Radiant Barrier Savings Calcs

[OOC]

I know there is a monster radiant barrier thread here http://hvac-talk.com/vbb/showthread.php?t=112101 , I thought it best to start a new one for my specific question.

Anyone use the radiant barrier and increasing insulation savings wotksheet at the DOE ORNL site, here http://www.ornl.gov/sci/roofs+walls/radiant/rb_05.html ?

I'm not getting the calcs to predict great savings for my home that I expected. Anyone else look at those calcs? Did it persuade your decision on an RB or not?

[paul42]

Their calculations are hard to follow.

But, the more insulation you have in your attic, the less savings you will get from RB.

If your duct work is not in your attic, you will have less savings from RB.

The more efficient your AC is, the less savings you will see from RB.

The payback on the RB sheathing I put in my house appears to be more than 20 years.

[OOC]

Is that 20 years based on their calcs, on your actual utility bill savings?

[paul42]

Is that 20 years based on their calcs, on your actual utility bill savings?

Their calcs.

The house was built with RB sheathing, so I don't have any before numbers.

[OOC]

OK, thanks.

[jax1]

Paul,
Did you pay more for the radiant barrier in your new house? RB attached to roof decking at the factory should be minimal or no more than plain plywood decking.
My payback was 0 yrs (no extra charge) for RB.

[paul42]

Paul,
Did you pay more for the radiant barrier in your new house? RB attached to roof decking at the factory should be minimal or no more than plain plywood decking.
My payback was 0 yrs (no extra charge) for RB.

According to my lumber yard, the RB decking is about $5 a 4x8 sheet more expensive than the plain decking. That added about $800 to the cost of the roof decking for my house.

[Shophound]

I have spray-on radiant barrier on my roof decking, and blown-in fiberglass to R-30, both applied by Efficient Attic Systems in the DFW area. We bought the house last year, and had EAS out mid-October, so we don't have a general summer to summer comparison for cooling costs and performance. That being said, for as much glass as this house has, it is not difficult to keep comfortable when outdoor temps reach or exceed 100 degrees. Our ductwork is in the attic, so the RB is providing a capacity gain to our a/c in that heat gain to the ducts has been reduced.

Even so, I don't think I'd be seeing the good performance I now do on hot days had I not removed every single supply diffuser in the house, and sealed glaring gaps between the supply duct boot and the ceiling drywall. I also sealed the supply plenum above the A-coil, which had been leaking a considerable amount of air. It's my opinion that radiant barrier and high insulation levels in an attic are very good things, but are crippled if there is not a reasonably airtight seal between attic and house. In many homes it can be difficult to create this seal, but the payoff is worthwhile, and the cost is not high. More labor intensive than anything, mostly...unless there are a lot of recessed can lights that require careful sealing to not create a fire hazard.

Create a barrier between attic and house, install gaskets on electrical outlets and switchplates, create an insulated, sealed attic hatch/access door, you'll then have a very good handle on reducing excess air infiltration/exfiltration of your house. Then add RB and good insulation to attic and you'll then be doing much better, both from a comfort and energy cost standpoint.

[texasmax]

Hound- what did you use to seal the gaps between the supply boots and the drywall? I'm about to do the same thing (as much as from above as from below because of ceiling heights), and thinking about the foam route.

[Shophound]

Hound- what did you use to seal the gaps between the supply boots and the drywall? I'm about to do the same thing (as much as from above as from below because of ceiling heights), and thinking about the foam route.

I used duct mastic and a cheap paint brush. I was generous with the mastic, as I had some rather large gaps to fill. Be sure to let the mastic dry thoroughly before screwing the supply grills back on. Here's a picture for you, a before and after shot of a round diffuser boot sealed with mastic: