Radiant Barriers in the attic

[spirilis]

well, this summer I decided to try my hand at a DIY install of AtticFoil (http://www.atticfoil.com/) over the attic floor. it's perforated so it doesn't provide a complete moisture barrier (important for attic floor installs) although I'm not sure how much that matters for me b/c my attic has boards nailed down over most of the surface, but anyhow I've done measurements with an IR thermometer to see if the stuff's just a gimmick or does anything and I gotta say, it's not bad.

a rough apples-to-apples comparison in the summertime:

5/3/2010 - pre-atticfoil install - outside temp ~92F, A/C was running at around 82% duty cycle with the thermostat set at 76F.
8/8/2010 - post-atticfoil install - outside temp ~90F, A/C was running at 58% duty cycle with the thermostat set at 76F.
both days were sunny and clear.

using the IR thermometer, I took a few example readings in the attic.

8/8/2010 - outside temp ~90F, roofing boards 130F, attic floor underneath foil is at 105F
8/21/2010 - outside temp ~77F, roofing boards 110F, attic floor underneath foil is at 86F

on the cooler side:
9/15/2010 - (7AM) outside temp ~53F, roofing boards 53F, attic floor underneath foil is at 60F
9/16/2010 - (7PM) outside temp ~65F, roofing boards 70F, attic floor underneath foil is at 75F

FYI I did try pointing the IR thermometer at the attic foil from above, usually I get the same reading as the roofing boards within +-1F or so.

Very interested to see how this changes things in colder <30F weather. I know that in really cold sub-25F weather around here a firing of my VC Defiant with 6 woodbrickfuel + 1 slat of pallet wood yields a ~12F increase in thermostat reading around 3hrs after ignition, and a fire started around 9-10PM will keep the heat pump from kicking on until ~3-4AM (thermostat set at 70F) for an average 3degF/hr drop in temperature off the peak so I'll use that as a basis for comparison.
Beyond the foil, my attic has fiberglass batts up to the ceiling joist height with boards over most of the attic except some spots. I've covered the attic floor as best as I could, overlapping the sheets where necessary.

as for cost, I bought the foil in 500sqft increments (thought I only needed 500sqft based on a rough measurement of the attic, but turns out I was wrong and there was some overlapping I didn't consider) and ended up buying 2 of them, each roll cost ~$84 shipped so I blew ~$168 on the project. the site claims it qualifies for the 30% tax credit so I'll look into that come tax time. not too pricey at all! I still have most of the second 500sqft roll left too for any fun projects I dream up involving the stuff ... lol

 

[btuser]

You could make some tin foil hats.

 

[spirilis]

sure could, but why bother--my whole house is a big tinfoil hat right now

 

[Slow1]

I'd be very interested in seeing your followup numbers here. This whole radiant barrier in the attic idea seems interesting and when I read up on it it seemed 'too good to be true' to a large degree. I wonder if it is so simple, inexpensive, and effective - why isn't it more widely accepted as standard practice?

Anyway, looking forward to hearing more results from your real-world test.

 

[spirilis]

I'd be very interested in seeing your followup numbers here. This whole radiant barrier in the attic idea seems interesting and when I read up on it it seemed 'too good to be true' to a large degree. I wonder if it is so simple, inexpensive, and effective - why isn't it more widely accepted as standard practice?

Anyway, looking forward to hearing more results from your real-world test.

That was my thought exactly. Sounds too good to be true, but then it's also fairly cheap IMO, so I figured why not. I will definitely let you all know what things look like come <30F weather

Also for all I know this may be one of those things where science preceeds conventional wisdom (actually isn't that always the case?). NASA uses radiant barrier with all kinds of stuff, the obvious one being astronaut space suits. Maybe the word is just finally getting out. But one thing that doesn't help is of the research I've found, there seems to be a lot of uncertainty about its usefulness in cold climates, although its usefulness is just about undisputed in hot climates. So we'll see.

 

[benjamin]

I wonder if it is so simple, inexpensive, and effective - why isn't it more widely accepted as standard practice?

.

Standard building practice is adequate insulation and ventilation for attics. If insulation and ventilation are missing then a radiant barrier may be the most effective solution for the money, but when you look at what energy costs these days, I think you'll realize that doing it right is cheaper in a relatively short time frame. Radiant barriers have little to offer to a house with good insulation and ventilation.

