to ventilate. R40 is ALOT of insulation. If you watch out for hot spots, you can forgo the ventilation in my opinion.

Also depending on how serious you are about re-roofing to metal, you can add roof ventilation at that time as well.

My word of caution, foam insulation is crazy hazardous in a fire. They melt when they burn, so thats a design consideration. If you want to lose your whole house in a fire or just a portion, foam is the way to go :twisted:
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