Let's beat a dead horse! (Attic cooling)

[Eric Larsen]

I've read most of the threads about radiant barrier insulation, powered gable vents, etc.

The reason I'm starting a new thread is I haven't read anything conclusive that allows me to make up my mind one way or the other about radiant barriers.


I live in the Sonora desert, and my attic temperature today was 160f. (The outside temp was 108f). That's entirely too hot. When we fire up our air conditioner, the winds of Hades blow out the registers for a couple minutes because all the ducts are cooking in the attic. I can SMELL the heat. It's rather unpleasant.

It's also VERY hard on our central air conditioners (it takes two to cool this place) -- we had to replace a coil today. Good thing it was under warranty, it would have been expensive.

I've got a few companies coming over to bid on a powered gable vent, thermostat controlled. I realize this probably won't save money on electricity, but I think it will probably save wear and tear on our AC system. That's reason enough for me. (I'm not getting up on a 30-foot ladder, or playing Quasimodo up in the attic. I'd rather let a licensed, INSURED company do it. Money well spent, as far as I'm concerned.)

Currently, our attic "ventilation" consists of soffit vents, and a tiny little non-powered roof vent, with a 4-inch pipe. That's woefully inadequate as far as I'm concerned.

What about more aggressive solutions?

Anyone have that blown-in radiant barrier chip system? Everything I read about it is either from companies selling the stuff, homeowners that scream "it works great" without providing any hard data confirming their praise, or people who flatly say "it doesn't work at all, a total waste of money" without providing much in the way of citations.

July and August here is miserable, just miserable. The soles of my shoes melt on the pavement. I can put a cast iron pan in my yard for an hour and fry eggs on it. My main goal is comfort more than savings, but both would be ideal.

My main fear is all that airflow up in the attic might suck conditioned air from the living area and blow it outside. That would be very bad.

Any thoughts?

[K. L. McReynolds]

The only radient barrier material with which I have experience is the reflective type. It looks like heavy aluminum foil and was installed on the ceiling rafters of the warehouse built for the local HfH chapter. It made a big difference. The roof for the warehouse was metal. I seem to remember something about the reflected heat being detrimental to shingles.

Another option, if your house has a gable roof and soffits is a ridge vent. Once installed, a properly configured and installed roof vent---in conjunction with adequate siffit vents----will use natural convection to keep the attic space cooler.

[Mike Cutler]

Eric

Each area of the country seems to have it's own unique heating and cooling requirements. So, what works for me may not work as well for you. We can get sustained periods in the 90's in Connecticut, but not 108. It's humid here too.

I installed a gable vent fan 2 summers ago, as well as re-insulated the attic. the amount of radiant heat from the attic, through the ceilings on the second floor of our house was too much for the AC units.
The gable vent fan took a lot of the stress off the AC units and they can run the house cooler now, and keep up, so I know they worked. As far as cost, well it's a small motor, I think 1/3 HP, so when it runs its like a 250 watt light bulb being on.
I don't know what a "blown-in radiant barrier chip system" is, but if it's just standard blown in attic insulation, like I have, they can keep it. It will settle and compact over time and lose it's loft, and insulation properties. At the same time I installed the gable vent fan, I installed rolled insulation over the blown in stuff. If I could have done it, I would have removed the blown in stuff. I don't like it at all.

[David G Baker]

When I lived in Northern California I installed three of those wind turbine vents evenly spaced just below the roof line. They are powered by the heat rising from your attic. I also added a thermostatically controlled whole house fan in my hall way. This fan had two 10 inch fans and an automatic insulation cover that would close the top of the unit with 3 inches of rigid foam insulation when not in use. I also installed a thermostatically controlled fan at the cool end of my attic gable that drew air into the attic in stead of venting it. I started cooling the house early in the morning while it was cool out and my wife would follow the Sun by drawing the blinds on the hot windows as the Sun moved through out the day. I added solar screens on the windows that the Sun beat on the hardest. I didn't do it but spray foam under the roof between the rafters would probably do quite a bit to keep the attic cool.
Beating the heat is not cheap and it may be cheaper to move to a more temperate climate if you have that option. One thing in your favor that I don't have in Michigan is you probably have very low humidity.

[Dave Wagner]

When we put our new roof on, I made sure they put in the ridge vents on all the peaks, the Turbines (non powered, they move just with heat and wind), and they seem to run almost constantly. I also take the window out in the gable end and put a fan to pull air in to push the hot air out. It is temp. Controlled too.

Once we start painting the house, I will drill small soffit vents in under the eves since there is none and I will have to check and pull the insulation back away from the soffits in the attic.

I do have some of the radiant barrier I bought a while ago, the heavy aluminum foil stuff with tyvex webbing in it. I just haven't put it down yet.