 

[btuser]

They can reflect infra-red and cosmic rays. A satellite in space with no atmosphere to protect it could benefit. Usually heat rays will travel through the air in your house and hit drywall, wood and insulation, heating them up. I don't know if a radiant barrier is going to bounce those back into the material, because heat will continue to move towards cold and by then your material has already gained. I could see it working to block heat from entering a house because depending on how you installed it you'd be bouncing the heat back into a free-flowing air space. As long as your roof is properly vented there would be a natural convection picking up the returned heat and exhausting it. I'm not sold on the application saving heat in the house. I'd think the attic is too far to edge of the thermal mass to return the heat to the living space. Maybe under the ceiling drywall.

But I'm cheering for you. I'm up for any trick that comes along.

 

[spirilis]

I believe the radiant barrier primarily shifts the balance of the system, i.e. heat still migrates from the ceiling to the top of the insulation, but now instead of radiating its heat directly onto the attic air & roofing boards its only vector of heat movement is conduction (or maybe some convection, for air that touches the foil). Conductive heat transfer across the foil (say from the middle of the attic to the edges, where it can heat up the ceiling joists and rafters at the edge) will provide heat loss but it may also prove to be a bottleneck--as heat concentrates around those edges it will raise their temperature higher than it would w/o the foil, which will have the effect of sending some of the heat back to the insulation in addition to sending some of it out of the structure (into the rafters, into the roofing boards, which radiate or convect the heat out into the atmosphere). The net effect is keeping the top of the insulation warmer than if the foil wasn't there, which should slow the rate of heat loss from the ceiling to the attic.
In addition, the website (www.atticfoil.com) has a diagram showing his own theory as to how it helps in cold weather--for those with exposed insulation he claims there is "convective looping" within the fiberglass insulation, where cold air sinks down to the bottom of the insulation and robs heat directly, rendering the insulation mostly useless. Putting the foil on top provides a bit of a windbreaker to slow or eliminate that. I don't think that would apply to my situation much because my attic has floorboards nailed down almost everywhere, so my scenario will rely exclusively on the thermal properties of the foil.

I'll let you know how it works come wintertime.

 

[spirilis]

Ok, reporting my first cool(ish) temperature reading. Good fall data:

Date: 10/17/2010
Time: 7:35AM
Outside temp: 45F according to thermometer next to house, IR temp of the deck reads 40F, IR temp of neighbor's house reads 39F
(all temperature readings via IR gun from here-)

Roofing boards: 41F on west-facing side, 37F on east-facing side

Exposed attic floor temp (a ~12x12" square that just happens to be uncovered bare floorboard): 42F

Lifted the attic foil at one spot and got a good reading of the attic floorboard ~1.5ft from the edge of the foil: 49F

Thermostat currently set to 74F in heat mode (air-source heat pump), heat runs for ~10min once an hour or more.

So yeah, seems to be having some tangible effect of keeping the top of the attic floor warmer by about 7*F at this stage.

 

[Slow1]

I don't suppose you would be willing to pull a section of the barrier out so that you can do the same attic floor temp readings on a section that does not have it? Ideal would be a section that has the same construction above/below of course - insulation drywall etc. Comparisons between the two as the seasons progress would be quite interesting to see.

 

[spirilis]

I don't suppose you would be willing to pull a section of the barrier out so that you can do the same attic floor temp readings on a section that does not have it? Ideal would be a section that has the same construction above/below of course - insulation drywall etc. Comparisons between the two as the seasons progress would be quite interesting to see.

I do sorta have that, although it's a small section, see my reading from the above post:
Exposed attic floor temp (a ~12x12” square that just happens to be uncovered bare floorboard): 42F

It's basically a part where various larger pieces come together (but none of them were cut long enough to cover it) and I was too lazy to cut a small square of foil to fit over it. I could make that section a little larger though. It is in the center of the house, just a foot or 2 away from the attic hatch opening so being in the center of the house it's a good test IMO as it should be warmer than the edges. In this reading it was only ~1F higher than the nearby roofing boards.

 

[Slow1]

Gotcha - I didn't quite understand. Thanks for the clarification.

 

[spirilis]

On another fun note, after the sun came out I went outside and tried to measure the sun's temp haha. The highest reading I could get from the IR thermometer was 355F. That was around 9AM. Pointing at the clear blue sky towards the north it read -11F. The IR thermometer is such a fun toy
The thermometer near our kitchen window was registering 65F, the wood siding near there registered 120F (sun was shining on it for a little while), deck only read 70F, the neighbor's house read 52F, grass in the shade read 48F.

Decided to take another reading in the attic. It's 10:10AM and the sun is shining brightly. Wunderground.com shows the local temp at 54F.

Attic temps:
Date: 10/17/2010
Time: 10:10AM
Outside temp: 54F according to http://www.wunderground.com/US/MD/New Market.html

Roofing boards: 61F on west-facing side, 79F on east-facing side, clear skies with bright sunlight

Exposed attic temp (no foil on this): 57F

Unexposed attic temp (covered with atticfoil): 53F

Just goes to show the effect does work both ways.

