We live in the central plains area and have been experiencing many days this summer where the temperature is 100 degrees and greater. Our attic gets so hot that it sets off the fire alarm (the alarm folks fixed the problem by installing a higher temperature alarm). We have known for some time that the venting in the attic is inadequate. There are no gables where a gable vent would be appropriate, so we will probably have to put in a ridge vent. Insulation is blown and not very thick. After researching the problem, I have determined that to solve the hot attic problem and improve our cooling bills, we can (1) add insulation in the form of blankets perpendicular to the floor beams, (2) staple a radiant barrier to the bottom of the roof rafters, or (3) put in a ridge vent along two of the main ridges.
Eventually I think that we will have to do all three, but right now I only have the time and money to do one of the three alternatives. My question is this: given the choice, which alternative is the most cost effective in terms of lowering attic temperature and electric bills?
Replies
We have a vent fan mounted through the sheathing & shingles, between
rafters. Works nicely. I think they're the best way to remove attic heat. Put 'em on a in-attic thermostat & forget it...
-- Brooks
The additional insulation will not make the attic any cooler, but out of the three you mentioned, it is the most cost effective way to lower your electric bills.
Other cost effective solutions.
If you have unshaded single pane windows facing west - add reflective window film and or sun screens.
Replace all filament bulbs with compact flourescents.
If your refrigerator is more than 10 years old, the payback on a new one can be as short as 5 years.
Web Law Man,
A hot attic is not necessarily a problem. If your insulation is adequate, and if your ceiling plane is air sealed (no ceiling leaks), and if you have no ductwork or HVAC equipment in your attic -- and you shouldn't -- then the temperature of your attic is irrelevant. Many studies have shown that powered attic ventilators (attic fans) use more electricity than is saved in air conditioning bills. Moreover, powered attic ventilators can suck conditioned air into the attic through air leaks. (I know you aren't considering installing an attic fan, but another poster suggested it.)
What is your problem? High cooling bills? Then be sure you have no air leaks between you house and your attic, and install more insulation on your attic floor. If you have any ductwork up there, that would need to be addressed first, of course. Remember, high cooling bills have many causes -- is your cooling equipment efficient? Do your east and west windows have a low solar heat gain coefficient?
MartinHolladay,Some more information. We would like to use some kind of passive cooling before we go for such things as powered vents, etc. But the thought of making sure that the ceiling plane is sealed is a good one. The only problem is: how do you get through all that loose pack, blown insulation to check for leaks? I know that our HVAC system has a plenum in the crawlspace for distributing air to the rooms, and the returns are all in the attic. So reducing attic temperature would seem to be desirable. One thing I had not thought of until now is to check the attic ductwork for leaks, but again, the ducts are wrapped with their own reflective insulation so if there is a leak in the ducts, it wouldn't be found easily. I don't believe that the heating/cooling unit is sealed on the top side from the attic space, so I need to check that.Sometimes I think we can feel the heat radiating through the ceiling, but that may just be something subjective. Does anyone know if radiant barriers are more or less effective than blanket insulation?
FWIW, I built a house in central Florida and installed a radiant barrier (FiFoil) which was manufactured locally. The difference in attic temps was like night and day. The problem you may encounter is accessibility. Mine was new construction and gable ended.
The story about cooking the shingles may be true but no one has proved it yet. Hurricanes got mine before the heat anyway. The story goes any amount of radiant barrier is good, but if done correctly it provides a path for the air to rise from soffit to ridge.
The ridge vent (or some kind of higher vent) is important, followed by a good insulation barrier above a sealed ceiling plane, as others have mentioned.
A ridge vent will not help much unless you have soffit vents and free air flow between one and the other.
Radiant foil on bottom of the rafters wil help with the immediate problem but could shorten the life of the shingles. Also, if the plywood sheathing is from mid seventies and fireproofed, the heat will accelerate the aging of it.
blowing in cellulose or chopped fiberglas on top of what you have will help keep the heat out of the house and will pay for itself in heating savings come winter.
What you need is a combination of more insulation and a complete ventilation system. Adding ridge vent only can sometimes create other problems in winter.
