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Best way to seal off attic above drop ceiling? Plywood and seal, 1 or 2" foamboard?

I am trying to come up with a solution to seal off the attic of this 5800 square foot house. The top attic is two parts: one part that is drywalled (300 sq ft) and other that is not (420 sq ft). It has a drop ceiling and if you move the tiles away you can just push up through the 9 inch fiberglass batts into the attic.

So they technically are heating an additional 700 sq ft of space until I close it off to shrink the envelope to the roof of the third floor. I was originally thinking 2 inch Thermax, then blow an additional 14 inches of cellulose. But Thermax was getting expensive.

Would I be able to just do drywall or 5/8 plywood and spray a coating to give it a vapor barrier? Or can I use 1 inch foam board then seal it and then put down plywood?

What is the most cost-effective and efficient way to close off that attic? (Note* there is fiberglass ductwork up there.)

Asked by Jeff Alligood
Posted Wed, 03/19/2014 - 13:49
Edited Wed, 03/19/2014 - 14:12

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8 Answers

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1.
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5800sqft homeowners can afford ....

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 03/19/2014 - 13:57

2.
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Jeff,
I understand that you have a suspended ceiling with no air barrier above the suspended ceiling. That's a problem.

However, before I can understand your proposed solutions, I need to understand whether you want to solve the problem by working from below (attacking the ceiling) or from above (attacking the attic floor).

Of course, there is also a third option: attacking the sloped rafters.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 03/19/2014 - 14:30
Edited Wed, 03/19/2014 - 14:31.

3.
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Martin,

I was going to work from above and attack the attic floor because working from below would be tough without taking down big sections if not all of the drop ceiling. So I would like to solve the problem from above. Third Option - I ruled out because spray foaming the area was extremely costly because its 45ft by 14' sloped sides.

Thanks.

Answered by Jeff Alligood
Posted Wed, 03/19/2014 - 15:24

4.
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installing and sealing plywood will work to air seal the attic, provided the edges of the floor framing below the plywood is sealed and insulated from outside air, and provided the air barrier between the eave sealing and attic floor sealing is continuous. Often easier said than done, since the rafters typically sit on the top plate of the wall, below the attic floor.

Answered by Bob Irving
Posted Wed, 03/19/2014 - 18:00

5.
Helpful? 1

Jeff,
By far the easiest place to locate the air barrier is on the underside of your ceiling joists. The easiest way to do this would be to remove the suspended ceiling and discard (or recycle) the components. Then install a drywall ceiling.

If you really don't want to do that, you can try to create a plywood air barrier -- a subfloor -- on the attic side of the joists. As Bob Irving correctly noted, the problem with this approach is that it is very difficult to create an air barrier at the perimeter of the attic -- an air barrier that connects the plywood subfloor with your wall air barrier (in most cases, that means the drywall or plaster on your walls below).

Finally, you really have another problem that needs to be addressed -- the fact that you have ducts in your attic. It doesn't make much sense to solve one serious thermal problem while ignoring another serious thermal problem. I strongly recommend that you consider creating an unvented, conditioned attic by installing insulation along your sloping rafters. For more information, see Creating a Conditioned Attic.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/20/2014 - 05:05

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Bob, That is what I was worried about trying to continue the barrier from the top to the bottom and make sure I continuous barrier.

Martin,
That is the direction that I was going. Instead of taking down the whole drop ceiling, would it be possible to take out just the panels and maybe use 1" or 1/2" foam board. And screw them to the bottom of the rafters...Not so worried about R-Value given they already have 9inch batts and will be blowing in an additional 14 inches.
Also, if I was to actually put wood on the top part of the rafters, which I opted out of given the input from Bob Irving. Wouldn't the wood act as a vapor barrier given me a double vapor barrier with the batts which have paper facing down (sorry I forgot to mention that) and then create a moisture problem.
But I would love to bring the whole attic into the envelope given the ductwork but I think its getting to costly if we were to spray foam the attic ceiling.
Thank You everyone for your input this is my new favorite forum!
-Jeff

Answered by Jeff Alligood
Posted Thu, 03/20/2014 - 09:36

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Also, does the material that we use on the underside of the rafters need to be a fire barrier, when would be have to have a fire barrier in the case?

Answered by Jeff Alligood
Posted Thu, 03/20/2014 - 09:52

8.
Helpful? 0

Jeff,
Yes, you can use rigid foam as an air barrier, as long as you can find a way to effectively seal the seams. The easiest type of foam to tape is foil-faced polyisocyanurate. Remember, though, that most building codes require that rigid foam needs to be protected by a layer of 1/2-inch gypsum drywall -- and if you are going to do that anyway, the drywall could just as well be your air barrier.

According to a code discussion forum, "A suspended ceiling won''t function as a thermal barrier, because a fire can get between the ceiling and the insulation it''s supposed to delay from reaching ignition temperature."

Don't worry about vapor retarders; if you decide to install plywood as subflooring in your attic, it won't do any harm. For more information on this issue, see:

Do I Need a Vapor Retarder?

Vapor Retarders and Vapor Barriers

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/20/2014 - 11:27

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