[Dan Mages]

I have no knowledge of their quality or performance, but I have seen solar powered roof vents. This will probably save you money not only from operations, but also from installation. Sherwin Williams offers a paint called E-Barrier that is supposed to be a termal barrier that you paint on the roof decking in the attic. They claim a 15% reduction in heating and cooling costs.

2 bits... probably not worth much, but something to think about

Dan

[Lee Schierer]

When we lived in South Carolina (Charlestown) we had a house with a dark shingled roof and felt the attic space was too hot. I purchased and installed two wind turbines just below the roof line on the back (west side) of the house and they worked wonders. As I recall the attic temperature dropped to within 10-15 degrees of the outside within a short period of time. Desipite the heavy rains we had, we never got a drop inside through the vents.

With regard to ventilation, you need as much or more inlet air opening for every square inch of exit opening. If you have a powered exhaust you need more entrance air opening that exit opening.

You may also want to consider adding more insulation around the duct work and in the ceiling itself.

[Eric Larsen]

We can get sustained periods in the 90's in Connecticut, but not 108. It's humid here too.
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I don't know what a "blown-in radiant barrier chip system" is, but if it's just standard blown in attic insulation, like I have, they can keep it.

I've found it's hard to explain to people just how hot it gets here. It's 6 a.m. as I type this, and it's 80f. That's the coolest it will be today. SWMBO and I wake up between 5 a.m. and 6 a.m. to start working on the house. We knock off before 10 a.m., when it's usually more than 100f. By 2 p.m., it's around 110f in the shade, and we live in a new development, without any shade. We haven't had a high of less than 103f for more than three weeks, and we won't have a low of less than 80f for another three weeks.

The "blown in radiant" is little cubes of radiant barrier which is layered on top the regular insulation (we have both batt and blown for that). Supposedly, the layers prevent dust from ruining the thermal properties of the barrier. I'm, well, skeptical.

When I lived in Northern California I installed three of those wind turbine vents evenly spaced just below the roof line. They are powered by the heat rising from your attic.
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I also added a thermostatically controlled whole house fan in my hall way.
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I added solar screens on the windows that the Sun beat on the hardest. I didn't do it but spray foam under the roof between the rafters would probably do quite a bit to keep the attic cool.

The HOA won't let us have those wind-powered vents. I'm against them because when it's not too hot, it's too cold. A thermostat controlled gable vent seems to be the best option for this heat.

Whole house fan? Not a chance. I don't want to replace the climate inside with the 108f dustbowl outside.

I've got the solar screens. In addition, I use 1 inch of foam insulation on the INSIDE of the windows -- and my electric bill is half that of any of my neighbors because of it. But this month, when it got to 117f, the heat buildup shattered one of the tempered windows on the west side of the house. ($150 to fix.) Spray foam under the rafters would probably cause the roof tiles to crack, like the foam insulation caused my window to crack. (Again, it's hard to explain the heat. Here's an idea. Imagine setting your oven to "warm." Put some fiberglass insulation and plywood in the oven. Let it cook for six hours. Then open the door, stick your head inside, and take a deep breath. THAT's my attic.)

I have no knowledge of their quality or performance, but I have seen solar powered roof vents. This will probably save you money not only from operations, but also from installation.

Unfortunately, I've rejected solar out of hand (and I'm a big fan of solar-powered anything). The vent will do the most good after sunset. The temperature drops from 108f to 90f in the span of 30 minutes, but the built up heat in the attic takes approximately five hours to vent with my current anemic "ventilation" system. Getting the attic down from 150f+ quickly at night is one of the main reasons for installing the powered vent.

[John Eaton]

I thought I'd add some info as I went through this recently on my own home, a friend's home and a rental property.

First some information on Roof Venting - basically you need to have either an unobstructed or a slightly positive air flow from the lowest part of the roof (near the ceiling or soffits) to the top vents where air rises to escape the attic - most either have a ridge vent (older homes) or those stack type vents. If the system is designed properly air from the lowest areas is supplied by vents in the soffits so it's pulled across the attic to escape in the vents near the peak. In older homes there might only be gable vents on either end and this allowed the hottest air to escape. During the 60's many homes were built with soffit vents to allow hot air to escape via the ridge vent. Finally those stack, spinning units became popular, and finally the powered unit.

The idea is to create a balance of unobstructive air that will naturally flow from bottom to the release vent in the top (with the theory that hot air rises) - or in worse cases some positive flow can be created using powered venting - but this only helps when there are obstructions to the existing flow.

