Figuring if good is good, better must be better, I bit the bullet and opted
to install a radiant barrier in my attic.
There are two types of radiant barriers: Paint and foil. The paint is
ghastly expensive - one of the best, Radiance E025 - is a bit less than $500
for five gallons. While it can be rolled on, or applied with a brush, the
preferred method is an airless sprayer. That would be, for me, another $200
from Harbor Freight.
The other type is (a heavy) aluminum foil. The effective difference is 93%
reflectivity for the foil vs. (best case) 75% for the paint. So then, the
advantages of each are:
Paint
* Ease of installation
Foil
* Price
* Efficiency
I finally opted for the foil and got mine from www.atticfoil.com. I got
2,000 sq ft for $150 plus another $40 for sales tax and shipping. Two
thousand feet will not be sufficient for my house (3000 sq ft floor space),
but I'll be able to better estimate how much more I'll need. I plan to do
the south and west sides first. I already owned an air compressor and staple
gun, so no additional expense for another tool. I also own a manual staple
gun, but I figured the pneumatic one would be easier.
The site mentioned above has bunches of videos on installing the material
and comparisons with other products.
I'm not particularly looking forward to getting up before the sun this
weekend, but I am excited about the savings in air conditioning costs.
I'll report back next week on the completion of the project.
Addendum:
If you HAVE to work in a hot attic, here are a couple of tips to make the
environment a tad more comfortable:
* Put a lawn sprinkler on the roof to wash away all the icky heat.
* Disconnect one of the attic a/c ducts and let it TRY to cool the attic.
Don't forget to leave the stairway open so the air can circulate.
That's a good idea. I have to go up there soon to install a celling lamp
line. I have to battle the insulation.
Not clear where the op is installing the reflective foil. I stapled it to
the joists in my garage. Works well. I had bought perforated foil from
insulation 4 less.
Greg
Interesting. I'm waiting for HB to report how well his works. I might just
do that next winter, in addition to insulating the basement walls (at least
the studded out part).
I'd be very interested to hear your test results.
Plot the attic temperature over the course of a hot/sunny/windless day.
Install the stuff.
Plot the attic temperature over the course of an equivalent
hot/sunny/windless day.
Post the results.
Heat flows from hot to cold.
The sun is VERY hot.
So, your roof absorbs radiant energy from the sun. Unchecked,
the roof will eventually reach the temperature of the sun reduced
by the attenuation of the atmosphere.
The good news is that the roof also radiates back into space and
conducts into the air both inside and outside the attic.
Space is very cold. At some temperature, fortunately for us,
very much colder than the sun, the absorption from the sun
is exactly balanced by the conduction through the roof plus
the re-radiation back into space plus conduction to the outside air.
If you put something in the way, like a tree, you can significantly
reduce the absorption radiant energy from the sun.
If you paint the roof white, that equilibrium temperature gets lower.
Silver plating the roof would be even better if you keep it smooth.
The problem is that different light frequencies pass through different
materials differently. How's that for ambiguous?
The INSIDE surface of the roof is heated mostly by conduction.
It radiates way down in the infrared region, so that radiation
from the inside surface
can't get back out thru the roof easily. Heat flow is
by conduction to the air. I didn't say "convection" because I
think it confuses the issue.
Park your car in the sun with the windows up. It gets very hot inside.
The visible radiation from the sun passes thru the windows.
AS it heats up the seats, it gets re-radiated way down in the infrared.
Problem is that infrared can't get back out thru the glass.
(Visible reflections do get back out.)
The result is that the inside heats up until the conduction thru
the glass and metal matches the radiant energy absorbed by the sun.
Putting a reflective shade inside the window helps because you're
re-radiating
frequencies that can get thru the glass. If you replace the windows
with something opaque, like shingles and wood, the reflector wouldn't
help much.
About the only option you have is to prevent the inside air molecules
from banging into the hot inside of the roof. The only thing that helps
that is the insulating value of whatever you put there. Aluminum foil
won't do much, cause the inside heats up to the outside pointing surface
temp and you're back where you started.
So, put insulation on the inside surface of the attic. That will reduce
the inside air temperature while raising the roof temperature. Think a
little
about how that affects the life of the roof.
