thoughts on radiant barrier in attic

[thatguy]

I am always looking for ways to save on heating/cooling cost.. I stumbled an article talking about 'radiant barrier' recently and wanted to get some of your thoughts..

We have a 1.5 story house - the top floor is about 1200 sq ft that could be finished and there is attic space above that (it is all open right now).. Currently the entire attic is unfinished (except for a plywood floor which is insulated as an exterior wall. The attic gets scorching hot in the summer and I have installed an exhaust ventilation fan in the house eave, but I cant really tell much of a difference. We also have a vent across the roof line.

I see that Reflectix offers an radiant barrier (4' x 125' rolls @ ~$70/roll) that can be installed across the roof joists leaving an air space the depth of the roof joist. From reading different articles this would help keep the heat from coming into the attic space and thus keep the house cooler.

Do you think this would help keep my house cooler? any first hand experience?

The reflectix website says that the barrier can cause the shingles to get 5 to 10 degrees hotter since the heat is trapped between the roof and the radiant barrier. How bad is this extra heat on shingle life? Our shingles are black.

Comments - thoughts - opinions?

I have included a pic of the house so you can see how much roof we have - its huge..

thanks

brian

 

[/pine]

on new construction the barrier can be installed on top of the rafters under the sheathing...allowing it to bag down between the rafters...on existing structures it can be installed on the underside of the rafters which leaves a bigger more uniform airspace...

the former is a little eaiser (and cooler) installation process the latter is more difficult (working in the enclosed attic space)

Applying it on top of the rafters at the time of construction does add to the time it takes to sheet the roof...

In Florida I never met an architect that did not recommend a radiant barrier...I've also done work for several of them and all had them installed on either new or remodel construction projects for themselves...

off the top of my head ...I think I recall the time it took to regain the expense averaged about two years which makes it a very worthwhile investment...

[edit]

How bad is this extra heat on shingle life? Our shingles are black.

without proper ventilation the extra heat will cause the shingles to curl prematurely...

 

[thatguy]

Thanks for the reply /pine

I am pretty sure our builder did not use any radiant barrier when they built, but I need to verify that..

I would need to install it to the underside of the rafter..

this is from the reflectix website and what I am thinking I would need to do, if i choose to do it..

Reflectix Inc. | Do-It-Yourself | DIY Attic

How much 'proper ventilation' is adequate to keep the shingles from premature aging?? From the pic you see we have a 'hip' roof all around the house that would not be covered with the radiant barrier (cant get to them even if i wanted to) .. We have soffit vents all the way around the house (under the hip roof) and the ridge vent.. Do you think that would give me enough ventilation?? The instruction say to run the barrier close to the ridge vent - so it would probable draw pretty well from the soffits up into the hip roof and exit through the ridge vent..

thanks

b

 

[tanglefoot]

Brian,
I believe it would make a difference for you. When we built our house 15 years ago there was a new product on the market called Cool-ply. I had to transport the material we were going to use for roof decking to their factory to have it laminated with the foil backing. It increased the cost of the roof, but I believe it has paid for itself. We have a black roof also, and three dormers, similar to yours.
Butch

 

[/pine]

I think what you have is a "Boston Hip" and they do leave part of the roof inaccessible...even full hips make it very hard to get to the corners inside the attic...

applying a radiant barrier after the fact does make it better to get good ventilation to remove the heat (more air space) but Boston hips often make ventilation a problem...Ridge vents are great as long as the lower part of the roof is open to the main attic space to allow for convection flow of air from the soffit venting out through the ridge vent...

Working in an atic that has blown type insulation is much more difficult that those with batts so that may be a factor too?

another option is vent fans that work on a temperature switch (mostly seen on Gable roofs) they do make them for hips too...

 

[Patriotic Stabilist]

I don't know about that kind but when hubby redecked, he used Tech Shield decking. Our attic is cool enough in the heat of the day you could go up there. Used to be you opened the attic access door and it was like heat out of an oven. Now our house stays pretty much 10 to 15 degrees cooler. We can go longer in the spring with no AC and earlier in the fall. Our light bill last month with over 100 degrees everyday was $135, in the rent house kept it between 75 and 78 and it was $94. Great stuff. I think the kind in a roll would work too, but we put on a metal roof not shingles.

