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Exhaust fans and attic ventilation questions ChrisW 06-27-2007
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Posted by on June 28, 2007, 8:30 pm
On Thu, 28 Jun 2007 15:14:43 -0700, Abby Normal

>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 ?
>>

>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.

        Yep.

>The second hottest thing in the attic will be the top of the
>insulation as the sheathing is radiating at it. Heat conducts downward

        Depends where the insulation is. Very often, it is not at the
roof rafters but at the top of the ceiling.

>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.

        No, it's also going to radiate in, strongly.
>
>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.

        OK.

>
>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.

Solar powered attic fan is an interesting product.

>
>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 forgot - you live in a swamp :-)

>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!

        From another post, it appears it ought to be 'Insulate !
Insulate !' I mean, what is this 1 1/2 " of styrofoam R-7 BS ? If
you're going to insulate, do it right.


--
Click here every day to feed an animal that needs you today !!!
http://www.theanimalrescuesite.com/

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'
'With sufficient thrust, pigs fly just fine.'
HVAC/R program for Palm PDA's
Free demo now available online http://pmilligan.net/palm/

Posted by Abby Normal on June 28, 2007, 10:11 pm
On Jun 28, 7:30 pm, .p.jm@see_my_sig_for_address.com wrote:
> On Thu, 28 Jun 2007 15:14:43 -0700, Abby Normal
>
>
>
>
>
> >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 ?
>
> >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.
>
> Yep.
>
> >The second hottest thing in the attic will be the top of the
> >insulation as the sheathing is radiating at it. Heat conducts downward
>
> Depends where the insulation is. Very often, it is not at the
> roof rafters but at the top of the ceiling.

Yes for this discussion the insulation is at the top of the ceiling.
Building Science and others are saying that heat radiating from the
underside of the roof will heat up the top of the insulation, to a
temperature hotter than the air in the attic space. The heat radiating
to this insulation conducts down to the space and convects up to the
air. Heat also convects from the underside of the roof sheathingl to
the air.

>
> >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.
>
> No, it's also going to radiate in, strongly.

Was a bit of a typo there that I corrected in my second response to
KJ, but what I was talking about was while the sun is shining heat is
not really transmitting from the air into the insulation but the other
way around. The roof is radiaiting a lot of heat to the insulation.

>
>
> >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.
>
> OK.

>
>
> >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.
>
> Solar powered attic fan is an interesting product.

Yes a solar powered fan would help there. Another point about the fans
is that they 'suck' as hard on the ceiling plane and wall cavities as
they do on the air vents, and I have seen them depressurize structures
causing massive condensation.

>
> >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 forgot - you live in a swamp :-)

Yah, I got it worse than the gulf coast but not as bad as the Persian
Gulf

> >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!
>
> From another post, it appears it ought to be 'Insulate !
> Insulate !' I mean, what is this 1 1/2 " of styrofoam R-7 BS ? If
> you're going to insulate, do it right.

I would have had to have sprayed icynene foam on the underside of the
roof to improve there. I was limited in space on what I could use
above the roof for insulation, but I think I am going to have good
results because I am reflecting away the solar radiation in the first
place.

I got thermal mass on my side, the attic is the warmest just after the
sun goes down. Prior to starting up the AC, in the heat of the day the
sealed attic was one degree hotter than the ambient air.

Without the thermal mass I think the attic temp would have peaked
while the sun was shining at maybe a few degrees higher. The roof is
actually sloping concrete. I hiltied pressure treated 2x4s on the flat
so I would have something to screw the white metal roof to. I put the
foam in between the 2x4s on the flat was limited to 1.5", and it is
really looking like it is kicking the hell out of fibreglass below a
140F sauna with an 80 dewpoint :-)

In my first round of tests, logging temp and RH every 5 minutes for a
week, the attic peaked at 84.8F, about an hour after the sun went
down. I think it also trapped some heat from surface mounted ceiling
lights. Maybe 4000 cubic feet of air up there, does not take too many
watts to heat that air up a couple degrees.

I was keeping the space below the ceiling betwen 79 and 80F.

Consider this, a single story commercial building with a flat roof.
Maybe there is 2 inches of foam above the roof deck and perhaps a
light coloured membrane. Bar joists could be holding up this roof
deck. They will drop a t-bar ceiling below. That space where you run
ducts can be a plenum sometimes.

I am calling a similar type of space my attic.

Code says I am supposed to condition it, I just have not got around to
doing it yet, I wanted to see what would happen if I did not..

> --
> Click here every day to feed an animal that needs you today
!!!http://www.theanimalrescuesite.com/
>
> Paul ( pjm @ pobox . com ) - remove spaces to email me
> 'Some days, it's just not worth chewing through the restraints.'
> 'With sufficient thrust, pigs fly just fine.'
> HVAC/R program for Palm PDA's
> Free demo now available onlinehttp://pmilligan.net/palm/- Hide quoted text -
>
> - Show quoted text -



Posted by on June 28, 2007, 10:40 pm
On Thu, 28 Jun 2007 19:11:38 -0700, Abby Normal

>On Jun 28, 7:30 pm, .p.jm@see_my_sig_for_address.com wrote:
>> On Thu, 28 Jun 2007 15:14:43 -0700, Abby Normal
>>
>>
>>
>>
>>
>> >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
>>

>>
>> >The second hottest thing in the attic will be the top of the
>> >insulation as the sheathing is radiating at it. Heat conducts downward
>>
>> Depends where the insulation is. Very often, it is not at the
>> roof rafters but at the top of the ceiling.
>
>Yes for this discussion the insulation is at the top of the ceiling.
>Building Science and others are saying that heat radiating from the
>underside of the roof will heat up the top of the insulation, to a
>temperature hotter than the air in the attic space.

        Certainly, yes.

> The heat radiating
>to this insulation conducts down to the space and convects up to the
>air. Heat also convects from the underside of the roof sheathingl to
>the air.

        Some will, some will not, of course, depending on R value,
which is why I sniggered rudely at your R-7 :-)

        IMO, it should be a radiant barrier under the roof, insulation
under that, minimum maybe R-21, but R-40 would be better. IOW, as you
are saying, keep the damned heat out to start with.

        Then a vented eave, and more insulation ( R-20 maybe ) at the
ceiling of the occupied space.

>>
>> >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.
>>
>> No, it's also going to radiate in, strongly.
>
>Was a bit of a typo there that I corrected in my second response to
>KJ, but what I was talking about was while the sun is shining heat is
>not really transmitting from the air into the insulation but the other
>way around. The roof is radiaiting a lot of heat to the insulation.

        Yes on the direction of flow, but no in any case where the
above might suggest heat flow from colder to hotter ( don't work like
that ), of course.

        So, during sun, the roof is heated by radiant in the visible
band, it keeps some heat ( gets hot ), re-radiates and reflects some
back out in the infra-red and visible bands, and radiates some in the
infra-red band to the insulation, which keeps some ( gets hot ) and
passes the rest via convection and conduction.

>> Solar powered attic fan is an interesting product.
>
>Yes a solar powered fan would help there. Another point about the fans
>is that they 'suck' as hard on the ceiling plane and wall cavities as
>they do on the air vents, and I have seen them depressurize structures
>causing massive condensation.

        Yep - inadequate intake area would do that to you !

        I also had a girlfriend once that had that effect sometimes.


>> From another post, it appears it ought to be 'Insulate !
>> Insulate !' I mean, what is this 1 1/2 " of styrofoam R-7 BS ? If
>> you're going to insulate, do it right.
>
>I would have had to have sprayed icynene foam on the underside of the
>roof to improve there. I was limited in space on what I could use
>above the roof for insulation, but I think I am going to have good
>results because I am reflecting away the solar radiation in the first
>place.
>
>I got thermal mass on my side, the attic is the warmest just after the
>sun goes down. Prior to starting up the AC, in the heat of the day the
>sealed attic was one degree hotter than the ambient air.
>
>Without the thermal mass I think the attic temp would have peaked
>while the sun was shining at maybe a few degrees higher. The roof is
>actually sloping concrete. I hiltied pressure treated 2x4s on the flat
>so I would have something to screw the white metal roof to. I put the
>foam in between the 2x4s on the flat was limited to 1.5", and it is

        Rafters ON FLAT ???? WTF ????


--
Click here every day to feed an animal that needs you today !!!
http://www.theanimalrescuesite.com/

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'
'With sufficient thrust, pigs fly just fine.'
HVAC/R program for Palm PDA's
Free demo now available online http://pmilligan.net/palm/

Posted by Abby Normal on June 28, 2007, 11:14 pm

I sent you some pictures.

I have been talking about what a regular attic is like then jumped to
what mine is like.

The 2x4s on flat are not structural. Something on top of roof
structure that I used to fasten the white metal roofing too. Pitcures
are a thousand words.

Here is another way of sealing attics

They start talking about a roof on page 23

http://www.buildingscienceconsulting.com/designsthatwork/hothumid/lakecharles/DTW_HotHumid-Creole.pdf

On Jun 28, 9:40 pm, .p.jm@see_my_sig_for_address.com wrote:
> On Thu, 28 Jun 2007 19:11:38 -0700, Abby Normal
>
>
>
>
>
> >On Jun 28, 7:30 pm, .p.jm@see_my_sig_for_address.com wrote:
> >> On Thu, 28 Jun 2007 15:14:43 -0700, Abby Normal
>
> >> >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
>
> >> >The second hottest thing in the attic will be the top of the
> >> >insulation as the sheathing is radiating at it. Heat conducts downward
>
> >> Depends where the insulation is. Very often, it is not at the
> >> roof rafters but at the top of the ceiling.
>
> >Yes for this discussion the insulation is at the top of the ceiling.
> >Building Science and others are saying that heat radiating from the
> >underside of the roof will heat up the top of the insulation, to a
> >temperature hotter than the air in the attic space.
>
> Certainly, yes.
>
> > The heat radiating
> >to this insulation conducts down to the space and convects up to the
> >air. Heat also convects from the underside of the roof sheathingl to
> >the air.
>
> Some will, some will not, of course, depending on R value,
> which is why I sniggered rudely at your R-7 :-)
>
> IMO, it should be a radiant barrier under the roof, insulation
> under that, minimum maybe R-21, but R-40 would be better. IOW, as you
> are saying, keep the damned heat out to start with.
>
> Then a vented eave, and more insulation ( R-20 maybe ) at the
> ceiling of the occupied space.
>
>
>
>
>
>
>
> >> >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.
>
> >> No, it's also going to radiate in, strongly.
>
> >Was a bit of a typo there that I corrected in my second response to
> >KJ, but what I was talking about was while the sun is shining heat is
> >not really transmitting from the air into the insulation but the other
> >way around. The roof is radiaiting a lot of heat to the insulation.
>
> Yes on the direction of flow, but no in any case where the
> above might suggest heat flow from colder to hotter ( don't work like
> that ), of course.
>
> So, during sun, the roof is heated by radiant in the visible
> band, it keeps some heat ( gets hot ), re-radiates and reflects some
> back out in the infra-red and visible bands, and radiates some in the
> infra-red band to the insulation, which keeps some ( gets hot ) and
> passes the rest via convection and conduction.
>
> >> Solar powered attic fan is an interesting product.
>
> >Yes a solar powered fan would help there. Another point about the fans
> >is that they 'suck' as hard on the ceiling plane and wall cavities as
> >they do on the air vents, and I have seen them depressurize structures
> >causing massive condensation.
>
> Yep - inadequate intake area would do that to you !
>
> I also had a girlfriend once that had that effect sometimes.
>
>
>
>
>
> >> From another post, it appears it ought to be 'Insulate !
> >> Insulate !' I mean, what is this 1 1/2 " of styrofoam R-7 BS ? If
> >> you're going to insulate, do it right.
>
> >I would have had to have sprayed icynene foam on the underside of the
> >roof to improve there. I was limited in space on what I could use
> >above the roof for insulation, but I think I am going to have good
> >results because I am reflecting away the solar radiation in the first
> >place.
>
> >I got thermal mass on my side, the attic is the warmest just after the
> >sun goes down. Prior to starting up the AC, in the heat of the day the
> >sealed attic was one degree hotter than the ambient air.
>
> >Without the thermal mass I think the attic temp would have peaked
> >while the sun was shining at maybe a few degrees higher. The roof is
> >actually sloping concrete. I hiltied pressure treated 2x4s on the flat
> >so I would have something to screw the white metal roof to. I put the
> >foam in between the 2x4s on the flat was limited to 1.5", and it is
>
> Rafters ON FLAT ???? WTF ????
>
> --
> Click here every day to feed an animal that needs you today
!!!http://www.theanimalrescuesite.com/
>
> Paul ( pjm @ pobox . com ) - remove spaces to email me
> 'Some days, it's just not worth chewing through the restraints.'
> 'With sufficient thrust, pigs fly just fine.'
> HVAC/R program for Palm PDA's
> Free demo now available onlinehttp://pmilligan.net/palm/- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -



Posted by on June 28, 2007, 11:21 pm
On Thu, 28 Jun 2007 20:14:13 -0700, Abby Normal

>
>I sent you some pictures.
>
>I have been talking about what a regular attic is like then jumped to
>what mine is like.
>
>The 2x4s on flat are not structural. Something on top of roof
>structure that I used to fasten the white metal roofing too. Pitcures
>are a thousand words.

        Yep. Got 'em. replied.


--
Click here every day to feed an animal that needs you today !!!
http://www.theanimalrescuesite.com/

Paul ( pjm @ pobox . com ) - remove spaces to email me
'Some days, it's just not worth chewing through the restraints.'
'With sufficient thrust, pigs fly just fine.'
HVAC/R program for Palm PDA's
Free demo now available online http://pmilligan.net/palm/

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