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Discussion Topic Insulating from the inside with polyiso?


phil1284's Avatar
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CAL

09-16-12, 12:44 PM   #1  
Insulating from the inside with polyiso?

Hi -

I have a 2BR townhouse condo with some heat issues. I believe the problem is radiant heat coming from the roof. This is in a warm, dryish climate (Los Angeles.) The roofs and ceilings are sloped but there is no attic space above the upstairs rooms. So I imagine that there is fiberglass stuffed between the rafters above the ceiling. As far as I can tell, the roof is un-vented. I looked along the roof overhangs but couldn't find any vents. Looked on the roof too but didn't see any venting. I doubt there is a radiant barrier since the building is from 1978.

I'm looking for a cost effective way to add insulation and reduce the flow of radiant heat. I'm not allowed to modify or penetrate the roof and there is no attic, so any insulation I add would have to be from the inside in. Also, I'd like to leave the current insulation intact to save on costs (assuming it's in okay condition).

Based on some things I've read, I thinking of using polyisocyanurate boards (AKA polyiso) to accomplish this. I believe these sometimes come with foil on both sides. Here is my thought about how this would be done:
-Remove the existing drywall ceiling, but leave the existing insulation, vapor barrier, etc. in place.
-Install 3/4" deep furring strips perpendicular to the rafters. This is to create an air gap to make the polyiso's radiant foil work better
-Attach the polyiso boards to the furring strips
-Seal between the polyiso boards with some kind of vapor barrier tape (aluminum tape? house wrap tape?)
-Attach another set of furring strips to the polyiso boards (on the inside side of the room this time). Again to create a small air gap
-Attach drywall to the furring strips and finish the drywall.
Is this a sound idea? Would there be problems with moisture getting trapped?
Is it overkill to have two air gaps around the polyiso? I'm guessing that for me the air gap on the outside side of the polyiso is the more important one since my problem is heat coming into the living space more than heat going out. (Keep in mind that this is a warm climate.)

Thanks

 
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Bud9051's Avatar
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09-16-12, 01:19 PM   #2  
Hi Phil,
Radiant barriers are one of those challenging elements that we want to use but have a hard time identifying how much they will help. In your case, the radiant energy has already been converted to heat so your next best barrier is insulation. The one detail I did not see in your description is "how much" polyiso (PIC) you want to add, one, two, or more inches. I just printed off a chart that illustrated the net r-value of a foil barrier, 3/4" gap, heat moving down on a 45 slope. Sounds close. They determined it added a net r-2.09, call it 2. If you simply increased your polyiso thickness by 3/4" you would be adding r-5.

As for the polisocyanurate, it gets some of its r-value from being filled with a gas other than air. Unfortunately, it leaks and over time ends up back to the r-5 the other foam insulations have to start. If the pink and blue are less expensive, again, how much would you have to add to equal the r-value of the PIC.

One more thought. If you are adding enough rigid foam to the inside, do you need to remove the drywall?

Bud

 
phil1284's Avatar
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09-16-12, 03:31 PM   #3  
Thanks Bud.

Yeah, I wasn't sure about thicknesses. I've read that 1" of polyiso with a 3/4" air gap is good for R-8.8, so I was thinking somewhere in the 1" - 2" range should be good.

I'm thinking of removing the ceiling drywall because I would like to also add LED recessed lighting. There is no lighting in the ceiling at all now.

 
Bud9051's Avatar
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09-16-12, 04:51 PM   #4  
Since you are going to pull the drywall down, go ahead and cut an inspection hole up high to see what you have for space. 2 x 6's or 2 x 8's or trusses of some sort. You will also see what is up there for insulation. As an fyi, the bottom of the roof sheathing is the place to install the radiant barrier. Up there with an air gap below it traps the heat at the roof where it radiates back into space and cools by outside air flow. They make plywood for new construction with foil on the bottom just for this purpose. In your case you could re-insulate with some better insulation (even foam if your budget allows) and end up with a really good ceiling.

Bud

 
Nashkat1's Avatar
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09-16-12, 09:39 PM   #5  
Welcome to the forums, Phil!

I have a 2BR townhouse condo with some heat issues. I believe the problem is radiant heat coming from the roof. This is in a warm, dryish climate (Los Angeles.) The roofs and ceilings are sloped but there is no attic space above the upstairs rooms. So I imagine that there is fiberglass stuffed between the rafters above the ceiling. As far as I can tell, the roof is un-vented. I looked along the roof overhangs but couldn't find any vents. Looked on the roof too but didn't see any venting. I doubt there is a radiant barrier since the building is from 1978.
The only effective insulation is dry insulation. Even in your relatively dry climate, an unventilated, insulated space above the top floor ceiling, up against the roof, is unlikely to stay dry enough to provide an effective barrier against heat transfer, either in or out.

I would replace the existing soffit material with a continuous vent covering. I would add a ridge vent. At this point you would have what we have on our 3-story townhome, which has vaulted ceilings on the top floor. The top floor of our townhome stays about 4[SUP]o[/SUP] cooler than the middle level all through the summer.

Remove the existing drywall ceiling, but leave the existing insulation, vapor barrier, etc. in place.
If you decide to do more work, after creating the ventilation, do it so that you only have one vapor barrier when you're finished.

-Install 3/4" deep furring strips perpendicular to the rafters. This is to create an air gap to make the polyiso's radiant foil work better
-Attach the polyiso boards to the furring strips
-Seal between the polyiso boards with some kind of vapor barrier tape (aluminum tape? house wrap tape?)
-Attach another set of furring strips to the polyiso boards (on the inside side of the room this time). Again to create a small air gap
-Attach drywall to the furring strips and finish the drywall.
That sounds like a lot of effort. I would just add a couple of inches of foam board to the face of each rafter/joist, to stop the bridging effect (heat transfer through the rafter/joist itself). Then I would add a couple of inches of new insulation, with a vapor barrier, over the existing insulation, after removing or slitting any vapor barrier that was there when I took the drywall down, and install new drywall with screws long enough to reach through the rock, the added foam board on the rafters, and another 1-1/2" or so into the wood. Say a 4-1/2" screw.

I'm thinking of removing the ceiling drywall because I would like to also add LED recessed lighting.
If you do that, you might as well not fool with any of the other work. Cutting holes in the drywall and sticking recessed light can up into them is the equivalent of adding a chimney, or vent, from the conditioned space into the unconditioned space. Even with the most air tight fixtures you can buy, and even with a sealant around the edge of each can, heat and moisture will transfer easily. Instead, add either surface or pendant fixtures. CFL, LED, halogen or incandescent isn't as important as maintaining the barrier between the conditioned and the unconditioned space.

 
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