Question about Radient Barrier


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Old 10-26-16, 10:22 AM
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Question about Radient Barrier

Hello,

My attic does not have vented soffits or a ridge vent. From what I can see the only ventilation the attic has comes from the two dormer vents on the north and south sides of the house.

I was thinking about putting in a radiant barrier across the roof joists. It's like tin foil, easy to put up, and not terribly expensive.

Because of the way my attic ventilates, will I get any of the desired results of a radiant barrier?

Thank you for any input and advice,

Pete
 
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Old 10-26-16, 10:51 AM
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Hi Pete,
The primary concern with attic ventilation is removing any moisture that seeps or leaks into that cold space where it can cause mold or damage the framing. In the summer, good passive ventilation will remove some of the heat, but the best defense against that heat is lots of insulation. The radiant barrier you suggest will do little for the heat and nothing for the moisture. The real attraction of the radiant barriers is exactly what you said, they are advertised as an "easy low cost solution, for which, in CT they are not. WAY down south they actually install plywood sheathing on the rafters that has a layer of foil on the attic side.

As for not having the traditional vents, there are many options. Tell us more about your insulation and I assume your concerns are about a hot attic.

Bud
 
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Old 10-26-16, 12:45 PM
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Thanks Bud

Bud,

Thanks for your reply.

Right now I am in a project to remove all the old insulation I have in the attic. That part is done (hard work!). Also, I have gone ahead and air-sealed everything and covered several large open spaces that went straight to the basement (house built in 1951).

I started putting down R30 bats. Should be done soon. I plan on doing two layers; R30 lengthwise and then another layer of R20 perpendicular to the bottom layer.

I wish I had talked to you before I bought the radiant barrier material. I have it, so am I wasting my time putting it up? My concerns were a hot attic but if insulation is enough, I am good with that and will eat the cost of the material.

Thanks again,

Pete
 
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Old 10-26-16, 01:29 PM
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Here's the kicker. Your RB across the bottom of the rafters will reduce the temp in the attic a few degrees, but with a lot of insulation that makes no (very little) difference on the inside of the house. If you have any ac ducts up there they will benefit from the added RB. But again, if all ducts are air sealed and well insulated no change.

If you install it, my suggestion would be to leave the bottom 2' and top 2' open to allow ventilation, assuming you have soffit and ridge vents.

Will it do any harm? None I can think of, so your choice.

Is your RB single or double sided?

Bud
 
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Old 10-26-16, 01:43 PM
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Again my thanks Bud for taking the time to explain all this to a rookie.

There are no vented soffits or ridge vents in my attic, just the two vents on the dormers. No duct work runs through the attic either.

The RB is double sided.

Sounds like this stuff my sell in my next yard sale.

Live and learn,

Pete
 
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Old 10-26-16, 02:42 PM
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Sorry forgot you mentioned the two dormer vents.
The standard recommendation is to have more low venting than high. With all high venting it relies on the wind primarily. The old myth that hot air rises and exits those high vents by itself doesn't happen. The moving force is the cooler (heavier) air from below that pushes in and displaces the warmer lighter air.

I would need an outside view of your home to make specific recommendations, but it has survived this long so just adding venting to your future list is probably good for now.

Best,
Bud
 
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Old 10-27-16, 06:06 AM
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So you mention added ventilation might be in my list. I found a brand new roof fan that was never hooked up to electric. Looks very similar to this:

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I have heard the pros and cons of them. Mainly, they are only effective if you have a well sealed attic. But, if as you say the insulation is doing its job, the attic being hot is not a problem, and the fan would only cool down the attic space and not the house (which is a ranch BTW).

What are your thoughts on theses types of fans?

Thanks again for the good feedback.
 
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Old 10-27-16, 07:16 AM
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Powered attic ventilation needs a lot of intake vent area to limit the negative pressure inside the attic. That negative pressure pulls conditioned air from inside your home through all of the cracks and crevices that are almost impossible to seal in a retrofit approach. New construction can get most, but not by crawling around in the attic with a can of foam or other.

The loss of conditioned air would exceed any benefits the fan might produce.

Bud
 
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Old 10-27-16, 07:26 AM
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OK Bud, that fan will stay as is. Not powered or to be used.

I appreciate all your input on this.

Pete
 
 

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