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-   -   Radiant barier DIY in existing house? (http://www.diychatroom.com/f9/radiant-barier-diy-existing-house-15458/)

JDuc 01-10-2008 09:37 PM

Radiant barier DIY in existing house?
 
If we had built it, we would have had the radiant barrier installed from the get go, but we didn't and it didn't get done since we bought the house as a foreclosure.

Being a pretty hard core DIYer, I want to know if it's possible to apply something like this on our own. Whether it's the spray on old construction, or using the new construction and securing it to the bottom side of the rafters.

I don't even know if that would give the same benefit...seems it would since it would create a void, but maybe not?

Thoughts? Options? Wanting to save some money but really would like this done before the summer heat (100+) comes back.

Ed the Roofer 01-10-2008 10:30 PM

This topic is not up my alley from a personal experience standpoint, but try out this link for starters.

http://rehabadvisor.pathnet.org/sp.asp?id=10292

Then try Google for the term; Radiant Barriers for Retrofit

Ed

JDuc 01-11-2008 08:31 AM

ok- so it looks like I was right to some extent:

Quote:

Preferably for retrofit construction, the radiant barrier is attached to either the faces or bottoms of the rafters or top chords of the roof trusses. In both configurations the space between the roof sheathing and the radiant barrier provides a channel through which warm air can move freely. If the radiant barrier is reflective on only one side, the reflective side should face toward the main attic space, since a surface facing downwards is less likely to have dust settle on it.
Looks like we're looking at a cooling cost savings of between 2% - 10% since we have R-19 in the attic.

Making me wonder if it's worth it now...

Guess I'd need to see how much it would cost to do our attic and compare that to the savings over 10 years or so.

jogr 01-11-2008 09:32 AM

JDUC, read the DOE report again. It says that theoretically you can save 16-42% of the ceiling losses. But it says ceiling losses are 15-25% of the house losses. Therefore, you could theoretically save 16-42% of 15-25% for a grand total of 2.4- 10.5% at maximum.

It also refers to a study that showed no actual gain in real buildings.

I'm not saying radiant barriers don't work (I am actually very interested in them) but before I lay out the cash I want to see some real good studies showing and quantifying real savings.

JDuc 01-11-2008 09:45 AM

Doh! You are right....that was a pretty confusing way to post some results.

if you find some results, would you mind posting them?

JDuc 01-11-2008 09:51 AM

Hrmmm - according to this page:

http://www.ornl.gov/sci/roofs+walls/radiant/rb_02.html

Quote:

The tests to date have shown that in attics with R-19 insulation, radiant barriers can reduce summer ceiling heat gains by about 16 to 42 percent compared to an attic with the same insulation level and no radiant barrier. These figures are for the average reduction in heat flow through the insulation path. They do not include effects of heat flow through the framing members.
Quote:

THIS DOES NOT MEAN THAT A 16 TO 42 PERCENT SAVINGS IN UTILITY BILLS CAN BE EXPECTED. Since the ceiling heat gains represent about 15 to 25 percent of the total cooling load on the house, a radiant barrier would be expected to reduce the space cooling portion of summer utility bills by less than 15 to 25 percent. Multiplying this percentage (15 to 25 percent) by the percentage reduction in ceiling heat flow (16 to 42 percent) would result in a 2 to 10 percent reduction in the cooling portion of summer utility bills. However, under some conditions, the percentage reduction of the cooling portion of summer utility bills may be larger, perhaps as large as 17 percent. The percentage reduction in total summer utility bills, which also include costs for operating appliances, water heaters, etc., would be smaller.
Much easier to read.:thumbup:

I'm going to go back and edit my initial comment so I don't confuse someone else. Thanks for pointing it out to me.

JDuc 01-11-2008 10:11 PM

Ok - so just went into the attic....we have 1900 sq. ft. of R30 and have air conditioning in ducts in the attic....using the calculations we would need to spend less than $209 on this...I don't think that's going to happen.

jogr - you might want to look at this page: http://www.ornl.gov/sci/roofs+walls/radiant/rb_02.html

And follow the calculations they give. It's a super easy way to determine if it's worth it or not.

For us it's not, for my mom it's not, for my husbands parents, it is. Might be a good present for his retired folks.


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