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Todd Burch
05-01-2014, 5:33 PM
I happen to have several square feet (2,576) of rigid foam insulation laying around that I'm not sure I'm going to use (it came with an order of stuff I will certainly use). I have a small house that I'm redoing (down to studs and back). The roof currently has 2 layers of composition shingles, and a roofer friend indicated the current top layer is about 12-15 years old. I think I also might have a leak in the valley over the front porch - I'll know for sure in the next week or so when I rip the porch ceiling out.

I was wondering, if I do choose to replace the roof, which would involve removing both layers and going back with either shingles or metal, would applying this 2" polyiso on top of the existing roof deck (1X6 T&G pine) make any difference whatsoever in the temp of my attic during the summer or winter in Texas? The 3 gabled ends would not have any insulation, and the will have new gable vents. Inside the attic, the ceiling would have blown (new) insulation. The polyiso is not foil faced.

Thanks, Todd

Craig Matheny
05-01-2014, 6:04 PM
My father in law has passed away years ago and was a roofer he did that and swore it made a diffrence

Bradley Gray
05-01-2014, 6:56 PM
I did this to my house a few years ago. I just had 1 layer of shingles to start with, so I ripped treated 2 x 4's to the roof angle for a starter strip, covered the roof with 11/2" rigid foam and covered that with OSB and new shingles. Cut my propane bill in half.

Steve Rozmiarek
05-01-2014, 9:04 PM
I wish I would have done it on my current wood shop build.

Jim Matthews
05-02-2014, 7:39 AM
Check the roof load rating, first.

The stuff is light, but adds weight.
It requires some extensive retrofitting, to make certain
each layer stays in place.

Poly iso with a foil backing is recommended for Southerly latitudes, to avoid radiant
heating of the space below the roof.

A less expensive method is to use a perforated foil membrane in the attic space.
(What I've done, considerably further North.)

The roofers I've asked about these techniques all warn that it could shorten
the lifespan of most asphalt shingles.

288548

Todd Burch
05-02-2014, 9:13 AM
Great point Jim. A quick search says composition shingles weigh 7X a metal roof, or up to 450 lbs per square. I don't think my shingles weigh 450 lbs per square, but the two layers that are on it right now might.

2" polyiso board weighs about 48 lbs per square.

A new layer of 7/16" OSB sheeting would weigh ~156 lbs per square, but I also just read that some metal roof manufactures make clips that allow their roofs to be installed right over the polyiso.

I'm glad you asked this question. Caused me to do research.

Nike Nihiser
05-02-2014, 9:54 AM
If the attic will have new, blown in insulation and the gables won't be insulated and are vented, I don't think adding the polyiso will make much difference. That's assuming you have an adequate amount (whatever that is) of blown in insulation. Why don't you cut the polyiso in strips to fit between the joist bays and seal it with spray foam, like "great stuff", instead of blown in insulation ?

Todd Burch
05-02-2014, 10:59 AM
My wife asked that same question Nike. I told her I had a life and didn't particularly want to spend a month doing that.

We have 4 replies for far:

2 who have done exactly this, or have first hand experience with someone who did it, and say it's works.
1 who wishes he would have done it.
1 who thinks it won't work.

And then me, who wasn't sure if it would work, so I asked.

Todd Burch
05-02-2014, 11:20 AM
Here are more of my thoughts on this…

In my part of Texas, it can get upwards of 120-140 degrees in an attic. Most people around here put their HVAC units in the attic, since we don't have basements. This means, working inside a 140 degree attic, an A/C unit has to supercool the air in order to be able to blow it out at ~75 degrees into the house (maybe even cooler). It just doesn't make a whole lot of sense to me. I've always thought that IF an attic could be kept at ambient temperature (even 90 degrees - which would be 50 degrees cooler than 140), the A/C system would not have to work as hard and I could run it less.

So, the question to myself has always been, how do I keep at uninsulated attic at ambient temperature?

I've thought about a tent over the roof to keep the sun off. Visualize the Denver Airport. But, the neighbors would probably complain.

Shade trees would work, but they take a long time. Not a great solution for a 2 story house either. Or for slab on grade foundations.

Bigger vent openings in soffits, gable ends and ridges. Seems houses are always built to "minimum" standards too, allowing for only the least allowable venting. And my understanding of venting an attic is that it isn't so much to keep an attic cool, but primarily to carry away the moisture. Also, like on my current small house, there are 3 gables and 3 gable vents. We're talking a 12" round vent, and two 14" x 24" vents. Roof is probably around 15 squares. No soffits for intake, and no ridge vent. Not sure how the builder planned on moving air through there, but I guess it does. On a gable, it seems to me that 2 vents per gable would be better. One (or two) at the bottom and one at the top. I've never seen this though.

And then a lot of us have asphalt shingles. Can you spell "heat sink"? Makes no sense.

The idea of using polyiso, to me, has merit. I've put my hand on the inside of a composition roof decking in the summer. It's hot. Real hot. Seems to be convection heat. I'm thinking the polyiso would stop this convection heat, or at least slow it down, and perhaps allow the attic to be more at norm with the ambient temperature.

Does this make any sense to anyone else?

Larry Edgerton
05-02-2014, 11:35 AM
I wouldn't.

One thing I would be concerned about is the Poly degrading because of the heat. I lived in Texas, I know what the heat is all about and if the foam collapses you have a big problem.

Do some research on under roof sheathing temps. They will be extreme in your area. Now you will be making the problem worse, by far. This will lead to sheathing failure above the foam and your shingles will boil the tar out of themselves in one summer.

