Radiant barrier question

On Tue, 01 Jul 2008 04:27:58 -0600, Bob wrote:

> I hope there are people here who install or have purchased radiant
> barriers. I am considering this.
>
> My questions:
> Are they really effective and reduce AC operating costs?
>
> I have read there are two types, sprayed (like a paint) and a film like
> thin plastic. Which is better?
>
> I have also heard that as dust collects on the surface, that they are
> less effective, which would seem to be a bad problem. (Who would go up
> in the attic and vacuum the underside of the roof). Is this true and
> is it a problem?
>
> What is the approximate cost - by square foot, or however?
>
> Thanks in advance for any information.
>
> Bob-tx

I just received samples of the following:
http://stores.ebay.com/Energy-Efficient-Solutions_Radiant-Barrier_ARMA-
Foil-Perforated_W0QQcolZ2QQdirZ1QQfsubZ17724604QQftidZ2QQtZkm

It looks promising. I might opt for this over paint for what I'm doing
(hydronic floor heat). If I ever get to doing the roof, I think I'll
apply the foil beneath the felt paper.



--

=================================================
Franz Fripplfrappl

On 2008-07-01, franz fripplfrappl <[email protected]> wrote:

> If I ever get to doing the roof, I think I'll apply the foil beneath
> the felt paper.

This won't work. In order for a radiant barrier to be effective, it
must face an air gap. If the radiant barrier is in contact with
another material, it will just conduct heat, and its radiant
properties won't matter.

So for a roof, you can use foil-faced plywood sheathing (foil facing
inside), or you could apply a film barrier to the underside of the
joists.

Cheers, Wayne

On Jul 1, 12:43 pm, Wayne Whitney <[email protected]> wrote:
> On 2008-07-01, franz fripplfrappl <[email protected]> wrote:
>
> > If I ever get to doing the roof, I think I'll apply the foil beneath
> > the felt paper.
>
> This won't work.  In order for a radiant barrier to be effective, it
> must face an air gap.  If the radiant barrier is in contact with
> another material, it will just conduct heat, and its radiant
> properties won't matter.
>
> So for a roof, you can use foil-faced plywood sheathing (foil facing
> inside), or you could apply a film barrier to the underside of the
> joists.
>
> Cheers, Wayne


If that's true, how can it work on roof sheathing with the radiant
barrier facing in towards the attic? If it only works as you
describe, it would seem to work backwards, only providing a barrier
from heat escaping the attic, not coming in from the roof.

On 2008-07-01, [email protected] <[email protected]> wrote:

> On Jul 1, 12:43 pm, Wayne Whitney <[email protected]> wrote:
>
>> This won't work. In order for a radiant barrier to be effective, it
>> must face an air gap. If the radiant barrier is in contact with
>> another material, it will just conduct heat, and its radiant
>> properties won't matter.
>>
>> So for a roof, you can use foil-faced plywood sheathing (foil facing
>> inside), or you could apply a film barrier to the underside of the
>> joists.
>
> If that's true, how can it work on roof sheathing with the radiant
> barrier facing in towards the attic? If it only works as you
> describe, it would seem to work backwards, only providing a barrier
> from heat escaping the attic, not coming in from the roof.

Radiation is how heat transfers across a vacuum or air gap (or any
transparent insulator). In the absence of an air gap, heat will
simply be transferred by conduction, which is what normal insulation
is designed to reduce.

By being "shiny", a radiant barrier does two thing: first, it reflects
most incoming radiant heat, and second, when it gets hot, it doesn't
radiate heat very well. If you have a shiny metal roof, you would be
taking advantage of the first property: it will reflect most of the
incoming solar radiation.

If you instead have a radiant barrier on the underside of the roof
sheathing, with an air space below, then you are using the second
property. The roof and roof sheathing and radiant barrier will heat
up from the sun, but the radiant barrier won't radiate much into the
attic.

Of course, where the roof sheathing contacts the roof framing, the
radiant barrier won't help, as the heat will be conducted to the roof
framing, which will radiate into the attic. I'm not sure if this is a
big deal; I think you still get most of the benefit anyway. You could
counteract this by painting the roof framing with a radiant barrier,
or you instead use a continuous film radiant barrier on the underside
of the rafters.

BTW, as previously mentioned, a radiant barrier facing up to an air
gap is typically not effective in the long-term, as dust will
accumulate and reduce the "shininess".

Cheers, Wayne

"Wayne Whitney" <[email protected]> wrote in message
news:slrng6l2jj.7i7.whitney@pizza.private...
> On 2008-07-01, [email protected] <[email protected]>
> wrote:
>
<snip>
> Radiation is how heat transfers across a vacuum or air gap (or any
> transparent insulator). In the absence of an air gap, heat will
> simply be transferred by conduction, which is what normal
> insulation
> is designed to reduce.
>
> By being "shiny", a radiant barrier does two thing: first, it
> reflects
> most incoming radiant heat, and second, when it gets hot, it
> doesn't
> radiate heat very well. If you have a shiny metal roof, you would
> be
> taking advantage of the first property: it will reflect most of
> the
> incoming solar radiation.
>
> If you instead have a radiant barrier on the underside of the roof
> sheathing, with an air space below, then you are using the second
> property. The roof and roof sheathing and radiant barrier will
> heat
> up from the sun, but the radiant barrier won't radiate much into
> the
> attic.
>
> Of course, where the roof sheathing contacts the roof framing, the
> radiant barrier won't help, as the heat will be conducted to the
> roof
> framing, which will radiate into the attic. I'm not sure if this
> is a
> big deal; I think you still get most of the benefit anyway. You
> could
> counteract this by painting the roof framing with a radiant
> barrier,
> or you instead use a continuous film radiant barrier on the
> underside
> of the rafters.
>
> BTW, as previously mentioned, a radiant barrier facing up to an
> air
> gap is typically not effective in the long-term, as dust will
> accumulate and reduce the "shininess".
>
> Cheers, Wayne
>

Sounds like good info, Wayne - thanks.

Although you did not specifically discuss the effectiveness a paint
barrier, I get the impression that it is not very effective.

There are so many newspaper and radio ads for radiant barriers that
I am wondering if it isn't a case of companies taking advantage of
concerns over high prices of energy, and pushing their product.
Either that, or it is becoming a fad.

The dust factor is also a concern - no attic remains completely dust
free, so in time, its effectiveness will diminish.

What do you think?

Bob-tx