Radiant barrier Question for Nick Pine

[Astro]

Hey Nick,

Here's a question for you. I tried on another group, but noone took a stab
at answering it.

If you insulate the hot water pipes, what added advantage do you get by
adding an aluminized bubble-wrap style radiant barrier as well?

I've been trying to figure out the effectiveness of those radiant barriers
but I'm a bit confused about exactly how to handle the conduction vs.
radiation issues. So any of your wonderful mathematical treatments would
be greatly appreciated.

This goes for attic usage of radiant barriers as well. If you believe the
ads, putting radiant barrier in your attic above the insulation will do
wonders for heat gain. But does it?

Thanks for your ideas...

 

[[email protected]]

If you insulate the hot water pipes, what added advantage do you get by
adding an aluminized bubble-wrap style radiant barrier as well?

About R1 for the air space. The R-value of a radiant barrier depends on
its position and the depth of the airspace next to it and its reflectivity
(1-emittance) and the direction of heatflow and the temperature.

The ASHRAE HOF says a 3.5" deep 0.05 emittance (foil) barrier with a 90 F
mean and 10 F differential temp has R2.66 (U = 1/R = 0.376) for upward
heatflow and R3.4 (U0.294) for horizontal heatflow and R8.19 (U0.122) for
downward heatflow. The average U value is 0.272, so I'd figure the radiant
barrier adds about 1+1/0.272 = R4.68 to whatever insulation is under it.

It may not save much energy if the pipes cool off between usages. A foot
of 1/2" pipe around 0.0845 pounds of water has RC = 4.68x0.0845 = 0.396
hours, ie 24 minutes, so the water will lose most of its heat in an hour.

...If you believe the ads, putting radiant barrier in your attic above
the insulation will do wonders for heat gain. But does it?

Yes, with an air space. Facing down works best, so it doesn't get dusty.

Nick

 

[daestrom]

Hey Nick,

Here's a question for you. I tried on another group, but noone took a stab
at answering it.

If you insulate the hot water pipes, what added advantage do you get by
adding an aluminized bubble-wrap style radiant barrier as well?

I've been trying to figure out the effectiveness of those radiant barriers
but I'm a bit confused about exactly how to handle the conduction vs.
radiation issues. So any of your wonderful mathematical treatments would
be greatly appreciated.

This goes for attic usage of radiant barriers as well. If you believe the
ads, putting radiant barrier in your attic above the insulation will do
wonders for heat gain. But does it?

PMJI,

I was looking at radiant barriers recently and ran across this...

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

The sellers of these make a big point of ORNL tests showing that radiant
barriers "...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."

But they sort of skip over the next paragraph where they say, "THIS DOES NOT
MEAN THAT A 16 TO 42 PERCENT SAVINGS IN UTILITY BILLS CAN BE EXPECTED."
(their emphasis, not mine).

Read that ORNL paper front to back and you'll see they aren't nearly as good
as some marketers would like you to know.

daestrom

 

[Astro]

thanks for the reference. It's always hard to know what to believe. What
seems to make sense often turns out to be a non-cost effective solution.

Best,

 

[News]

PMJI,

I was looking at radiant barriers recently and ran across this...

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

The sellers of these make a big point of ORNL tests showing that radiant
barriers "...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."

But they sort of skip over the next paragraph where they say, "THIS DOES NOT
MEAN THAT A 16 TO 42 PERCENT SAVINGS IN UTILITY BILLS CAN BE EXPECTED."
(their emphasis, not mine).

Read that ORNL paper front to back and you'll see they aren't nearly as good
as some marketers would like you to know.

daestrom

Some makers say it is the equivalent of 200mm of rockwool in performance.
With the lack of independent evidence to support the claims I would give
this stuff a wide berth. It is nor exactly cheap either.