Everything I've read suggests that insulating paint is "snake oil".
My thermo book suggests that putting a radiant barrier where there's
no radiation at frequencies that can pass thru the exit medium does nothing
more than the "R" value of the barrier.
Adding roof insulation can reduce attic temperatures if you live
in a region that doesn't get enough sun to melt your roof.
I'd love to see test results that prove me wrong. I'd install some in
a heartbeat.
It's not 'snake oil,' but it is probably not as good as is claimed.
The 'R' value of aluminum foil is virtually zero. A radiant barrier is NOT
an insulator. It reflects the radiant heat from the shingles and decking
back up.
Go to www.atticfoil.com - they've got lots of test results. Alternatively,
check the Wikipedia entry, which reads, in part,
"Radiant solar energy strikes the roof, heating the shingles, felt paper and
roof sheathing by conduction, and causing the underside of the sheathing and
the roof framing to radiate heat downward through the attic toward the attic
floor. When a radiant barrier is placed between the roofing material and the
insulation on the attic floor, much of the heat radiated from the hot roof
is reflected back toward the roof and the low emissivity of the underside of
the radiant barrier means very little radiant heat is emitted downwards.
This makes the top surface of the insulation cooler than it would have been
without a radiant barrier and thus reduces the amount of heat that moves
through the insulation into the rooms below the ceiling."
And
"According to a study by the Building Envelope Research Program of the Oak
Ridge National Laboratory,[13] homes with air-conditioning duct work in the
attic in the hottest climate zones, e.g., the deep south of the U.S., could
benefit the most from radiant barrier interventions, with annual utility
bill savings up to $150; whereas homes in milder climates, e.g., Baltimore,
could see savings about half those of their southern neighbors. On the other
hand, if there are no ducts or air handlers in the attic, the annual savings
could be even much less, from about $20 in Miami to $5 in Baltimore.
Nevertheless, a radiant barrier may still help to improve comfort and to
reduce the peak air-conditioning load."
Before I did my garage, I would go in and just feel the heat from the
ceiling. After, it was wonderful. Sure, the garage heats up during the day.
I also painted south brick wall yellow vs the old dark red. Would be nice
if I could push a button for winter.
Greg
back up to where? That radiation ain't going back up thru the wood or
asphalt. No, it didn't come thru them in the first place. It was
conducted.
All the reflector can do is heat up the inside surface of the roof.
yep, that's the radiation. if you're making a living space in your attic,
you might benefit from reducing the radiant component in summer.
The radiant energy you feel from your bedroom ceiling
is NOT the sun's radiation. It's the radiation from the inside
surface of your ceiling based on its temperature. The path from the roof
surface to your ceiling is mostly conduction...and its sister
convection plus a little radiation and some more conduction thru
the insulation and sheetrock.
After, it was wonderful. Sure, the garage heats up during the day.
Exactly.
The experiment isn't hard. Stand in front of a radiant electric heater.
Feel the burn. Now, hang a slab of sheetrock between you and the
heater. Got colder
didn't it. But, if you measure the air temperature, you made zero
difference
in the heat coming into the room.
Wait long enough and the sheetrock will absorb energy, heat up and you'll
be able to feel its radiation.
You can if the button is on the top of a paint spray can. ;-)
Or if the button is the one that lowers the white shade over your black
wall.
The primary difference here is that you're radiating energy back into
a medium that can propagate that frequency radiation: free space. Put
the shade
on the inside and you're not reducing the energy coming into the space.
Well, the inside temperature of the brick will rise if you don't let
it radiate, but since the sun is very hot, the outside brick surface
will heat up too. Total BTU/HR thru the brick won't change much.
Not a button, but a shade tree that loses its leaves in winter can be
helpful.
Follow along:
1. Heat from the sun heats the roof shingles via radiation.
2. Head from the shingles heats the felt and decking via conduction (mostly)
3. Heat from the decking heats the air in the attic via radiation
4. Heat from the attic heats the insulation/ceiling via conduction
5. Heat from the ceiling heats the house
Now insert the following:
2a. Radiated heat from the decking is reflected back to the decking via a
radiant barrier
Yep. The heat that was destined for the attic now goes back up to the inside
surface of the roof.
