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Old 03-16-2018, 12:36 PM   #1
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Converting to Sealed Attic


I live in Las Vegas, NV and am researching converting attic from vented to a sealed attic. Currently the attic has two vents on the side of the house and a air handler and heater in the attic.

This idea mostly came from me shopping for a new central AC and wanting to replace ducts from old R4.3 ducts that have no radiant barrier to the new standard R8 silver jacket covered ducts. However this would take lots of work or cost at least $2500 to install. Also, R8 is really nothing when its like 140F in the attic for the heat to pass thru.

So my second idea is to instead of switching to new ductwork, insulate under the rafters and switch to a sealed attic and keep the old ducts. The attic temps would be much less then allowing savings on AC bill. Also, all the loud popping sounds from when house cools and heats up should be reduced.

According to the new 2012 IECC, the insulation requirement for my zone 3 is R38 for ceilings. I am thinking using closed cell spray foam to get the insulation even with the 2 X 4's about 3.5 inches for R value of about 21.7 then use insulation fabric, staple to rafter 2 X 4s and blow in an additional 4 to 5 inches of cellulose insulation so that it will sag down in center holding the newspaper insulation against the existing foam insulation and be close to R38. I already have some loose cellulose insulation on attic floor so figure just throw some of this in there and buy less. All cellulose and no foam would not work because would require about 11 or 12 inches which is too heavy and too much volume. All spray foam would be very expensive and thermal barrier would be needed since I have some boxes up in attic. The cellulose can act as thermal barrier as I understand.

Vents would have to be covered over with fiberboard and insulation and gas furnace intake pipe has to be added to do conversion as I understand.

Does anyone have any thoughts or comments on this?
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Old 03-16-2018, 12:56 PM   #2
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Re: Converting to Sealed Attic


What you describe is a common solution for mechanical equipment in the attic and as you researched there are guidelines.

One of the primary precautions is to keep moisture laden air away from a cool roof deck. In your location that is less of an issue than here in Maine but that is the purpose of using a closed cell rigid foam in direct contact with the bottom of the roof. Going unvented will also include any end walls and will require conditioning that added space.

Note, when they fill those rafter cavities you don't get a neat 3.5" of rigid insulation. The results are very bumpy with either more or less than the 3.5" IMO, less is better if completing the job with a blown-in product. More can get in the way.

Others here can better advise on the "how to" but basically you are headed in the right direction.
Here's some good reading if you haven't found it.
http://www.greenbuildingadvisor.com/...ditioned-attic

Bud
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Old 03-16-2018, 01:14 PM   #3
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Re: Converting to Sealed Attic


Thanks Bud,

Yes, from what I've read open cell insulation is problematic because it can lead to roofs holding moisture but closed cell insulation doesn't have the same issue. Also, it has a higher R value per inch which will help keep the weight and insulation volume down.

There is a builder here, Pulte homes that I think uses a similar insulation method in their attics. My GFs house had it in attic and the builder puts down the insulation fabric on top side of rafters during construction and has the fabric hanging down between rafter 2X4s loaded up with cellulose. The picture can be seen in PDF below if you scroll thru it. The image they show it looks much less saggy than one I remember seeing in person. I think was R30 total. Perhaps they are using foam board on top of roof in addition to the hanging insulation underneath. Regardless, I figure this is really only option for insulating under rafters other than doing lots of spray foam $ then spray some type of thermal barrier paint on underside.

https://www1.eere.energy.gov/buildin...ebb_hotdry.pdf

Yes, well unless using foam boards, spray foam will be lumpy or not perfect which is OK for attic. This insulation fabric is somewhat transparent so can see thru if the loose insulation gets hung up on the lumps to make sure it all goes down in place.

Thanks for the link.
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Old 03-16-2018, 01:31 PM   #4
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Re: Converting to Sealed Attic


But the space will need to be conditioned and then you have the rule of foam in conditioned space should be covered with drywall, not sure if anyone does that but I always look at the dust you are now going to pick up in the attic and try to filter out of the air.
I would discuss it with locals that have had it done for a few years and see if they would do it again.
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