I would also worry about sheathing degradation from moisture build up. Sun will drive water vapor through your roofing and with no way to evaporate on the backside it will collect in the sheathing. Remember shingles are waterproof, not vapor proof. The perm rating is low but it is still there.

I wish you were closer I would just buy the foam from you........

This guy has a book on warm climates, I use his cold climate book as a reference. http://www.buildingscienceconsulting.com/who/lstiburek.aspx

Larry

Pat Barry
05-02-2014, 1:49 PM
So, the question to myself has always been, how do I keep at uninsulated attic at ambient temperature?

Bigger vent openings in soffits, gable ends and ridges. Seems houses are always built to "minimum" standards too, allowing for only the least allowable venting. And my understanding of venting an attic is that it isn't so much to keep an attic cool, but primarily to carry away the moisture. Also, like on my current small house, there are 3 gables and 3 gable vents. We're talking a 12" round vent, and two 14" x 24" vents. Roof is probably around 15 squares. No soffits for intake, and no ridge vent. Not sure how the builder planned on moving air through there, but I guess it does. On a gable, it seems to me that 2 vents per gable would be better. One (or two) at the bottom and one at the top. I've never seen this though.

And then a lot of us have asphalt shingles. Can you spell "heat sink"? Makes no sense.

The idea of using polyiso, to me, has merit. I've put my hand on the inside of a composition roof decking in the summer. It's hot. Real hot. Seems to be convection heat. I'm thinking the polyiso would stop this convection heat, or at least slow it down, and perhaps allow the attic to be more at norm with the ambient temperature.

Does this make any sense to anyone else?
Yes - I agree 100% that keeping the attic temperature clsoe to the outdooor ambient temperature is ideal.. Here in MN, in the winter for example, if the attic is warm compared to the outside you can get melting of the snow from underneath and that will re-freeze on the overhang because its an unheated space under there (outside). Therefore - ventilation - very important to keep this from happening. Likewise, in the summer, you don't want an oven overhead radiating heat into your home. Sure the insulation will slow it down, but it won't stop it. Again, ventilation is very important.

With regard to moisture removal - first the vapor barriers should keep that to a minimum, but you certainly don't want wet insulation. So, its most important to stop the moisture penetration into the attic and then for sure exhaust it if it happens to be there with ventilation.

Mike Lassiter
05-02-2014, 2:38 PM
You might look into a radiant barrier that you could fasten to the bottom side of the rafters that would reflect the radiant heat from the shingles preventing it from getting into the attic space. This could help lower attic temps a lot, but you would want / need to vent that hot air trapped in the rafter air space to keep from cooking the shingles. Heat, as mentioned will cut shingle life considerable if the attic isn't vented good. That is a factor in roof warranty too. If the shingles look like they have been cooked and brittle, the warranty will be basically nothing. Warranty requires other stipulations to be met to ensure the shingles can last the expected warranty time.

I have saw a roof done on a log cabin decades ago now, that had tongue and groove pine laid on the beams, them what we call around here "blue board" insulation sheets then plywood on top of that for the shingles to be nailed to. I cannot say if it worked or didn't. I know the log cabin is still there, but don't know if the roof has every had problems. It is old enough now that the shingles should have needed to be replaced if everything was just right anyway.

Todd Burch
05-02-2014, 7:04 PM
...
One thing I would be concerned about is the Poly degrading because of the heat. I lived in Texas, I know what the heat is all about and if the foam collapses you have a big problem.

Do some research on under roof sheathing temps. They will be extreme in your area. Now you will be making the problem worse, by far. This will lead to sheathing failure above the foam and your shingles will boil the tar out of themselves in one summer.


Good point. Seems there are lover and hater websites of polyiso out there. Found this:


Thermoset vs. Thermoplastic —
Is Your Value Melting Away?

Foam plastic insulations are either thermoplastic or thermoset materials.
Polyiso insulation products are thermoset, which means that once
manufactured, they are rigid, will not soften or melt and remain strong,
even at elevated temperatures. In fact, they can withstand elevated
temperatures without losing their insulating power.

Extruded polystyrene is a thermoplastic material which softens at 165°F
and melts in the 200°F to 210°F range. These temperatures are extremely
close to those found on walls and roofs. If the material is gone, so is the
insulation value.

Todd Burch
05-02-2014, 7:32 PM
I would also worry about sheathing degradation from moisture build up. Sun will drive water vapor through your roofing and with no way to evaporate on the backside it will collect in the sheathing. Remember shingles are waterproof, not vapor proof. The perm rating is low but it is still there.
...
This guy has a book on warm climates, I use his cold climate book as a reference. http://www.buildingscienceconsulting.com/who/lstiburek.aspx

Larry
I enjoy listening to Lstiburek. He's very knowledgeable and funny too. He has a $45 book and ASHRAE has a $110 book that seem to be the same topic.

Not sure about the whole hot sun driving down water vapor concept. I didn't make it that far in school. ;) Rigid foam seems to be the (THE) most popular commercial roof insulating material, hands down. I wonder what the commercial roofers do. Apparently, no insulation works at all with water in it.

If the sun drives water vapor through roofing (regardless of what is underneath it), building where the sun is out might be problematic…

It appears foil faced polyiso insulation has a lower perm rating than the fiberglass-cloth faced insulation that I have.

A lot of houses down here use a radiant barrier roof decking product called "Tech Shield", which is essentially OSB with a sheet of foil glued to it, facing the attic. I would reckon it has a pretty low perm rating, and Tech Shield decked roofs are not crumbling into the attics they are covering. Help me understand this concept.