The problem with proper venting came to a head with new home design, where the architect was more concerned with adding high ceilings (through doming, vaulting, etc) and not allowing for air movement in the attic - basically dead-air areas are formed by protrusions in the attic floor - air just sits there and get's hotter and hotter as there isn't any place to go. The powered vent was created to off-sent this by creating positive/negative pressure so air is drawn out of the attic to offset these pockets. If you have these vaults or any other vertical walls in the attic the powered vent will help a lot.

There are a couple of different problems regarding older homes - some have been retrofitted with ridge vents to work with gable vents - if the ridge vent is closer than about 2 feet from the gable vent the air just sucks in and goes right out, providing very little air movement to the rest of the attic - this can be fixed by covering part of the ridge vent closest to the gable vent. A different problem occurs when people cover their soffits with cheap vinyl - the holes in the vinyl over the existing soffit vents drastically cut down the amount of air flow and throw the attic balance off - this can be fixed to removing or cutting out the vinyl over the soffit vents so there's less obstruction.

Finally, there's an inert membrane product that can be added to the attic to reflect radiant heat - in the South (I live in ATL) during the summer about 70% of the attic heat is produced by the sun - by adding a reflective membrane most of that heat is reflected back through the roof - as mentioned this can reduce the life of the shingles, but the immediate cost savings usually offsets the cost of roofing. There are basically three different membranes available: the first is just a continuuous membrane that can be added to the roof rafters - it's stapled horizontally (comes in 4' wide rolls) and bounces the radiant heat right back out; the second looks like the first only it has small slits cut throughout and is designed to be laid right on your existing insulation (it's the easiest to apply) and stapled to the ceiling rafters - attic floor (the slits prevent moisture from building next to the insulation which can cause spore intrusion) - this helps by keeping the heat out of the house, but the attic is still hot; the last is an insulated product where small air pockets are sandwiched between two layers - this is used to insulate the attic doors or spaces where regular batts can't be applied (it's the same stuff you buy to cover the water heater).

All are good products and can cost around $3-4 per square foot if you pay someone to apply them. Or you can buy them directly from the manufacturer, if you don't mind getting dirty and very hot. This is the company I used:

http://www.radiantguard.com/

I did our rental property for less than $300 with shipping. They also sell this stuff (seen it in Lowes down in Florida) but I'm guessing it depends on where you live - I haven't seen it in my local borg stores.

One other thing I might mention - thermostat placement - if you have high bills due to the AC running all the time, and it's always freezing in the house in most rooms it could be due to the thermostat being placed in a bad area - for instance if you have really large windows and sunlight hits the thermostat during part of the day it could read that's it's hotter than it really is, forcing the thermostat to kick on when it's not needed - relocating the thermostat can fix that issue. Another common problem, for some reasone the attic access is usually in the hallway near the thermostat - if the attic door isn't insulated it can radiate heat down which will also fool the thermostat causing it to kick on all the time - insulating the attic door (use the bubble foil - you can buy smaller sheets as water heater covers and cut to fit) can help that out a lot.

Anyway, just my 2 cents from my own research and experiences. Oh and on the rental unit - it would often hit 160 degrees in the attic prior to the film - now it averages around 120 degrees or less, so there's a measurable difference, plus the AC units don't work as hard, making the tenants happer due to cheaper bills.

-- John

[David G Baker]

Eric,
Ever been in Needles California? Now that is hot! A few years back it reached 125 degrees F. Had some relatives that existed there and I would have to listen to them put me down when I complained about the heat in my area of California.

[Dennis Thornton]

Good post and advice by John.

Another thing you should consider - Ductwork in attics can cost a lot in lost energy. Leaky ductwork can be a major portion of your energy costs so you should also focus there. Make sure they're sealed and not leaking. The most common type of ductwork in newer homes is the flexible type. You put R30+ in your attic, and then run ducts that have 1" of fiberglass insulation above the R30+. No wonder when the AC kicks on you get a blast of superheated air.

Flexible ducts are now available with more insulation, but it's still nothing with a high R value. A common solution is to wrap the ductwork with a radiant barrier (the type with the bubble core works well for this) and then cover them with insulation.

A good HVAC contractor has the equipment to test the ductwork for leakage.

A properly constructed home will have multiple things done to improve efficiencies. Ductwork that is inside the conditioned space, HVAC equipment sized right (contrary to a common misconception, your AC should be running for longer periods), good sealing, reflective roofs, etc. Unfortunately many of these aren't an option once the home is built and the mortgage payments have started.

One thing to remember - radiant barriers reflect heat and insulation slows the heat flow.


