PAV with Attic Tent

[Joe Mascitti]

Hello folks..I'm trying to get as much info as I can B-4 getting an Attic Tent. Over the past summer, there was a lot of negative comments about using PAV's. The main negative was it causing negative pressure in the home, either sucking in hot outside air in , or sucking cool air thru the attic stairway. With an attic tent, would that not eliminate the PAV's causing a negative pressure inside the home, thus making sure the PAV's only exhaust 100% attic air. Here in Atlanta, August was a very hot month, by shutting my PAV off, my upstairs remained cooler and the unit ran in shorter cycles. I know I have A LOT of air leakage around my attic pulldown stairway.

Thanks for your opinions and advice.

Joe

[skippedover]

PAV's are not recommended under almost any circumstances. The attic access alone is not the only source of treated air leaking into the attic. A PAV speeds the process. You've alrealdly stated that not running the PAV made the system more comfortable and by running shorter cycles, more economical. So my suggestion is to make the investment in the attic tent. Then disconnect the PAV once and for all. The attic tent will likely help even more and leaving the PAV disconnected saves on the electricity to operate it as well as making the 2nd floor more comfortable and the system more economical. You just need to get your head around the fact that a little hot air in the upper parts of the attic are NOT hurting your AC system, unless the return air ducts leak like a seive. That's a good reason to seal the return air ducts and while you're at it, seal the supplly side too!

[Joe Mascitti]

Makes sense to me...so many people say that PAV's are the way to go, but I guess they really do not understand the harm they actually cause...Will not break my heart to keep them off....

[skippedover]

Yeah, the ones who always tout them are the people selling them. We actuallly have a company in our are called The Fan Man and you only see the truck in the summer. There was, admitedly, a time and place for whole house fans. But that's long gone by methinks and selling such things as whole house and PAV fans is doing the whole world an injustice. Good for you for understanding the issue and doing something about it. Congratulations, you're in a minority.

[Swampfox]

Install a ridge vent and ditch the fan

[Joe Mascitti]

Every ridge on my roof has total length ridge vents...a couple of years ago, I was told to add a PAV for extra ventilation...at the time, I did not know they cause more harm than good...


Joe

[mark beiser]

Disconnect the PAV!

It may be ok to use it if you were to seal EVERY hole where wires penetrate the top plate of walls, seal the holes in and caulk every electrical box, seal the edge of every piece of drywall where it meets the top plates of the walls, seal the back sides of knee walls with insulative sheething, etc..

While these things are good things to do anyway, it ain't gonna happen for most people, and even so, several studies have shown that a PAV still wouldn't save you as much as it costs to run/maintain it.

[Shophound]

The theory behind a PAV is to reduce attic air temperature by drawing "cooler" air from outside the attic. It probably does this to an extent, but not without the caveats already mentioned in this post.

What it also can't do is reduce the radiant heating load eminating from the roof deck, unless a tremendous amount of considerably cooler air could scream through the attic, turning it into a virtual wind tunnel. The average PAV doesn't deliver that kind of velocity, so any amount of air it brings inside will be warmed by the solar heated roof deck undersides, rafters, and trusses.

Let's say on a 100 degree day, the underside of the roof deck is 140 degrees F. Optimally with a PAV, the ideal absolute best heat transfer to be hoped for would be a 40 degree drop from the roofdeck to the 100 degree entering air. In reality, the incoming 100 degree air will tend to rise as it absorbs heat from the 140 degree roof deck. If it is not quickly expelled from the attic, the rise will continue and lessen the amount of heat transfer available from roof deck to PAV induced air. As already stated, a tremendous volume of air would be required to reach the ideal attic condition of being at or around outdoor ambient conditions...it's not going to happen.

Solution? Reduce attic heat gain at the source...the roof deck! Radiant barrier, exterior shading via mature trees, soffit and ridge venting.

We had our "new" house sprayed with a spray-on radiant barrier this past autumn. When warm weather returns I'd like to place a remote thermometer up there that can send a signal down to a recording unit in air conditioned comfort...I'll want to track the average difference between attic and outdoor air temperatures to see how effective the radiant barrier is...only thing is I wish we had the house during the past summer without the barrier to act as a baseline comparison. But I do anticipate a reduction in heat gain, overall, and our a/c demand to be lower. Time will tell.

[eee]

Expect a 5% or less temperature drop from a power vent. What a waste of money! Spend electricity to do nothing, except make money for the power vent company and installer.

Ridge vent, if you must. But lighter color shingles would work better.

Seal up the pull down with the attic tent or equivalent.

Contact me if I can be of help.

eee