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Foil (on rafters) vs. spray on radiant barrier for retrofit application


bigboy's Avatar
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Join Date: Mar 2005
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TX

10-12-12, 05:01 PM   #1  
Foil (on rafters) vs. spray on radiant barrier for retrofit application

Seeking some wisdom:

I have done a lot of reading on the Internet (therefore - know enough to be dangerous LOL) and it seems clear that foil is generally a better material for radiant barrier application.

I am talking to a contractor though that made me wonder about it. Basically, the reason he is recommending spray-on is as follows:

- the radiant barrier should really prevent the heat from entering the attic; if foil is installed on rafters, there is 4-6 inches of space between the foil and the roof. Hence, the heat has already entered the attic, and wind turbines and soffits will not vent it efficiently enough
- there is no question that foil is "better" but not necessarily for the retrofit application; in that case, the values on the paper are not "real life" values, and the spray-on is a more effective way to go (as the reflectivity of the spray-on would supposedly be better than of foil installed on rafters)
- while his company does both, he is therefore suggesting that I go with HeatBloc Ultra

About the home: it is a large (~2500 sqf) single story, with cavernous attic space. I mean cavernous. I could play basketball up there. Built in 2002. I also plan to add a few inches of insulation at the same time. Located in DFW, Texas. We just moved into the home about 2 months ago and plan to stick around, so I figure it is money well spent.

Does what he says make sense? Somehow - it does to me but again - I am not an expert, so I am asking some.

Thanks!

 
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Bud9051's Avatar
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ME

10-12-12, 06:08 PM   #2  
One of the questions I have for the spray on radiant barriers is, what is their emissivity? In other words, how good of a radiant barrier are they? Did he give you any numbers?

Now, where is the heat going? The sun hits the roof, shingles/sheathing get hot, then the RB gets hot but doesn't radiate into the attic, so where does the heat go? The answer is that the shingles and the sheathing have to get hotter, ie the heat builds up. And that is how it dissipates some of the heat, increased radiation back into the air and increased convection across the shingles. But the sprayed on RB does get hot, same as the sheathing, and that heat is also convected into the attic.

Add a RB to the bottom of the rafters, reflective on both sides, and the radiant energy from the sheathing will be reflected back to the sheathing. The sheathing will dissipate the additional heat to the outside as described above and will resist radiating it across the attic as desired.

Both approaches will still allow considerable heat to enter the attic, so good ventilation is a must. But either approach will work fine. Note i prefer the double sided RB over the single sided.

Price the RB, add two guys for one day in the attic and you should get $400 plus materials, say $0.15 per sq ft. I'm just guessing at these numbers, but a 2,500 sq ft ranch would require say 4,000 sq ft of RB, so you coming in with a $1,000 charge, very roughly. Now you pay the salesman and that is why he likes the spray on, because you can't price it out and he can quote you $4,000 for the job.

Forgive me if I have exaggerated, as we don't use RB's way up north, but tell us how far off I am.

I do like RB's and can even find places where they will work in cold country, so adding one where you are is a definite, just don't pay too much. Either will improve your attic heat, but if the price is too high, better to put it into extra insulation and air sealing.

Bud

 
bigboy's Avatar
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10-12-12, 06:57 PM   #3  
Thank you, Sir... you bring up some good points.

The spray-on product that I mentioned (HeatBloc Ultra) has emissivity of .19 (their site, 3rd party site). So you'd say that this is over 80% effective at blocking heat. While not as high as foil-types of products, I was wondering how installation impacts it all, as that is something I did not consider originally. It made sense to me that the installation method would have an impact. Especially because my attic is so tall that they'd create a "false ceiling" up there out of foil, at about 8 feet height (with holes for ventilation of course).

I have received some quotes on foil type also and we are looking at $2.5K as opposed to $1.3K for HeatBloc installation. The effectiveness (in best case scenario) of foil is ~95% or so, but I am not sure if that difference in price is worth it.

Finally - I realize this is somewhat outside of "do it yourself" question. I looked at my attic long and hard and have decided that due to layout, amount of air ducting I have up there (house has 5 air returns, just so you get an idea) - I am going to pay someone else to git'er done. Plenty of other do it yourself projects left for me though.

 
Nashkat1's Avatar
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10-12-12, 08:40 PM   #4  
How about this: How about applying the foil RB across the attic face of the rafters and providing a passage for some of the intake air to enter the rafter bays from the soffit. With a ridge vent for exhaust, it seems that the reflected heat would be vented up and out, along with any heat built up below the RB. This should keep the attic itself cool as well as dry (the primary purpose of attic ventilation).

 
Bud9051's Avatar
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10-13-12, 05:08 AM   #5  
I'm not sure the false ceiling concept is a good idea. You either need to ventilate the attic or encapsulate it with no venting, but that is a lot more complicated than a layer of RB. Nashkat's thinking of installing the RB all the way up, but allowing ventilation to flow above and below the it would be better.

I keep forgetting you southern folks like to install your HVAC in the attic. In most cases, the ducts were lightly insulated but never sealed. Air leakage into or out of those ducts is a major issue and worth looking into.

An emissivity of 0.19 does not mean it is blocking 81% of the heat transfer, although they like to imply that. Everything above absolute zero radiates energy, that means everything is receiving as well as emitting energy. The heat transfer is the difference between what an object radiates and what it receives. At 20% emissivity, there is more energy being transmitted across that attic than convected. Granted it is less than no RB, but it is still trying to equalize all temperatures inside that attic and radiant energy works rather fast.

If you have been reading, then you have no doubt run into the importance of sealing and insulating that system up there. Your payback for doing that would be much better.

When they build a house with the RB already on the bottom of the sheathing, it is an inexpensive way to reduce energy costs a little. To retrofit a RB or spray on a partial RB is more expensive and provides less results. At least those prices aren't over the top, IMO, from way up north.

Bud

 
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