Radiant barrier other than paint

[keith ouellette]

The name of the product is silver shield radiant barrier. To me it looked like a thin, silver thermal blanket but instead of keeping heat in it reflected the thermal energy back out. I have been in two seperate home attics that used it. If you have ever been in the attic of a home in florida durring july you would know it is at least 120 deg. even with ventilation. These two homes with the radiant barrier were only a few deg. above the outside temp.
It attatches to the trusses. It just gets stapled in place and in florida type climate you leave a foot of space at the bottom so the soffit vents will work. I am going to put some in this winter.

[Russ Filtz]

This issue was hashed out in another thread. My take on it is I don't want to be cooking my shingles from both sides. Which is basically what the goal is by reducing attic temp. I doubt much of the attic heat gets translated into indoor AC load, but I'm sure some does. I'd still go with attic ventilation and spec lighter color shingles next time.

[Rob Russell]

I doubt much of the attic heat gets translated into indoor AC load, but I'm sure some does.

I can tell you from practical experience that a hot attic does indeed translate into AC load.

We lost power last summer at about 7PM on one of the hottest days of the summer. We don't have central AC, so had gotten home and just fired up the attic fan to start moving some air when the power died. That night, I tried to sleep in our 2nd floor bedroom and could feel the heat radiating down from the ceiling. That heat was coming through a typical New England insulated attic ceiling - blown in insulation completely covering the joists - at least R25 - R30.

[keith ouellette]

This issue was hashed out in another thread. My take on it is I don't want to be cooking my shingles from both sides. Which is basically what the goal is by reducing attic temp. I doubt much of the attic heat gets translated into indoor AC load, but I'm sure some does. I'd still go with attic ventilation and spec lighter color shingles next time.

I had posted there also. Just wanted to pass along some good info. In Fl. most homes have the duct work in the attic and heat directly effects the temp. of air flowing through the ducts. I have been in many attics in my state and other than having a blower vents don't remove enough heat and lighter colored shingles didn't help much either. Keeping the thermal energy from getting in the space is the best way to go.

[Russ Filtz]

Guess I'm not arguing that the cooler attic will help the AC load, just that by doing that you are probably shortening the life of the roof (possibly drastically). You need to weigh the cost of new roofing more often vs the AC energy costs. Or other options like attic fans, or better insulated duct work.

If your Florida house is like mine, it's virtually impossible to access the attic at all. With high, angled ceilings I've got nothing more than a crawl space up there between the ceiling and roof. I'd have to rip the whole roof off and start over!

[keith ouellette]

My house was built with flat cielings. This product I'm talking about would be a lot more difficult to install in your situation. As to heating the shingles from both sides... The shingles don't get any hotter. The energy that passed through them has already effected the shingle and doesn't add to the heat on the way out. Think of adding boiling water to already boiling water. The new boiling water doesn't ad extra heat.

[Russ Filtz]

Au contraire! I think it's more about where that energy goes, or gets trapped. With no interior coating the energy would be radiated out into the attic space. With the coating it trapped and reflected back into the shingle. This is analogous to using an interior sunscreen in your car window. It helps a little by reflecting some of the light back out the window, but the air space between the shade and the window still heats up and thereby heats up the interior some (and the window itself). An exterior window cover would keep the car interior much color in this instance.

Another way to look at it is a pipe carrying hot liquid. Uninsulated (unpainted roof) the exterior surface of the pipe will typically be at some temperature lower than the fluid. This is due to convection and radiation at the pipe exterior. Now if you add insulation to the pipe (your radiative paint layer) the pipe surface temp will approach, if not equal, the fluid temp. Thereby increasing the average wall temperature of the pipe (shingles in the roof's case). Simple heat transfer theory.

You aren't adding energy. You're correct that all the energy that's in the "system" already went through the roof the first time. But it's a matter of how that energy is handled from there that can affect the temperature of the shingles.

What you really need is a high emissivity coating (think black paint) on the inside to shed the energy into the attic airspace, then attic airflow to remove the "heated" air from inside. Ever wonder why a black object inside a room is cool to the touch (think black granite or even a cast iron skillet). That's high emissivity talking!

[Russ Filtz]

Think of adding boiling water to already boiling water. The new boiling water doesn't ad extra heat.

Oh but it does! Doesn't add any temperature rise, but the total enthalpy of the "system" is increased (i.e. total thermal mass).

You're straining my 25-years ago studies in Thermo and heat transfer, now quit it!

[keith ouellette]

[QUOTE=Russ Filtz;668659]Oh but it does! Doesn't add any temperature rise, but the total enthalpy of the "system" is increased (i.e. total thermal mass).

