Do power attic fans help?

[echo8287]

One thing I don't think anyone mentioned is your eave vents can be blocked by your insulation in the attic. Even if you have continuous eave vents. If you don't have blocking they make styrofoam baffles that you can slide into the eaves and staple to the underside of the plywood roof. This will not let the insulation block the air flow. David

[Joe Mascitti]

Hey Miniboy...yes I have soffet vents and made sure they are not blocked by the insulation...105 is rare for us in Atlanta, on a normal summers day around 90-95, the attic temp is right around 100-105

I few more soffet vents probably would not hurt...


It would be interesting to look in the attic with and infrared detector on a hot summers day...

[MiniBoy]

One thing I don't think anyone mentioned is your eave vents can be blocked by your insulation in the attic. Even if you have continuous eave vents. If you don't have blocking they make styrofoam baffles that you can slide into the eaves and staple to the underside of the plywood roof. This will not let the insulation block the air flow. David

David:
As I mentioned in an earlier post, my recollection is that the soffits are not blocked with insulation .. whoever insulated did a neat job and the bat insulation is cut and fitted just to the top of the exterior walls .. although I am going to confirm this on Monday. In the event of blockage, I had the same idea to use the styro baffles as the easiest way to open the rafter bays to the soffit.

Russ

[MiniBoy]

Hey Miniboy...yes I have soffet vents and made sure they are not blocked by the insulation...105 is rare for us in Atlanta, on a normal summers day around 90-95, the attic temp is right around 100-105

I few more soffet vents probably would not hurt...


It would be interesting to look in the attic with and infrared detector on a hot summers day...

Joe:

Thanks for the feedback.

Your soffit to ridge convection seems to be working well, even with the presence of passive gable vents, IMO. If I can achieve a 10 degree higher attic to outside to inside temp, I will be very satisfied. Here in New England we are talking max outdoor temps of 95 degrees.

I do have an infrared detector, and had a lot of fun in some un-insulated basements this past fall / winter. Most folks don't realize that an 8 in cement block foundation wall has about the same R value as a sheet of 1/4 in. plywood. I will certainly be using the infrared detector in attics this spring / summer.

Russ

[Shophound]

Shophound:

Thanks for the reply ..

Being from the "Old School", and reading a couple of these threads, my thinking has changed regarding the PAV route. The intent of this job was to lower the attic temp in the summer, to reduce the A/C load. I have moved away from the PAV theory to the passive thermoconvection, as it won't use more energy and require fan and control maintenance. I have a couple of issues on this job .. cutting in soffit vents seem inevitable, but I am not clear on whether the gable vents could not be left to be passively functional, rather than block them up, as one contributor has suggested. They are at least 2 ft. from the ridgeline, and it would seem that the soffit to ridge vent "chimney effect" would convect a goodly amount of the hottest "source heat" from under the roof deck between the rafters out the ridge vent, while the gable vents would allow a certain passive cross flow of cooler outside air closer to the floor, on a breezy day at least. However, I have to recall a phrase from another poster on this thread .. and this is not a direct quote .. if air did what we would like it to do, or hope it would do, life would be easy . So, I am mindful that what we hope and want to happen is not necessarily what does happen ! For certain, the idea of sucking conditioned air from the living space to the attic is self defeating, to say the least !!

I need to go back and do a little more investigating .. my recollection of the job indicates no blocking between the rafters at the top of the eaves walls, as well as the bat insulation appears to end at the top of those walls, and the rafters do NOT have birdsmouth cuts at the top of the wall, rather sit on top of the wall, so there should be a couple of inches of space between the top of the insulation and the bottom of the roof sheathing .. but this has to be confirmed the first of the week.

Thanks for your taking the time to post .. appreciate your input .. it's 1:00 A.M. here in Massachusetts .. goin' ni-nites ..

Russ

Re: PAV's...we learn by experience. Often an idea is conceived and executed with sound reasoning, and a PAV sure sounds good on paper. Alas, the devil is in the details, and with PAV's, demons of air pressure differences abound.

Re: gable end vents remaining when ridge and soffit venting are also in place...my take is that if the ridge is seeing a good amount of heat escaping it, enough to cause the entire attic air pressure to go negative in respect to atmosphere, all openings below the ridge will draw in ambient air. Gable end and soffits will both draw when the ridge exhausts.

A caveat would be a gable end vent that faces prevailing winds. In my area, that direction would be from the south to the north. Although wind forcing air into a south facing gable end vent might seem beneficial, the caution is when the attic is not well sealed from the house interior. The wind will pressurize the attic space and force superheated air into the house through ceiling penetrations, wire chases, etc. Overall I don't think facing a gable to the south in a hot climate is a great idea, due to higher solar exposure such a facade would receive in summer. It can't always be avoided, understandably.

Re: adding soffit vents to a house not currently with them installed. We already discussed one aspect of ensuring clearance between top plate and roof deck; lack of blocking or holes drilled in blocking. The next item to check, sans blocking, is that insulation does not obstruct these passages. The insulation will still pass air even if it does block the passage, but at a reduced rate (which also nullifies the R value of the insulation at that location). There are baffles that can be installed between the rafters at the top plate that will keep insulation out of the passage and direct air toward the ridge from the soffits. Home center stores sell them.

[Jopopsy]

Okay, so now I'm depressed. My new home has a attic fan already installed. It has a ridge vent and soffit vents all along the bottom of the roof overhang (its a new construction home).

So, since its already installed, do I run it or do I just unplug it? I thought I was being given a solid by having this thing installed for me. Now I think I have a mushroom covering a hole in my roof ...

[platchford]

Okay, so now I'm depressed. My new home has a attic fan already installed. It has a ridge vent and soffit vents all along the bottom of the roof overhang (its a new construction home).

