It's Hot Upstairs More Insulation or Attic Fan?

[Bubba]

"Kids", Before you get too "bent" on going out and adding more insulation to an attic that is already insulated, you might want to get the facts. The biggest change in your heat loss through the attic is going from

0" of insulation to 3". After that, its really not that big of a change in btu loss. Dont believe me? Have a Right J load calculation done on your house and plug in the correct amount of insulation you currently have ( 3", 6" etc.?) Now take that same exact calculation and change it to 12" or 18" or whatever you wish. Now look at the btu loss difference. Its not that great so now you have to figure is it worth spending $1000 etc to save very few btu's. Its your money and your figurin. Bubba

[Ollie N.]

So Bubba, since we're talking about gaining heat, not heat/cooling loss, how much heat is being gained?

How did you arrive at a $1000 amount to insulate?

It would be utterly foolish to attempt fixing an insulation problem, by adjusting and wasting BTU's.

[jmeehan]

Tom covered this very well.

First make sure you have enough vents for the area and that they are clear and working.

I have never had much luck with fans, but they can help at times. It sort of depends on the area where you live.

[Bubba]

I dont know. I didnt and wont do a load calculation on that house unless someone wishes to pay me for it. Will you?

A number I pulled outta my ass. I paid a bit less than that several years ago so Im sure the price has risen to that about now for my size home.

Not sure what you are gettin at but certainlly wasting btu's anytime is foolish.

Bubba

[nicksanspam]

Sounds like more insulation or a radiant barrier under the roof would help.

Warm air rises. Whatever a fan can do, more attic ventilation can do... For instance, 250 Btu/h-ft^2 of sun shining on a 1200 ft^2 roof with 60% solar reflectance over an attic with a 1000 cfm fan and a roof with a US U2 (2 Btu/h-F-ft^2) conductance to 100 F outdoor air makes an equivalent electrical circuit like this, viewed in a fixed font:

T = 100+I/1000 = 135.3 F attic temp | 0.5/1200 | 1/1000 ----------www----------www------ 100 F outdoor air | ------------------->

| I = (150-100)/(0.5/1200+1/1000) = 35.3K Btu/h | | 100+250(1-0.6)0.5 = 150 F sol-air temp --- - | -

We can also make 35.3K Btu/h flow with 2 A ft^2 vents with a 35.3 F temp difference and an 8' height difference if 16.6Asqrt(8)(35.3)^1.5 = 35.3K, ie A = 3.58 ft^2, eg 2'x2' high and low gable vents with doors to close them up in wintertime.

Nick in PA

[Tom in PA]

Thanks, everyone, for all of the advice.

We do have some blocked vents (but not a lot) so I'm clearing those out. Also plan to add insulation as a first step, then maybe do the attic fan (gable vent fan) as well.

Here's another question. The only 24" width fiberglass insulation (which is the size I need) available at the local home stores has the kraft paper backing. I'm putting this on top of existing insulation so if I use it I guess I'd have an additional vapor barrier between the original and the new insulation. Is that a problem?

Thanks again!

Tom

[ransley]

You dont want another vapor barrier, use unfaced only on top, what zone are you, what R value do you have now. To see what is optimal manufacurers have optimal tables on R values.

[Oren]

I've been known to pull the facing off of insulation. If the OP is limited in supply, it is a option. (don't know cost difference in faced and un-faced).

[jmeehan]

Or just slit it a few times with a knife.

[jmeehan]

Not at all useless. Just compare R values. That is the measure of their value. Good well applied foam can be better per inch, but it is the total R value that counts. The best for the money is usually fiberglass. Blown in has some additional advantages but mostly in the winter, not summer.

[jmeehan]

On Jul 6, 1:08=A0am, mm wrote: =2E..

Likely because the vent system is poorly designed or executed and you are not getting enough natural air flow. Too little vent area or partly blocked vents or poorly placed vents causing short cuts.