I see you are much more educated than i am because I havn't had much skooling ( and I bet it shows) but adding to the thermal mass will not add to the temp. Increasing the thermal mass only means there is more of the hot stuff there. A larger thermal mass may mean that when the heat source is taken away it might stay hotter longer.

Your example of the car winow and screen is not the same because we are trying to see if the window (shingles) are going to get hotter because the screen reflects thermal energy back through the glass. Covering a house with something to sade it like a car cover would be best but its hard to grow house sized trees very quickly.

In the example of the pipe carrying hot liquid the surface of the pipe will stay hotter and approach the temp of the liquid ( or equal it with perfect insulation) if it is insulated. True. But we are talking about wether or not the pipe can get hotter by having thermal energy bounced back through it.

In all three cases shingles, window, pipe the amount of thermal energy is constant and un focused and no matter how many times it goes through an object the object will get no hotter.

Reflecting the heat back out of the attick will keep it from collecting in the attick.

As far as emissivity is concerned I will admit i know what it is but do not understand your example and how it will help the attic temp. or the shingle temp. but would like to hear more.

[Russ Filtz]

[QUOTE=keith ouellette;668706]

In the example of the pipe carrying hot liquid the surface of the pipe will stay hotter and approach the temp of the liquid ( or equal it with perfect insulation) if it is insulated. True. But we are talking about wether or not the pipe can get hotter by having thermal energy bounced back through it.

In all three cases shingles, window, pipe the amount of thermal energy is constant and un focused and no matter how many times it goes through an object the object will get no hotter.

Wait, you just agreed and disagreed at the same time on my pipe analogy! You agreed the pipe wall would get hotter with insulation (normally a pipe carrying hot (or cold) liquid will have a temperature difference from the inside surface to the outside surface). Adding insulation basically makes the temperature change go away. We're not really "bouncing" energy back and forth, creating heat out of nothing, just minimizing it's loss to the surroundings (attic in this case).

It's more of an insulation effect which causes the average temp of the pipe (roof) to be hotter. It's that average temp rise, due to the insulation effect, which I'm saying can cause shingle damage, not that we're "adding" heat somehow.

If getting an attic too hot can cause shingle damage, I don't see why trapping the heat directly in the roof material can't do the same!

The only way to know for sure would be to do some controlled tests and take temperature measurements of the shingles.

Emissivity is really kind of simple. It's a measure of how much energy can be radiated back away from the body. Black tends to be very good at both absorption and radiation (emissivity is just a measurement of how well the radiation is "emitted"). A perfect material which can absorb and emit 100% of all energy is called a "blackbody".

[keith ouellette]

[QUOTE=Russ Filtz;668800][QUOTE=keith ouellette;668706]

Wait, you just agreed and disagreed at the same time on my pipe analogy! You agreed the pipe wall would get hotter with insulation (normally a pipe carrying hot (or cold) liquid will have a temperature difference from the inside surface to the outside surface). Adding insulation basically makes the temperature change go away. We're not really "bouncing" energy back and forth, creating heat out of nothing, just minimizing it's loss to the surroundings (attic in this case).

It's more of an insulation effect which causes the average temp of the pipe (roof) to be hotter. It's that average temp rise, due to the insulation effect, which I'm saying can cause shingle damage, not that we're "adding" heat somehow.

If getting an attic too hot can cause shingle damage, I don't see why trapping the heat directly in the roof material can't do the same!

The only way to know for sure would be to do some controlled tests and take temperature measurements of the shingles.

I didn't agree and disagree. I agree that insulation will keep the outside of the pipe hotter and close to the temp. of the fluid within but it can't make the pipe hotter than the internal temp. That is what I am saying in the case of the shingles. The radiant barrier moves the thermal energy out of the attic but can't increase the total heat.

One thing I agree on COMPLETELY WITH YOU. Controlled tests are the only way to go.

[Russ Filtz]

OK, my brain hurts now from trying to remember my heat transfer and thermo classes!

[jeremy levine]

I make no claims ...

http://www.ornl.gov/sci/roofs+walls/radiant/rb_03.html Has a section on the effect on roof temp.

[Russ Filtz]

Thanks, according to them the roof temp does rise as expected, but it's only 2-10 degrees with a radiant barrier attached to roof.. May or may not impact shingle life much.

[keith ouellette]

Well. I was wrong acording to those field tests. 2 to 10 deg. differance is a lot greater than the no differance I claimed. I still like the product. I've seen it work. Thanks for the info Jeremy. Do you miss Bender's show or is that Flexo without the mustache?