Here's what the proper use of radiant barriers, insulation, and ventilation can do:

I used to live in Tucson in a SW adobe style home. I had AC. When it was hot outside I could be sitting in the living room with the AC on 70 and it wasn't comfortable. I made several changes:
1. Applied a white elastomeric coating on the roof (flat roof).
2. Added a radiant barrier to the roof rafters.
3. Added R40 to the attic (originally had R6).
4. Added a radiant barrier on top of the insulation.
5. Added a powered vent for the attic.

After that was done I could sit in the LR with the AC on 78 and it was very comfortable. Electric bill dropped a lot. The house was much more comforable in the winter too. I didn't do anything with the ductwork (fortunately it was all withing the conditioned space).

[Eric Larsen]

[QUOTE=Dennis Thornton;1188041
Flexible ducts are now available with more insulation, but it's still nothing with a high R value. A common solution is to wrap the ductwork with a radiant barrier (the type with the bubble core works well for this) and then cover them with insulation.
[/QUOTE]

The ducts are wrapped with radiant barrier, and have 4 inches of insulation around them. They've been checked for leaks.

Doesn't matter how well insulated the ducts are. If they're cooking in 160f heat for six hours, R60 isn't going to stop the air inside them from reaching 160f as well.

(We turn off the AC from 1 p.m. to 7 p.m. because we're in the power company's "time of use" plan. It saves us a small fortune on electricity. For instance, my across the street neighbor has a smaller house. He pays $500/month to keep his house at 78f. We pay $150/month, but we suffer during the day with 87f. Basically, we crank the AC to max from 10 a.m. to 1 p.m. Then we turn it off for six hours. Then we crank it back to 11. Most air conditioners only go to 10. Ours goes to 11. During the winter, we still pay 3 cents less per KwH, so our bill is $50 less than everyone. It saves us at least $2,000 per year.)

Two year old house, built by a large commercial builder.

My problems:

1) Reddish-brown tile roof -- not economical to replace it, can't paint it.

2) HVAC ductwork in the attic that looks like something out of the movie "Brazil." The attic itself has a gentle slope, so there isn't much room for me to move around. (I'd recommend being 5-foot tall to get around my attic.)

3) The sheet radiant barrier isn't a viable option because of #2. It would be nigh impossible to install. And radiant barriers don't eliminate heat, they just send it in another direction. In this case, it would be back toward the roof tile, which I don't think is wise. I would be OK with radiant barrier on the attic floor (over the insulation). But again, that should have been done during construction. That's why the "blown in" barrier cubes solution intrigues me, even though it's triple the price of sheet barrier.

4) Ventilation at the house (I'm sure I mentioned it before) consists of a good line of soffit vents, and a single 4-inch diameter pipe exhausting out the middle of the attic. No fan, no wind power, just the laws of thermodynamics moving the air.

It looks like the second from the right, except brown in color:

[Lee Schierer]

4) Ventilation at the house (I'm sure I mentioned it before) consists of a good line of soffit vents, and a single 4-inch diameter pipe exhausting out the middle of the attic. No fan, no wind power, just the laws of thermodynamics moving the air.

Now that's your problem. Convection air movement will be extremely slow through a 4" pipe. That's on .54 square feet of opening. If ther air moved at 44 feet per minute (30 mph) you would only be moving 23.7 cubic feet of air each minute. A 5' peaked roof on a 20 x 30 house will contain over 1500 cubic feet of air. That means that it would take over 1 hour for the air to be replaced once at 30 mph exit velocity. Most likely you are not getting that much air velocity, in fact it is probably down in the range of 5 feet per minute or less and your attic air isn't replaced even once per day by convection flow.

Add more vents or add power. For $600 you can add one of these (1250 scfm) and it will suck the air out of that same attic once every minute and and half.

[Dennis Thornton]

Everything I've read about concerns with roofing and radiant barriers was with asphalt shingles. If the tiles are actual tile (not a composite) they wouldn't be affected by a few more degrees they would see with a radiant barrier.

A 4" vent isn't anything near what code requires. IIRC, code requires nearly equal amounts of venting for intake (soffit vents) and exhaust (ridge/gable/etc). The only way a 4" vent would be sufficient is if it has a powerful fan connected or your attic is something like 10sqft.

My current home (CA ranch, 2 story) has approximately 4sqft of venting in the roof and another 1 sqft of gable vent in a 500sqft attic portion (soffit vents) and that still gets too hot for my liking.

maybe there's a ridge vent too?

[Eric Larsen]

A 5' peaked roof on a 20 x 30 house will contain over 1500 cubic feet of air. That means that it would take over 1 hour for the air to be replaced once at 30 mph exit velocity.

It's worse than that. The roof is more like 6.5" feet (which is why you'd want to be five feet to walk around up there). And the attic is more like 30 x 60.

This site tells me I need a fan that moves appx. 1000cfm.