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Piffin,Thanks for your input. There seem to be plenty of soffit vents, but for some reason the original builders didn't provide a way for the air to flow out again. The house is actually two 1920's bungalows that were carted out to site to a raised foundation, and then a new addition was tacked onto them. The original roof sheathing looks to me like 1 x 12 planks, but the newer part of the house has plywood. There might be a small gable vent in one or the original bungalow gables, but I'm not sure.I have read a number of pros and cons with radiant barriers with respect to the longevity of the shingles and/or sheathing. Would you know of any horror stories along those lines?
Why not put the radiant barrier on top of the blown in insulation?
Reflects attic heat into the ventilated attic... Reflects heat back into the home.
In order for radiant barriers to perform effectively, there must be at least an inch of free air space in front of the surface, so it is doubtfull that it would reflect much heat back into the house. Further, it would act as a vapour barrier ON TOP of the other insulation and trap moisture there to feed mold and render that insulation ineffective. Not a good idea.
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Piffin,
Have you had any experience with sealing the space and foaming the rafters in retrofit cases?
I use foam any time the customer can afford it
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What I was really asking was your experience in dealing with redoing existing attics where under eave spaces are harder to get at.
maybe my head is tired. I'm not sure what the question is.
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There are some radiant barriers that are permable so that is not a problem, but it needs to be checked out.And the radiant barrier will "reflect heat back" indirectly.It is also a low emissive surface so that it warm up instead of radiating the house heat out towards the cold roof at night.That is the same reason that radiant barriers work on the bottom of the roof with the foil side down. Because it is also a low emssive (non radiating) surface.The big problem with radiant barrier on the attic floor and pointing up is that it will quickly be covered with dust and lose it's radiant properties.
No horror stories.There are testing studies showing that shingles kept hotter dry out the volatile oils faster. These primarily over SIPS with insulation behind them. they find that the life span is reduced but by such a small percentage that it is pretty much negligible. But when you use a radiant barrier, it is reflecting the heat back at them. Somehow, my mind imagines that adds more heat to the shingle than just keeping them hot with foam ins. behind.Maybe that isn't so. I don't know. If you were careful to run it tight and continuopus, it would theoreticly provide a channel for the airflow and that would give you increased velocity for cooling with a ridge vent. And reflect heat away from the interior at the same time.I just haven't seen anything that demonstrates that radiant barriers are cost effective.
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We used to have a cedar shake roof, and the attic was fairly cool because of the fact that air can easily pass thru shakes (over skip sheating). Since we stripped the cedar and reroofed with new plywood and asphalt shingles the attic is much hotter. We have four gables and almost no soffit for venting, so I'm going to throw in one of these:
http://www.solatube.com/res_roofmount.php
That is a pretty sharp item...wonder if the motor will last longer than the conventional fans. Not having to run electricity resolves some issues, but the ones I have installed have motor failure before I can recover my cost!
I grew up in central Kansas. Temp -10 to 112. Not every year, but often enough. Many houses used turbine vents.
Now I'm north of Seattle and have installed turbine vents on three of our houses. Soffit and gable vents for air flow keeps the attics noticeably cooler.
The 14" turbine cost about $40 at the local Home Depot. Three vent an approx. 2500 sq. ft. attic. Best of all they are wind not electrially powered. If installed properly(color & position) they are not obvious from the street.
The wind turbines DO NOT power the air out of the attic. If you remove the spinning hood, there will be more air coming out. The turbine spins because of the hot air rushing out of the attic. The turbine reduces the airflow.
To increase the flow of hot air out of an attic, just place a "Chimney" stack over the opening at the highest portion of the attic. A thermal siphon is what I call it.
A long time ago when Macintosh computers were air cooled, there was a chimney hat that was place on top of the computer to help draw more air thru it, it had no electrical hookup.
Hi,
"I have determined that to solve the hot attic problem and improve our cooling bills, we can (1) add insulation in the form of blankets perpendicular to the floor beams, (2) staple a radiant barrier to the bottom of the roof rafters, or (3) put in a ridge vent along two of the main ridges."
I think insulation would likely give you the best payback of the three. I'd just blow more cellulose over the existing insulation. It will work better and be cheaper than adding FG bats.
