# Thread: Attic needs more ventilation

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again: solar radiation depends upon the 4th power of absolute temperature -- so not much difference between 155F and 105F, so not much affect of radiation either from 155F to 75F in thru the ceiling next to the attic.

this was affirmed by my best friend, a retired NASA rocket engineer, who was charged with determining the best paint for the fuel tanks for mitigating solar effects upon the fuel tanks during fueling -- Tom helped put man on the moon -- using slide rule & Merchant calculators -- BTW, the chosen paint was such that color had such little influence that the tanks could be of any color, or have any logo desired! Tom knew every flow system of the vehicle, and the heat transfers involved -- such that they could predict the preformance of new device designs.

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Cembsee--We are not talking about solar radiation in space. We are talking about radiant heat transfer in attic assemblies. The radiant heat transfer is from the hot roof sheathing to the cooler attic floor below. this can be calculated by the formula Q=s x (t1 to the 4th) - (t2 to the 4th) T1 being the roof sheathing. t2 being the temperature of the attic floor before insulation is put in place. If roof sheathing is 180 degress, the radiant heat transfer from the sheathing down to the floor (at 75 degrees) is: 147.1 Btu/h/sq.ft. In a 1500 sq. ft. attic that is about 222,500 btu/h! Of course that does not account for insulation which would lower that number. But 2%--no way.

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Uktra- I have an honest- no arguement question about attic heat. Please don't get defensive if this sounds like an arguement question, because it isn't. I am really hoping for some good information on how radiant heat effects my current home (no PV's).

Here tis': I have noticed that as the sun is about 1/2 (noon) to 3/4 down, when I would THINK the heat load to be the highest of the day, my AC does not run longer. BUT, when the sun is 3/4 or more, down to sunset and past, the heat load in the house seems to increase and the AC has a much harder time keepng a set temperature. This happens until about 10pm. IT CAN do it, it runs great, but I notice it runs more.

Some obvious points that should be known:

1. The later-sun facing part of the house has 2 medium-small solar-screen covered windows and a gable-end of the roof. The are lots of trees to block all sun about 1 hour before sunset.

2. System has been checked and is running optimally.

3. Roof is brown composite.

4. 1 Whirly-bird with gable end vents and some soffit vents (new).

My theory is that at sunset any wind stops and the latent heat buildup does not move much at all, just sitting in the attic with no place to go. Also, without the roof surface continuous heating from the sun, I lose any inherent stack effect to help keep the air moving at a decent pace.

It seems that once the sun is off the roof, the radinat effect would decrease dramatically as the roof could give it's heat off to the air air relatively quickly.

You are welcome to tell me that what I am seeing is not really happening, but I am hoping there really is something to this that I may be able to cure with more soffit vents or something.

Let me know what you think-

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What you are seeing is the stored up heat in the building products of the home. The roof sheathing and other wood framing members takes time to cool down. If you have ducts in the attic, you may want to look at a radiant barrier. If you can do it yourself, it would be cost effect. Of course sealing ducts and sealing holes in attic floor is the first priority in lowering heat ransfer to the living areas.
Hope this helps

[Edited by uktra on 06-20-2005 at 04:36 PM]

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Originally posted by uktra
What you are seeing is the stored up heat in the building products of the home. The roof sheathing and other wood framing members takes time to cool down.
So the guts of the house are absorbing heat on a linear scale...and they take longer to "add" to the heat issue?

I would agree with that. Is radiated heat an issue at this point? Wouldn't it help to "flush" out this latent heat? I just seems that the house materials would cool off much faster, and since the problem is no longer radiation- at least not as much- that this could be a good point to try to get rid of the heat. That would help bring the temperature of the insulation down a bit faster.
I beleive there are tests that have shown that after outside temps go down, the core of fiberglass insulation can stay very hot for a while and this can radiate into the house for some time.

If there is no longer a heat source to continue heating the roof surface, it seems some sort of ventilation would be beneficial at this point? NOT a PV, just "something".

I have been intersted int he RB for a while, and if I replace my roof someday I am definitely going to use it. BUT, concerning the RB, if there is no longer a radiant roof surface, it won't do much good for this time of day, true? Again- I know it works, that is not my point.