 

[spirilis]

tossing in one more data point for today:

Date: 10/17/2010
Time: 9:50PM
Conditions: Dark, very little wind, clear skies (can see the stars)

Outside temperature: 55F (wunderground.com), decking boards read 50F by IR gun, thermometer outside kitchen window reads ~58F.

Roofing boards: West-side 58F, east-side 54F

Exposed (no-atticfoil) attic floorboard: 61F

Unexposed (covered in atticfoil at least 1.5ft away from edge) attic floorboard: 67F

Thermostat reads 78F and we had the A/C on earlier set to 78 because it got warm outside (upper 70's/lower 80's). I love how cool it gets in the evening now.

 

[spirilis]

Reading on a rainy morning:

Date: 10/19/2010
Time: 7:45AM
Outside temp: 48F (wunderground.com), light rain. Deck IR temp: 47F, neighbor's house 47F, thermometer outside kitchen window reads upper 40's

Roofing boards: 54F (west), 50F (east)

Exposed attic floorboard (no atticfoil): 53F

Unexposed attic floorboard (covered with atticfoil): 57F

Thermostat reads 75F, set to 72F in heat mode, was reading 79F last night with downstairs woodstove crankin'

 

[spirilis]

ok, finally got a reading with the outside temp in the 30's.

Date: 10/30/2010
Time: 6:50AM
Outside temp: 37F (per wunderground.com), dark, clear.
Roofing boards: 35F (both)
Exposed attic floorboard (no atticfoil): 40F
Unexposed attic floorboard (covered with atticfoil): 48F
Thermostat reads 73F, set to 73F in heat mode.

 

[btuser]

Is that radiant barrier allowing moisture to escape?

 

[spirilis]

Is that radiant barrier allowing moisture to escape?

It has perforations for letting moisture escape, yes. How well that works I'm not sure. My attic already has sheets of plywood nailed down everywhere so I'm not sure how breathable it is in the first place.

 

[spirilis]

back for another data point-
11/1/2010 - Morning
Time: 6:50AM
Outside temp: Wunderground 34F, Deck IR temp 27F, neighbor's house 29F
Roofing boards: 27F (west), 27F (east)
Exposed attic floor temp (no atticfoil): 33F
Unexposed attic floor temp (with atticfoil): 41F
Thermostat reads 73F, set to 73F heat, heat pump kicks on for 5-10min once in a while.

 

[spirilis]

slightly more interesting point of data today. It seems radiative heat loss to the atmosphere is creating quite a disconnect between the outside "air temp" and the IR temp.

11/2/2010 - Morning
Time: 6:15AM
Outside temp: Wunderground 29F, Deck IR temp 20F, thermometer outside kitchen window 30F
Roofing boards: 20F (west), 18F (east)
Exposed attic floor temp: 26F
Unexposed attic floor temp: 36F
Thermostat reads 73F, set to 73F heat, running frequently.

The temperature differential between the atticfoil-protected floorboard and the exposed floorboard is a little wider than other data points here. The heat pump seems to be above 50% duty cycle (haven't actually measured it but it's definitely running more often than not...)

 

[spirilis]

ok folks, time for another one of these

11/29/2010 - Morning
Time: 5:30AM
Roofing boards: 17F (west), 15F (east)
Outside temp: Wunderground 23F, neighbor's house IR temp 17F, deck IR temp 11F
Exposed attic floor temp: 21F
Unexposed attic floor temp (protected by the atticfoil): 29F
Thermostat reads 73F, set to 73F heat. Heat pump running frequently but not continuously as I remember it doing with <25F temps last year.

General consensus so far: Seems to help in cold weather, the temperature difference isn't as profound as the summertime (where IR heat gain is more prevalent) but every little bit counts nonetheless. It's not *hurting*, at the very least, which brings me to conclude that it's a viable way for people in cooler climates to insulate their attic (after beefing up the traditional fiberglass/cellulose insulation which probably helps better with cold weather) if they are in such a climate where they need air conditioning during the summertime, when this stuff seems to help the most IMO. It's a no-brainer for folks in warmer climates including the mid-Atlantic area where I live.

 

[billb3]

Don't they put that barrier between the roof rafters to keep the shingle heat out of the attic down South ?

 

[spirilis]

Don't they put that barrier between the roof rafters to keep the shingle heat out of the attic down South ?

Yes that is one way to do it, helps with heat gain only from what I understand.
Putting it on the attic floor helps with hot and cold weather but it should be perforated to let moisture pass through.