So, since its already installed, do I run it or do I just unplug it? I thought I was being given a solid by having this thing installed for me. Now I think I have a mushroom covering a hole in my roof ...

Heh, you think you feel bad... I'm the one that cut the hole in my roof and installed mine.

[Shophound]

Okay, so now I'm depressed. My new home has a attic fan already installed. It has a ridge vent and soffit vents all along the bottom of the roof overhang (its a new construction home).

So, since its already installed, do I run it or do I just unplug it? I thought I was being given a solid by having this thing installed for me. Now I think I have a mushroom covering a hole in my roof ...

If you're REALLY attached to that PAV, you can go above and beyond the average and seal the crap out of every ceiling in your house that has attic space over it. Can lights, supply boots, fart fan housings, etc. Even then, that's no guarantee your PAV won't suck conditioned air from the house when it runs. It'll just suck less.

[MiniBoy]

Re: PAV's...we learn by experience. Often an idea is conceived and executed with sound reasoning, and a PAV sure sounds good on paper. Alas, the devil is in the details, and with PAV's, demons of air pressure differences abound.

Re: gable end vents remaining when ridge and soffit venting are also in place...my take is that if the ridge is seeing a good amount of heat escaping it, enough to cause the entire attic air pressure to go negative in respect to atmosphere, all openings below the ridge will draw in ambient air. Gable end and soffits will both draw when the ridge exhausts.

A caveat would be a gable end vent that faces prevailing winds. In my area, that direction would be from the south to the north. Although wind forcing air into a south facing gable end vent might seem beneficial, the caution is when the attic is not well sealed from the house interior. The wind will pressurize the attic space and force superheated air into the house through ceiling penetrations, wire chases, etc. Overall I don't think facing a gable to the south in a hot climate is a great idea, due to higher solar exposure such a facade would receive in summer. It can't always be avoided, understandably.

Re: adding soffit vents to a house not currently with them installed. We already discussed one aspect of ensuring clearance between top plate and roof deck; lack of blocking or holes drilled in blocking. The next item to check, sans blocking, is that insulation does not obstruct these passages. The insulation will still pass air even if it does block the passage, but at a reduced rate (which also nullifies the R value of the insulation at that location). There are baffles that can be installed between the rafters at the top plate that will keep insulation out of the passage and direct air toward the ridge from the soffits. Home center stores sell them.

This is the easy part .. except for venting the very complicated soffit structure .. the good news is that there is only 26 ft. on either side of the building.

Hounder: I'm doing an edit experiment .. bear with me .. It didn't work .. 1/2 hour of typing down the drain !!!

O.K. !! I got it ( above in red ) .. editing a quote is a bit tricky !! .. it's Ni-Nite time here in the sticks .. will report back tomorrow after building investigations are completed .. thanks for your input,

Russ

[printmanjackson]

If you're REALLY attached to that PAV, you can go above and beyond the average and seal the crap out of every ceiling in your house that has attic space over it. Can lights, supply boots, fart fan housings, etc. Even then, that's no guarantee your PAV won't suck conditioned air from the house when it runs. It'll just suck less.

You could install a avariable speed control so it won't pull so much. If you do, just make sure it has enough amps to cover the motor. The ones for most ceiling fans are not enough and don't use a light dimmer.

Here is one I found at a fair price:
http://electrical.hardwarestore.com/...ch-239053.aspx

[printmanjackson]

Okay, so now I'm depressed. My new home has a attic fan already installed. It has a ridge vent and soffit vents all along the bottom of the roof overhang (its a new construction home).

So, since its already installed, do I run it or do I just unplug it? I thought I was being given a solid by having this thing installed for me. Now I think I have a mushroom covering a hole in my roof ...

Most new homes have a switch in the attic that will turn off your PAV. Thay way it's easier to change out the motors. Just turn it off and cut a piece of 1/4" plywood to cover the hole.

[Jopopsy]

If the major reason for not running a PAV is b/c the average home is not sealed tight enough to thwart conditioned air being sucked from the house and into the attic, could then a argument be made to only run the PAV when the house A/C is NOT on?

In other words, if its warm enough to heat that attic but the homeowner chooses NOT to turn on the AC that day, would turning on the attic fan mitigate some heat transfer from the attic to the living area? Its not like you are pulling conditioned air that you paid to maintain into the attic ...

???

Also, for the life of me I cannot understand how the PAV can create such a pressure differntial as to suck air from the living areas. Between the ridge vent, gables, and eaves of a newly constructed home, how can that large a number of openings to the outside not be sufficient to faciliate a air exchange between the PAV and the outside?

[Shophound]

You could install a avariable speed control so it won't pull so much. If you do, just make sure it has enough amps to cover the motor. The ones for most ceiling fans are not enough and don't use a light dimmer.

Here is one I found at a fair price:
http://electrical.hardwarestore.com/...ch-239053.aspx

Where would you mount that rheostat device? In the attic? Rheostats (such as a light dimmer) generate a lot of heat when in operation. They need an adequate heat sink to disperse that heat or they'll cook to death. If the rheostat is installed in the house, room temperature is often sufficient to cool stats that are used for light duty. Have you ever seen commercial installations of rheostats/dimmers, such as in restaurants or banquet facilities? They have finned heat sinks on the surface of the wall, and those sinks get HOT. And that's just for dimming the lights in a restaurant. I couldn't imagine sticking a "dimmer" on a PAV in an attic, even if the thing was rated for it. If I put it in the house, that rheostat is going to add heat to the house...just what I don't need.

If your heart is set on a PAV, get a solar powered one and seal the crap out of your ceiling adjoining attic spaces.