[Edited by old yeller on 06-20-2005 at 04:48 PM]

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Originally posted by uktra
What you are seeing is the stored up heat in the building products of the home. The roof sheathing and other wood framing members takes time to cool down. If you have ducts in the attic, you may want to look at a radiant barrier. If you can do it yourself, it would be cost effect. Of course sealing ducts and sealing holes in attic floor is the first priority in lowering heat ransfer to the living areas.
Hope this helps

[Edited by uktra on 06-20-2005 at 04:36 PM]
IF my ducts are sealed, is there anything I can do to protect them from radiant heat directly? RB on my entire roof is close to impossible right now. MAybe cover them with radiant barrier material? They are much easier to get to than my roof decking.

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Old Yeller:

You can do what I did: spray a lot of loose fill cellulose over the attic ducts, and bury them...you won't get all of the duct-runs covered, but a majority of the ducts can be protected from "heat" (radiant or convection, chose your poison)....only costs about \$200 at Home Depot for the insulation, and they give you the blowing machine free for 24 hours if you buy more than 20 bags (about \$7 per bag)....just be prepared for a lot of mess when you spray this stuff, and leave your AC system OFF....

PS: Lots of debates about cellulose vs fiber glass, but I'm not touching that one here...

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Originally posted by uktra
Cembsee--We are not talking about solar radiation in space. We are talking about radiant heat transfer in attic assemblies. The radiant heat transfer is from the hot roof sheathing to the cooler attic floor below. this can be calculated by the formula Q=s x (t1 to the 4th) - (t2 to the 4th) T1 being the roof sheathing. t2 being the temperature of the attic floor before insulation is put in place. If roof sheathing is 180 degress, the radiant heat transfer from the sheathing down to the floor (at 75 degrees) is: 147.1 Btu/h/sq.ft. In a 1500 sq. ft. attic that is about 222,500 btu/h! Of course that does not account for insulation which would lower that number. But 2%--no way.
Uktra. I am talking about the Suns radiation. Radiation is heat transfer by the emission of electromagnetic waves which carry energy away from the emitting object. For ordinary temperatures (less than red hot"), the radiation is in the infrared region of the electromagnetic spectrum.

The relationship governing radiation from hot objects is called the Stefan-Boltzmann law. The roof of the house could never exceed the tempurature of the surounding air without heat grain from the Sun's radiation. Reducing the amount of radiation that makes it into the attic will result in much lower temperatures in the attic.

If the man had a steel roof, good insulation (reducing both conduction and convection heat transfer) his attic could be around 20 degrees warmer then his second floor. Adding that gable fan, and his attic could be the same temp as the ambient air outside.

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Originally posted by robnjr
Old Yeller:

You can do what I did: spray a lot of loose fill cellulose over the attic ducts, and bury them...you won't get all of the duct-runs covered, but a majority of the ducts can be protected from "heat" (radiant or convection, chose your poison)....only costs about \$200 at Home Depot for the insulation, and they give you the blowing machine free for 24 hours if you buy more than 20 bags (about \$7 per bag)....just be prepared for a lot of mess when you spray this stuff, and leave your AC system OFF....

PS: Lots of debates about cellulose vs fiber glass, but I'm not touching that one here...
I agree with burying them, but the radiation factor does still remain. I am actually looking at doing the blown in like you are talking about, then going over the entire attic with Radiant Barrier Chips. These are supposedly way better than trying to line the rafters or floor with solid sheets and WAAAY easier to do without tearing off the roof.

This seems like a big heat double-whammy. I'll keep the majority of radiation off the insulation, which can actually make your problem worse in some situations by holding retained heat against the ducts. Then the insulation will help retard the ambient heat in the attic away from the flex duct.

What do you think? What did you do with the air handler? Did you "build up" some insulation around the non-maintenance sections of the air handler?

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jimbo48236--I understand source of heat is the sun by radiant transfer. I agree with everything you say up to the point where you say insulation will reduce convective transfer. This depends on the type of insulation. The real problem is why put a power vent in, when the small reduction in attic air temperature, and the smaller amount of heat transfer from the hot air to rooms below will be offset by the energy to power the fan?

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1974 CollegePhysics by Sears& Zemansky Young:
temps to be in Kelvin
the quoted formula indicates temp to 4th power only, (t-sub1 & t-sub2)
the quoted formula is for heat transfer from small hot body to surrounding walls --

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cem-bsee--Actually the formula is in degrees R (Rankin--F+460). This formula calculates the radiant heat transfer from a higher heat source to a lower heat source.

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Originally posted by uktra
cem-bsee--Actually the formula is in degrees R (Rankin--F+460). This formula calculates the radiant heat transfer from a higher heat source to a lower heat source.
Kelvin is the more typical temperature scale for science. Whatever you use, make sure you have the correct Stefen-Boltzmann constant.

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