You mentioned getting access to the attic floor for sealing through the blown in insulation that is already there. I just used a small rake -- about 8 inches wide with a 3 ft handle. Find where the walls come up, push the insulation aside with the rake, seal up all the wiring and plumbing penetrations with the foam in a can. Do the same for vent fans, lighting fixtures, ... Its messy, but not difficult.
Obviously, do this before you add more insulation -- wear a good mask.
Good free sealing guide here:
http://www.builditsolar.com/Projects/Conservation/conservation.htm#Insulating
Be careful about not insulating right over can lights that are not rated for direct insultion contact.
Sealing duct joints with duct mastic would probably be very worthwhile. This article says duct that 30% loss from ducts is "typical":
http://oikos.com/esb/28/duct_losses.html
Laying the radiant barrier over the existing insulation does not work well because it gets covered with dust and loses its reflectivity. http://www.SouthFace.org has some good material on radiant barriers. They seem to point to a potential 5 to 10% saving (when applied to the rafters).
I'd agree with the don't use an attic fan advice, just increase the passive ventilation when you can.
The FSEC has found more reflective roof surfaces to save around 20% on the cooling bill.
http://www.builditsolar.com/Projects/Cooling/passive_cooling.htm#Roof
Gary
GaryGary,You make some good points. I think that the first thing to do would be to ensure that all penetrations into the attic, especially around the waste stacks and electrical wires through the top wall plates, should be sealed with foam. Also, since we have a loft in the attic area, I think I should take the advice provided in the Oikos article and make sure that that envelope is well sealed. I hate to get down into the blown insulation and grub around, but I think it would be well worth it.Finally, I'll have to check the duct work in the house. The return ducts run through the attic and the supply ducts run through the crawl space. Other than by visual inspection, I don't know how to determine whether there are leaks or not. For example, there could be two runs of ductwork that are not connected, but short of ripping off the duct insulation and checking things, I don't see how you could detect it.These are cheap, labor-intensive things that I need to do before spending any big bucks on insulation. One thing though. If you have plywood for a partial attic floor, as most attics have, the plywood is laid right down on the joists, so that the insulation underneath is going to be as thick as it gets. Does one just add insulation around the floored area and accept the fact that the floor area is going to be less efficient insulation-wise?
To test the return ducts, momentarily place plastic bags over the return vents while the fan is running. If air still comes out of the supply ducts the return ducts leak.
WebLawMan,
Out of your three original choices, I think the radiant barrier stapled to the rafters will be the most effective at reducing heat in the space below. Most of the heat transfer from the roof plane to the attic is radiant. Fiberglass batts will do very little for you because fiberglass is transparent to infrared radiation. The energy just passes through it and heats up you ceiling plaster/drywall.
The heat you feel from your ceilings is not your imagination. It can be quantified by a non-contact temperature sensor. Cellulose is opaque to infrared, so it is a much better choice than fiberglass, especially when loose-blown on an attic floor. If you use cellulose, you can dispense with the stapled-up reflective barrier.
Cellulose also helps to suppress convection, which minimizes the losses from ceiling plane leaks. In any case, it isn't the air leaks that are making you hot in summer; it's the radiant effect.
I agree with others who say that a hot attic is not such a big deal of itself. It is the hot living space we want to avoid. Where I live, a hot attic is cheap insurance against airborne termites. Some passive vents and a barrier to passage of radiant heat are all you need, IMO.
Bill
Bill,
Your statement, "Fiberglass batts will do very little for you because fiberglass is transparent to infrared radiation. The energy just passes through it and heats up you ceiling plaster/drywall," is untrue. Although similar statments are frequently stated by distributors of radiant products, there is no basis to them. Moreover, the concept is easily disproved. The next time you are standing in front of a wood stove cranking out heat, hold an unfaced 8-inch fiberglass batt in front of your face. Do you feel the same amount of heat as you did with no fiberglass batt? Nope -- the fiberglass batt has stopped most of the radiant heat that used to be warming your skin.
--"The next time you are standing in front of a wood stove cranking out heat, hold an unfaced 8-inch fiberglass batt in front of your face. Do you feel the same amount of heat as you did with no fiberglass batt? Nope -- the fiberglass batt has stopped most of the radiant heat that used to be warming your skin."---
Try this experiment, after feeling no warmth on your skin, wait an hour, now you'll feel warmth. Turn off the source of heat, an hour later a thick fiberglass batt will still be radiating heat.
