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Posted by Abby Normal on June 28, 2007, 6:14 pm
On Jun 28, 9:21 am, .p.jm@see_my_sig_for_address.com wrote:
> On Thu, 28 Jun 2007 05:57:12 -0700, Abby Normal
>
> >I would vent the fans to the outside, not let them blow in the attic.
>
> >Venting attics is for cold climates, to stop ice dams. Other areas
> >need to be sealing the attics and keeping the heat out. We need to
> >change how we build.
>
> Abby - seeing as how most of the roof load is radiant, it seems
> intuitive that a method to help reject it via a layer that is
> constantly changing air with the outside, makes sense. In effect,
> it's like standing in the shade on a hot sunny day - lots cooler.
>
> Then, you do your insulation ( insulating now against only the ambient
> DB, not ambient PLUS radiant gain ) under that.
>
> How do you see it differently ?
>
> IOW - take your 'sealed attic' building, and, keeping all things equal
> ( same day environment ), build a big shade tent over the top, 1 foot
> over the roof, open-eave. Just a big shade over the whole thing.
> What happens to your heat load ? It goes down HUGELY.
>
> That is how I see the usual vented roof, as commonly built. How is
> this not a good thing ?
>
> --
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> Paul ( pjm @ pobox . com ) - remove spaces to email me
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Let's consider a standard pitched roof, vented attic, a layer of
insulation at the celing plane.
>From what I have been reading, the hottest part of the attic is the
underside of the sheathing of that pitched roof. Darker the roof is,
the hotter the underside of the sheathing will be. It is radiating
like hell.
The second hottest thing in the attic will be the top of the
insulation as the sheathing is radiating at it. Heat conducts downward
through the insulation to the space and it also convects up from the
insulation to the third hottest part of the attic, which is that super
heated air.
So my thinking is there are only two ways heat from that superheated
air is going to make it into the conditioned space. 1) When a pressure
difference causes that air to physically move through the insulation
to the space below and 2) After the sun sets.
People are trying radiant barriers, but what I am trying, is to try
and keep all that solar heat out of the attic in the first place.
Venting will lower the attic air temp, and encourage a little more
heat from the top of the insulation to go up. I think the only time
power venting saves you anything is when you have crap insulation to
begin with. Otherwise the power running the fan could have ran a
compressor a little more.
So if they vent the attic down here to "flush out the heat" the
problem is that the air is so damn humid, that attic air could result
in gallons of water making it into the space. Dewpoint floats from 77
to 81 all rainy season here.
I have been reading a lot of what the Florida Solar Centre says ( they
even sent a buddy of Nick down here) I just heard the guy yap and
figured they would know each other-- sure as shit. I have also been
reading a lot of Dr Joe at Building Science dot com.
I have data loggers in my attic right now. Got some buildings
scheduled for demo across from my office, light coloured metal
roofing, zero attic insulation. Next sunny day going to measure how
hot the underside of that sheathing gets. Got a few techs with
infrareds taking some measurements for me when they pull attic duty
also.
The chorus of my new theme somng is Toys! Toys! Toys! In the attic!
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