Heat is transferred in one of three ways:
1. Conduction (hot-cold items must be touching each other)
2. Convection (an intermediate such as air transfers the heat from one
surface to another)
3. Radiation.
Instead of hanging a bit of sheetrock in front of the heater, hang a mirror.
Your hand behind the mirror will never get hot, all the radiant heat is
reflected back to the heater. That's the principle behind a radiant barrier.
Of course there has to be some way for this heat to escape, else you might
experience some kind of meltdown. Proper installation techniques of radiant
barrier foil accounts for this requirement.
Aside: Sheetrock is a piss-poor insulator. It has an R-value of 0.45.
I did an experimental wall in my basement. Up against cinderblock wall I
put reflective foam of 1/4 inch. 3 inch studs were then put up. On top of
the studs I added another 1/4 inch foam layer. That's another wall I could
feel the heat at the end of a sunny day. Overall I'm surprised how well it
works overall, and winter.
The polyethylene foam meets fire code, no drywall required. R 3.5 of the
cinderblock and who knows how much additional insulation factor, between
1/2 foam, no IR pass air space.
Greg
I'm sure they do.
I'm much more interested in YOUR test results.
Alternatively,
I think you can make a thermodynamic argument for this, but, if you have
the recommended amount of insulation, I think it's a small difference.
Say that radiant energy heats the top surface of your insulation.
The R-value of the insulation between that top surface and the ceiling
of your bedroom is, maybe R-30.
The R-value of the insulation between the top surface of the insulation
and the air in the attic is R-0. Where is most of the heat flow gonna
go? The insulation also radiates back toward the roof.
I think you'd be better off if you put the radiant barrier on top of the
insulation and installed a fan to vent the attic.
and thus reduces the amount of heat that moves
I'm having a great deal of trouble with the concept of impeding radiation
thru something that doesn't pass the radiation.
Put a mirror on your roof and it's easy to understand the value of
the mirror in reflecting energy back into space. Cover the mirror with
asphalt and wood and I'm not convinced that the mirror does much any more.
Maybe somebody with mad physics skills will chime in here.
$5 to $20 annual savings from a $190 + labor investment to fixup part of
your house fits well within my definition of "not much".
I like the wording, "nevertheless...may still help...improve comfort..."
Kinda like reading the EPA sticker on a car and believing you'll average
40MPG.
I hope you are successful and desire to hear about the test results.
I got a buddy that's a roofer and also does radiant barrier.
The kind he uses, he just lays in over the existing insulation, taping
the seams with foil tape.
He also wraps all the duct work with it. You might consider that
option, too.
He did it to his house and said he could feel a noticable difference in
the temp of the air vents.
Yes, putting the foil on the joists IS an acceptable way of installation.
Wrapping the a/c ducts is a good idea, but my a/c ductwork already HAS
aluminum surface (or at least silver colored).
To respond to an earlier criticism of the project, why would the duct
manufacturers even MAKE ductwork with aluminum-foil coverings if it didn't
provide some benefit?
the key word is "some" benefit.
My buddy wrapped the longest supply run, he said after doing so, the
20-30 seconds of warm air before the cold air started reduce
signifgantly(YMMV)
He also reroofed his house and added the barrier under the shingles, he
gets his from here http://www.energyq.com/index.php at about $34 per
100sq ft
I assume you are trying to reduce your a/c bills in the house and not
trying to help the attic as your main reason for doing this? If so,
just curious why you chose this way instead of more insulation for the
ceiling? Is this cheaper than the insulation? I would think tho
it's more labor intensive but I'm no expert in this stuff.
Good question.
I've already got R-33 fiberglass insulation on the attic side of the
ceiling. I've got a (smallish) ridge vent, four turbines, and an immense
amount of fascia vents.
Maybe I'm trying to put lipstick on a corpse, maybe things are as good as
they'll get. Still, for $250, it seemed like a worthwhile gamble.
I figure if the result makes no difference, I can take down the foil and use
it to wrap Christmas presents. For the next couple of decades. Plus, there's
always baked potatoes...
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.