Attic Insulation


Old 06-24-10, 02:57 PM
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Join Date: May 2010
Location: USA
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Attic Insulation

I live in the SF Bay area so we have a fairly temperate
climate. We are doing a complete remodel of our 1960's
single-story ranch style home.
We have a contractor building the shell and my wife and I will install all the insulation. The roof will be composition.

The rafters are currently 2x4. Some will be replaced with
2x6 or larger for a section of the house that will have a
vaulted ceiling. The ceiling joists are 2x6.

It is my understanding that R-30 is the guideline for attic
insulation. Originally I was planning on using the spray-in
foam but it is relatively costly and not really necessary for
our area.

Since parts of the roof have 2x4 framing, I can't put r-30
batted insulation between the rafters. I can't even
put it between the ceiling joists since they are not
tall enough. I would also like to be able to put down
some plywood for storage and I don't want to compress
the insulation.

Can I put part of the R-value under the roof deck and part
of it between the ceiling joists so that the total is R-30?

Is there an advantage to placing the insulation under the
roof deck versus between the ceiling joists?

Lastly, if I put batted insulation under the roof deck,
do I have to provide an air gap between the insulation
and the deck? I have seen plastic inserts that provide
an air channel that you staple under the roof deck before
you install the batted insulation.


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Old 06-24-10, 06:34 PM
Join Date: Oct 2008
Location: New England
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Hi Victor, a lot of questions, but we will try to get to them,
1. R-30 is the threshold where adding more is not cost effective, based upon todays energy calculations. if energy costs go up, so would the recommended insulation. In areas where you can't just add another layer, but can alter your choice before construction, it may be beneficial to invest is some extra for those areas.
2. Foan is expensive. Check mineral wool, or consider blown in cellulose. Both are very dense and eliminate any air flow through the insulation.
3. For storage above 2x4 ceiling joists, consider 2 or 3, 2x6's on edge run perpendicular to the current truss members. plywood/osb can go on top and the insulation below. For the rest of the attic area, blown in insulation goes between and over the joists.
4. I would not mix ceiling insulation and rafter insulation. One or the other, with ceiling being the traditional. Make sure you have good attic ventilation.
5. The plastic inserts you mention are primaring to keep blown in ceining insulation from totally blocking air flow from soffits to attic area. The best high vent is a ridge vent, next are gable vents, one or the other, not both.

Here is some reading for you, slow to open, but good:

Old 06-24-10, 09:54 PM
Thread Starter
Join Date: May 2010
Location: USA
Posts: 16
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Thanks Bud. The reference material you included was really helpful. I now have a better understanding of the importance
of sealing against air movement as well as thermal movement.

I figured that since all my ducting will be in the attic, it would be
helpful to insulate under the roof deck as well since it would do a
better job of insulating the ducts than the thin stuff typically
installed on the duct itself. There still has to be ventilation
in the attic though so maybe it is a wash. I was not
planning on completely sealing the attic.

I was also thinking that keeping the entire attic insulated would
minimize thermal bridging through the ceiling framing.

Old 06-25-10, 03:52 AM
Join Date: Oct 2008
Location: New England
Posts: 10,406
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My choice over the baffles for an air gap is to use one inch or so spacers on each side of the rafter, up against the bottom of the roof deck. I cut strips of rigid foam board. Then I can apply a layer of rigid and continue to fill the cavity or continue with some fiberglass or other. Then you can cover the entire bottom of the rafters with whatever thickness of additional rigid insulation desired.

Check codes to determine if a fire barrier is needed/required to cover all of that rigid. 1/2" sheetrock is common. There is also a commercial grade foil faced foam board, Dow I believe, that can give you the final covering, fire barrier, and some radiant protection as well. I have read about it, but never tried to find it so don't know the cost. It meets different codes than the big box stuff (foil faced polyisocyanurate) but again, check local requirements.


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