Comments on: Are Radiant Barriers Cost Effective in New Homes?

By: Kyle

Kyle — Sat, 01 Nov 2014 19:44:34 +0000

Very useful info. Working on
Very useful info. Working on plans now to build new home in Louisiana. Will probably add in the cost of RB since we are starting from scratch. Getting Christmas decorations out of the attic at Thanksgiving can be brutal here as it may well be 95 degrees. Energy savings for home and body should be considered this far south.

By: David Butler

David Butler — Sun, 25 Sep 2011 00:03:41 +0000

Michael: I
Michael: I'll buy that 20% may not be typical for seasonal average, for the same reason you cited for radiant barriers (night vs. day, average, vs. worst case). My comment was misleading in that sense.

My point was that conducted losses are also at play, which M.Johnson seemed to ignore by suggesting that the good leakage results of the study somehow refute the notion that attics are a bad place for HVAC.

M.Johnson: Regarding attic hvac, there's another factor at play that's often overlooked: every time the system cycles off, the air in the ducts and the mass of the AHU and duct system heat up. This thermal inertia must be overcome at the beginning of each cycle. The larger the system (relative to load), the more on-off cycles, and the more effect this has on overall efficiency.

Regarding the impact of radiant barriers on ducts, keep in mind that attic ducts are (or should be) located just above the insulation, not at the peak of the roof (upflow duct octopuses should be outlawed). So when evaluating the benefit of a RB, one must consider the temperature where the losses occur, not at the peak of the roof.

My home does not have a RB. I monitored temps at the top of the insulation last summer and found the daily peak temperature (T-max) never exceeded 25F above the day's high temperature. The T-max trendline is slightly less than 20F above the daily high.

That being said, 115F or 120F is still hot enough to cause serious efficiency losses in my hvac system.

By: Michael Blasnik

Michael Blasnik — Sat, 24 Sep 2011 22:57:19 +0000

Also, David Butler's example
Also, David Butler's example of 20% heat gains from conduction are nearly twice as large as you would expect for R-6 ducts and M Johnson's estimate of a 20F reduction in attic temperature may be in the ballpark for peak temperature but the average temperature reduction will likely be less than 5F because of a phenomenon known as night. FSEC measured about a 3F average decline.

By: Michael Blasnik

Michael Blasnik — Sat, 24 Sep 2011 22:26:11 +0000

M. Johnson - I seriously
M. Johnson - I seriously doubt that I am misinterpreting the report - I wrote it. The 86 cfm25 duct leakage is the average measured value - not some sort of estimate.

I don't understand why you think such a value isn't reasonable -- these are all Energy Star homes where duct leakage was being tested. Do you think ducts can't be made tight?

The 3% figure is reasonable from both an engineering perspective as well as being supported by the data.

By: M. Johnson

M. Johnson — Sat, 24 Sep 2011 17:53:46 +0000

@Mr. Butler: when you said
@Mr. Butler: when you said "...Let's say the delta-t across the coil is 20F and the supply air is 55F. If the supply air picks up 4 degrees by the time it reaches the room, you've lost 20% of the capacity to the attic..."

Your principle is very true. Now please consider with that the statement that RB (radiant barrier) can replace attic temperatures by up to 20 degrees. That would seriously reduce the delta-T between duct and attic air, and I wonder if you can maintain the plausibility of RB benefit only ~3% given this consideration.

P.S. Please forgive the all-bold typeface in my previous post. My intent was to only bold what was quoted, then evidently it did not turn off the way I thought it did.

By: M. Johnson

M. Johnson — Sat, 24 Sep 2011 17:43:08 +0000

@Mr. Blanik: you said
@Mr. Blanik: you said "The total duct leakage to outside averaged 86 cfm25 in the homes in the analysis."

My first impression was you were much more thorough and perceptive than I in reading this report. Now that I have read the section you reference, I must dissent.

I have not found anywhere in the report which tells us the actual measured leakage. What I did find was a section which said the 86 CFM25 was IMPLIED by the low correlation in their model.

These homes are in the Houston area where such low leakage ought to be headline news for the Building Science community. I just need to see some more data before I can believe Houston is on the high end of any part of construction quality, especially regarding duct work.

By association I hold out the possibility this report might not be based on a correct model, and when duct leakage is restated to a more correct number, the number for radiant barrier might not remain at 3% either. To me neither passes the intuitive baloney test.

By: Angela Brown

Angela Brown — Thu, 22 Sep 2011 16:43:50 +0000
This post and the comments
This post and the comments are really very informative. I learnt so many things about Radiant Barriers, thanks.

By: David Butler

David Butler — Tue, 20 Sep 2011 21:36:57 +0000
M.Johnson, even if ducts are
M.Johnson, even if ducts are as tight as these (a big IF for homes not being tested), there's still conducted heat transfer.

Let's say the delta-t across the coil is 20F and the supply air is 55F. If the supply air picks up 4 degrees by the time it reaches the room, you've lost 20% of the capacity to the attic. That's huge. And it's typical.

By: M. Johnson

M. Johnson — Tue, 20 Sep 2011 21:10:12 +0000
So does this not come close
So does this not come close to voiding the conventional wisdom that that putting ducts and equipment in conditioned space is important to efficiency?

By: Michael Blasnik

Michael Blasnik — Tue, 20 Sep 2011 21:04:10 +0000
The total duct leakage to
The total duct leakage to outside averaged 86 cfm25 in the homes in the analysis.