Cool roof system?

[mol1jb]

So my roof will need to be replaced in about 5 years so I have been doing some digging on energy efficient roof systems. Here is a great article on a cool roof system:

(broken link removed)

Now I don't live in the South but even in the Midwest summers are hot. And finding passive cooling options are limited. Definitely putting some consideration into this.

Has anyone here done something like this with good results?

 

[DBoon]

Summers may get hot in the Midwest for a few weeks at a time, but they can't compare to Houston...

I would say that if you like the look of a metal roof, then consider that. But also consider the entire roof/attic system. You might find more relief from heat using some better venting or more attic insulation, or air-sealing between the attic and the ceiling to reduce the stack effect of a hot attic.

You have plenty of time to figure this out. www.greenbuildingadvisor.com adn www.greenbuildingtalk.com are good sites worth exploring and getting soem advice from. They can both provide good information for how to spend your money most wisely.

 

[woodgeek]

If the plane between your attic and conditioned space is well airsealed and insulated, then the heat leak from your attic to your house in the summertime is actually tiny, and so would be the AC savings from a cool roof system. If your attic adjacent rooms get 'hotter' in the summer than you would expect...the solution is proper airsealing, not cool roofs or attic fans or whatever.

 

[byQ]

Thanks for the post/the write up. I'm getting close to building a roof. I'm not in Houston but it gets hot where I'm at, without the humidity (mountain desert). I'm going with metal because it lasts longer although more expensive.

I knew about colors and heat absorption. And it does make good sense to create a 'moving air system' between the metal roofing and the sheathing (sheathing = plywood or OSB) by creating a space between the 2 layers. News to me - the barrier he creates between the metal roofing and the sheathing. I wonder what he uses for this barrier? Rigid board insulation?

I was thinking of spray foam under the sheathing. But he talks about a barrier above the sheathing. It looks like something thin that you lay down?

 

[mol1jb]

Thanks for the post/the write up. I'm getting close to building a roof. I'm not in Houston but it gets hot where I'm at, without the humidity (mountain desert). I'm going with metal because it lasts longer although more expensive.

I knew about colors and heat absorption. And it does make good sense to create a 'moving air system' between the metal roofing and the sheathing (sheathing = plywood or OSB) by creating a space between the 2 layers. News to me - the barrier he creates between the metal roofing and the sheathing. I wonder what he uses for this barrier? Rigid board insulation?

I was thinking of spray foam under the sheathing. But he talks about a barrier above the sheathing. It looks like something thin that you lay down?

I know in this system there is a radiant barrier on top of the roof sheeting to reflect any residual radiant rays that get past the metal roof. Those reflected rays heat up the air space between the metal roof panels and the roof sheeting and then exit out the top roof vent.

 

[peakbagger]

Most of the Structural Insulated Panel (SIP) structures are built this way. After the SIP goes in place they strap across is and then installed standing seam roofs. The nice part for northern climes is no worry about ice damming as the entire roof is cold. I have seen some standing seams go in with two layers of strapping on running vertically and the second running horizontally and expect that was done to increase the air flow up under the steel.

I expect the radiant barrier shown is the aluminum bubble wrap that's overhyped for other uses.

 

[byQ]

Okay. Let's build the roof - in theory (I'm building a cathedral ceiling). The rafters are up. Now the OSB is nailed down. I lay the tar paper across the roof - starting at the bottom and fasten. Now I lay a radiant barrier foil on top of the tar paper and fasten.

Now I want to build a cavity, so I nail down some 1x2's (or similar) horizontally over the tar paper and foil. The light colored metal roofing is attached through these furring strips and into rafters. Next, I spray some expanding foam on the underside of the sheathing - just a couple of inches.

Finally, I attach sheathing to the underside of the rafters and staple a semi-permeable barrier on this sheathing. Next, I fill the cavity with cellulose fiber. Let's see what happens on a hot summer's day.

The sun hits the metal roofing and most bounces off of it but some energy is absorbed into it. This energy starts traveling downwards. It hits the air channels of flowing air and is horizontally moved out from under the roof. But some of this energy makes it downward - it hits the radiant barrier and some is reflected back upward. This reflected energy is absorbed into the metal roofing. This roof heating causes the air in the channels under the roof to move faster (a higher temp gradient).

However, some of the energy passes through the foil radiant barrier. But, it is absorbed into the 2" spray foam insulation under the sheathing. And the spray foam insulation heats up a little which heats up the cellulose a little.

 

[woodgeek]

And if my conventional attic is 700 sq ft, 140°F, and has R-49 on the floor (from 18" of cellulose, cuz I live up north), then the amount of heat leaking to living space is 700 * (140-70) /49 = 1000 BTU/h, or 1/12th of a ton of AC...a small fraction of what a window AC provides.

Hot attics heating living space is due to air leaks through the insulation (ice dams too). Airseal, insulate and done.

 

[mol1jb]

And if my conventional attic is 700 sq ft, 140°F, and has R-49 on the floor (from 18" of cellulose, cuz I live up north), then the amount of heat leaking to living space is 700 * (140-70) /49 = 1000 BTU/h, or 1/12th of a ton of AC...a small fraction of what a window AC provides.

Hot attics heating living space is due to air leaks through the insulation (ice dams too). Airseal, insulate and done.

I hear you. I have done the air sealing and attic insulation increase in my house to R 50. But I don't want to stop there. It may only decrease my cooling needs by a small margin, but pay that out over 30-50 years of the life of my future metal roof and I'll take that reduction and the money it saves me as well as reducing my carbon foot print.

 

[peakbagger]

In a cold climate I like a cold roof as ice damming can be significant issue even if attic ventilation is done well. Lot to be said for an unheated or cooled attic space acting as a buffer to the outdoors. Using a SIP system means skipping all roof framing just put the walls up drop the sips in place and get a membrane on them and the place is tight. Anytime I can get something like this done at factory compared to in the field I am for it. The only issue with SIPs I have heard is they expand and contract with temperature variations, some of the early uses used standard shingle and after few years the shingles telegraphed the joints. I think most SIP roofs are now standing seam so its not an issue.

 

[woodgeek]

1000 BTU/h = 200W in a 16 SEER AC. Figure the cool roof saves you half that, maybe 1 kWh per sunny summer day, maybe $20 a year?

What's the payback on the extra material cost? If you buy green energy....the cool roof could have higher carbon footprint. IOW, a waste of effort.

 

[mol1jb]

Okay. Let's build the roof - in theory (I'm building a cathedral ceiling). The rafters are up. Now the OSB is nailed down. I lay the tar paper across the roof - starting at the bottom and fasten. Now I lay a radiant barrier foil on top of the tar paper and fasten.

Now I want to build a cavity, so I nail down some 1x2's (or similar) horizontally over the tar paper and foil. The light colored metal roofing is attached through these furring strips and into rafters. Next, I spray some expanding foam on the underside of the sheathing - just a couple of inches.

Finally, I attach sheathing to the underside of the rafters and staple a semi-permeable barrier on this sheathing. Next, I fill the cavity with cellulose fiber. Let's see what happens on a hot summer's day.

The sun hits the metal roofing and most bounces off of it but some energy is absorbed into it. This energy starts traveling downwards. It hits the air channels of flowing air and is horizontally moved out from under the roof. But some of this energy makes it downward - it hits the radiant barrier and some is reflected back upward. This reflected energy is absorbed into the metal roofing. This roof heating causes the air in the channels under the roof to move faster (a higher temp gradient).

However, some of the energy passes through the foil radiant barrier. But, it is absorbed into the 2" spray foam insulation under the sheathing. And the spray foam insulation heats up a little which heats up the cellulose a little.

Close but we would have to do a vertical furring strips (for our bottom to top above sheeting ventilation) to the deck first then a horizontal furring strips on top to attach our metal roof to. In theory and looking at the research that organizations have posted on cool roof systems, your attic space (if you have one with cathedral ceilings) should be only a few degrees higher than outdoor temps.

In my situations I have a shingle roof currently. I am thinking of building this system over my existing shingles for several reasons. (I would be doing the roof install myself)

1. Less landfill waste
2. Less labor without removing
3. Regular 3 tab shingles are flat enough for this to work
4. Furring strip system can be screwed through existing shingles.
5. I know the current sheeting is in good condition.

Now I haven't decided for sure if over shingle install will be the route I take. But I think for my house it would work good.

 

[mol1jb]

1000 BTU/h = 200W in a 16 SEER AC. Figure the cool roof saves you half that, maybe 1 kWh per sunny summer day, maybe $20 a year?

What's the payback on the extra material cost? If you buy green energy....the cool roof could have higher carbon footprint. IOW, a waste of effort.

All good points. But I am assuming that you are doing these calculations on your house with the equipment you have. I believe my AC is only 13 SEER. Perhaps my air sealing is not as good as yours. I have only been in my house a year and there are still many energy improvements to be done. In my roofs situation if I leave the existing shingles on for the metal roof install I will have to use horizontal furring strips anyways. So in my situation I feel it makes good sense to add one more step of the vertical furring strips to achieve above sheeting ventilation and make my roof twice as efficient.