 

[dex3361]

The quick answer is yes it would make a big difference if installed right. The long answer is as follows. Putting a reflective barrier on the under side of the rafters would work well to stop the transmission of radiant heat. Heat transfers 3 ways conductive, radiant and convective. You would be changing the transmission of radiant heat by placing a barrier of poor emmisivity between the hot roof and your attic. Emmissivity is the ability of something to emit radiant (Infrared) heat and the lower the number the less it emits radiant heat. A good example is to take a large polished stainless or polished aluminum pot and apply a 4" X 4" square of electrical tape to the side of it. Taking all of the safety precautions not to get burned(safety disclaimer), fill the pot with water and bring to a boil now take a non contact Infrared thermometer (point and measure) and take the temperature of the shiny metal area and compare it to the area of black tape. The area of black will be giving off a lot of radiant heat and the areas of shiny metal will be giving off very little. Black electrical tape is a good emmiter of radaint and shiny metal is a poor emmiter. oh by the way do not touch the hot areas the shiny metal and black tape will be the same temperature to the touch(conductive). I know how everybody likes pics so I have included a few of the above mentioned demo taken with an Infrared Camera. I also included a pic of myself for those of you who dont know what I look like.

 

[Alan L.]

When we built our house we used Selection 500 foam in the walls and cathedral ceilings, but the flat ceilings contain traditional blown in fiberglass to R50. So we do have a regular attic space. We used the roof decking material that comes with a foil backing that is placed on the underside. Ours shingles are composition - a dark color.

We also have ridge vents.

I am a believer in it. I placed a thermometer in the attic and found that in the hot summer the attic gets only as hot as it is outside. One time I checked, it was 102 outside, and 102 in the attic. Previous homes I have lived in got MUCH hotter in the attic than the outside temp.

I don't get any of those 2-digit electric bills because we set our thermostat mostly right at 70, sometimes less, regardless of the temp outside. I can say that our electric bill is now less for a space 3 times as large as the one we moved out of. I'm pleased.

 

[Patriotic Stabilist]

It was over 100 for every day on our electric bill last month, it was $136 and a $17 credit for overbilling from Entergy, so for us pretty high.

That works out to $.08672 per kwh, used 1528

At $156.38 is $.1023 per kwh.

So for 30 days that's 59.93 kw's per day. I guess the freezer and fridge in the garage in this heat added up. Last summer for this month we used 1459, freezer was in the house, but it wasn't 100 degrees every day either.
This was for a 2000 sq ft house. Here we keep it between 78 and 81, we are quite comfortable at those temperatures.

At the rent house, it was kept between 75 and 78 for a month, water heater on and several lights left on 24 hours, they are cfl's. It was $94 and the rate is almost $.18 per kwh. Both houses are energy efficient, that one is only 1200 sq ft.

But we are doing ok I think.

 

[thatguy]

I think what you have is a "Boston Hip" and they do leave part of the roof inaccessible...even full hips make it very hard to get to the corners inside the attic...

applying a radiant barrier after the fact does make it better to get good ventilation to remove the heat (more air space) but Boston hips often make ventilation a problem...Ridge vents are great as long as the lower part of the roof is open to the main attic space to allow for convection flow of air from the soffit venting out through the ridge vent...

Working in an atic that has blown type insulation is much more difficult that those with batts so that may be a factor too?

another option is vent fans that work on a temperature switch (mostly seen on Gable roofs) they do make them for hips too...

Just had DUH moment as I was digesting your post.. - Although we do have the soffit venting, the front and rear of the house are the only ones open to the attic space and would have good air flow - the sides are not open (except maybe where the side meets the rear/front hips..

In anyone experience (cost effectiveness) - applying the radiant barrier to the underside of the roof rafter is it a DIY job or get a local contractor? I know that around here you can't even buy rolled insulation for the price the contractor will put it in for you..

As mentioned in another post - our attic gets much hotter than the outside temperature and I do have a vent fan in use. but Im not so sure how good that fan works..

thanks for the help