After the sun has gone down, a fully insulated attic will radiate heat from the insulation into the living space for hours
After the sun has gone down, a fully insulated attic will radiate heat from the insulation into the living space for hours
…and this is why I like powered attic vent fans. Mine keeps running long into the evening even though the outside temp was far below the temp I’ve set the thermostat for the fan at. (There is adequate soffit & gable vents for air flow.) I don’t understand why so many folks look down on the powered fans. (other than extra wiring and occasional motor failure which cn be a major PIA)
Forced attic ventilation is a commonly encouraged technique to reduce residential heat gains from the ceiling. However, even those who are in favor of increased attic ventilation have often warned that the energy consumption associated with the attic fan motor is likely greater than any realized energy savings from its use (Wolfert and Hinrichs, 1974). Also, an early detailed study showed that while forced attic ventilation did reduce cooling energy use, the reduction was quite small and outweighed by the energy consumption of the fan itself (Dutt and Harrje, 1979). Another study in two instrumented side-by-side homes in Texas came to similar conclusions (Burch and Treado, 1979). Forced ventilation was found to reduce ceiling heat gain by 1.1 Btu/hr/ft2 (328 W) over soffit venting and gains to the attic duct system by 94 W.(2) At a normal air conditioning COP of 2.5, the overall reduction in cooling energy use could be expected to be approximately 170 W against the measured consumption of 284 W by the ventilation fan. Measured reduction to the maximum cooling load was only 6% for R-11 ceiling insulation. Thus, the powered ventilation does not typically result in a net energy savings for powered vent fans unless the attic is uninsulated. Under this scenario, other means of controlling attic heat gain are preferable and more cost effective than forced ventilation. Other analysis, tends to verify this conclusion. Detailed simulations suggest that the heat transfer in an attic to a residential building interior in mid-summer is dominated by radiative gains from the hot roof decking directly to the insulation surface (Parker et al., 1991; Wilkes, 1991). This mode of heat transfer is more effectively limited by 1) increased attic insulation, 2) a truss-mounted radiant barrier or 3) a white reflective roof surface that limits solar gain to the attic structure.
Any refs to studies done in the last 10 years or less? I just got the electric bill and my own unofficial study says that it cost me ~$100 more last month to maintain the same perceived comfort level in the house while I went through the process of replacing the motor. outside temps were not unusual for the period.
if the fan saved you $100 in one month on your electric bill, then there is something very seriously wrong with the insulation in your attic or with your A/C unit.Taking things to extremes, if the attic fan lowered the attic tmeperature from 140 degrees to 110 degrees and you keep your house at 80, then the temperature differential across the attic insulation goes from 60 degrees to 30 degrees. If you have R10 insulation in your attic, then the difference is 3,000 btu / hr / 1,000 sq. ft. (30 degree differential times 1,000 sq. ft. divided by the R-Factor) If you have a 5,000 sq. ft. house, then that is 15,000 btu / hr (for the hottest part of the day).For a 10 SEER A/C unit, that is 1.5 kilowatt / hr of A/C. and at my electric rates, that works out to 23 cents extra per hour (during the hottest part of the day). At night, as the temperatures go down, the differential goes down as well.doubling the insulation from R-10 to R-20 would cut the savings from running the fan in half.I don't see how it could ever be anywhere near $100.
Martin,I agree that my statement is not true for ALL infrared wavelengths. Personal experience, however, shows it to be substantially true for the spectra put out by hot roof decks radiating down through attics to gypsum ceilings below. Inch for inch, cellulose is vastly more effective at blocking these wavelengths than fiberglass is. Male-pattern baldness makes me uniquely qualified to assess the radiant load from room ceilings, and I can feel a very large difference.It would be interesting to see the performance curves for these and other common insulating materials, graphed as % transmission per inch against black-body temperature. Anybody have a link?Bill
when adding roofing fans, you must already have good circulation
I've already added two roof fans and 4 years later neither one work any longer. It's because I already had inadequate ventilation. The best thing would be to get some air flowing thru there via ridge vents. I'm going thru the same thing. Good luck.
Hopefully he has figured this out considering the post is from a decade ago. Hope you got yours figured out nonetheless.