 

[ginab]

Avoid dark colored roof, it conducts a much higher temperature making the room more hot and thereby increasing your electricity bill. A cool roof is much cooler than a dark colored roof. It saves energy and reduces the utility bills. A white roof decreases the solar grain and the heat retention of your home. it also increases the comfort of your home. While getting a new roof it is better to get light colored shingles or asphalt.

 

[velvetfoot]

If you're gonna put metal roofing up, might as well get something that allows mounts for solar panels to be clamped on.

Come to think of it, would solar panels help or hurt with this issue with existing homes in the south? They're raised off the roof, but are dark-colored.

 

[mol1jb]

If you're gonna put metal roofing up, might as well get something that allows mounts for solar panels to be clamped on.

Come to think of it, would solar panels help or hurt with this issue with existing homes in the south? They're raised off the roof, but are dark-colored.

I believe most roof mount pv systems are a few inches above the actual roof and thus provide shade and a solar block.

 

[Hogwildz]

And if my conventional attic is 700 sq ft, 140°F, and has R-49 on the floor (from 18" of cellulose, cuz I live up north), then the amount of heat leaking to living space is 700 * (140-70) /49 = 1000 BTU/h, or 1/12th of a ton of AC...a small fraction of what a window AC provides.

Hot attics heating living space is due to air leaks through the insulation (ice dams too). Airseal, insulate and done.

&/or improper attic ventilation. You either have a tightly sealed attic at roof level, which the attic space is then conditioned, or a ventilated system(soffit & ridge vent) and a well air sealed and insulated attic floor area. On a sealed roof system, the air leakage is stopped at the roof level, not the ceiling level, except where the roofing structure meets the walls.

 

[Hogwildz]

Avoid dark colored roof, it conducts a much higher temperature making the room more hot and thereby increasing your electricity bill. A cool roof is much cooler than a dark colored roof. It saves energy and reduces the utility bills. A white roof decreases the solar grain and the heat retention of your home. it also increases the comfort of your home. While getting a new roof it is better to get light colored shingles or asphalt.

Unless you're in a colder climate, then the reverse would be true.

 

[mol1jb]

So I was looking for distributors in my area and found this above shingle retrofit system! To boot the distributor is only 1.5 hr from where I live. Very cool looking.

http://www.mcelroymetal.com/products/retrofit-systems/138t-shingle-recover.html

 

[woodgeek]

All good points. But I am assuming that you are doing these calculations on your house with the equipment you have. I believe my AC is only 13 SEER. Perhaps my air sealing is not as good as yours. I have only been in my house a year and there are still many energy improvements to be done. In my roofs situation if I leave the existing shingles on for the metal roof install I will have to use horizontal furring strips anyways. So in my situation I feel it makes good sense to add one more step of the vertical furring strips to achieve above sheeting ventilation and make my roof twice as efficient.

Not buying it. If your attic is not airsealed, airseal it. If it is underinsulated, insulate it. Both of those things are easier and cheaper than the cool roof system, and once they're done, the cool roof adds nothing to performance. It s a poor, partial band-aid solution to a problem that is better fixed other ways.

The problem is everyone **thinks** their attic is insulated and airsealed, while very few are. If your upper stories get hot in the summer, then you have a problem. An airsealed R-50 attic will not allow the attic heat in.

Your roof doesn't need to be 'efficient' It is supposed to shed water and simply be outside your thermal and ventilation envelope. Once it is, the additional ventilation or insulation is useless. It's like insulating an unheated shed. Does it make you feel good? Or a waste of time?

 

[mol1jb]

Not buying it. If your attic is not airsealed, airseal it. If it is underinsulated, insulate it. Both of those things are easier and cheaper than the cool roof system, and once they're done, the cool roof adds nothing to performance. It s a poor, partial band-aid solution to a problem that is better fixed other ways.

The problem is everyone **thinks** their attic is insulated and airsealed, while very few are. If your upper stories get hot in the summer, then you have a problem. An airsealed R-50 attic will not allow the attic heat in.

Your roof doesn't need to be 'efficient' It is supposed to shed water and simply be outside your thermal and ventilation envelope. Once it is, the additional ventilation or insulation is useless. It's like insulating an unheated shed. Does it make you feel good? Or a waste of time?

Well its a good thing Im not selling anything then. And I think we are all clear of your opinion of this matter.

Each efficiency retrofit is different based in the constraints of the existing structure. It is a bit narrow to assume that after sufficient air sealing and insulation that is all you need fir an efficient house. Many different areas beyond those can offer increased comfort and energy efficiency.

Can one go too far and is it a waste of time and money? That arguement can be made in any topic. It all depends what your end goal is.

And I will insulate my potting shed so my plants dont freeze in April