Replies

1. 
   BillHartmann | Jan 20, 2009 08:05pm | #1

   "Just looking for a way to even out the temperatures without running the AC when it's warm, but not hot outside. Any input would be appreciated."

   You don't need to run the AC. If the duct work as been done properly, SPECIALLY THE RETURNS, then just run the fan.

   However, lots of cases ducts are done right so auxiliary systems are used.

   McDesign had a thread last summer where he made a fabric tunnel that hung down a large open stair well.

   .
   William the Geezer, the sequel to Billy the Kid - Shoe
1. 
   JTBfromPA | Jan 21, 2009 04:01pm | #2

   Yeah, I can just run the fan to circulate air, but that draws air from every return and blows it out of every supply. The idea behind the attic to basement fan is to specifically pull the hottest air out and dump it into the coolest place.

   Thanks for the post.

2. 
   MikeHennessy | Jan 21, 2009 04:19pm | #3

   I remember seeing it (tho' I don't recall it as "attic to basement", just hi to low) -- around the same time as McDesign's fan post, IIRC, if that helps pinpoint the issue no.

   Mike Hennessy
   Pittsburgh, PA

3. 
   Mongo | Jan 21, 2009 06:14pm | #4

   Is your attic completely finished already?

   When I finished off my attic (which started with open framing filled with FG batts and 6-mil poly over that), I put 2" of foil-faced polyiso on the rafter faces and 1" on the walls.

   Eliminating solar gain via the roof plane resulted in the attic temp dropping from 127 at the peak to 77 over the next couple of days. The shocker was the cooling off of the bedrooms on the floor below the attic over the next few days.

   I was going to add a couple of loops of radiant floor heat up in the attic, but didn't bother doing it. It's very comfortable in the winter. The coldest it gets is around 66, once a few bodies get up there or the lights get turned on it can even get a little too warm.

   On hot summer days it stays around 75-77. Since the air handler is up there behind the attic knee wall I pulled some air off to condition the attic in summer. Either fan only, or air conditioning, in case it gets a little stuffy.

   That was my attempt at attacking the cause instead of treating the symptoms.

   I'd be wary of pumping moist air from the basement into the thermal envelope of the house. I don;t recall the FHB article about tdoing that.

   I do recall Forrest's fabric pipe, I thought he used that in an open "great room", something to minimize stratification in a 2-1/2 story tall open space.

1. 
   JTBfromPA | Jan 21, 2009 07:22pm | #5

   Mongo,

   Thanks for the suggestions - that's a whole different direction than I was even thinking about.

   We are in the process of the addition, so we've got nothing but framing, sheathing, and paper on the roof.

   Are you saying your foil-faced insulation was between the rafters? Are you aware of a product to install under the shingles - seems like it'd be easier to install there (& more thorough).

   Also did you notice the loss of solar gain in the winter?

   Basically we have the old half of the house w/ finished attic. It gets crazy hot up there in the summer - had to add a window shaker to handle the load. In the new half, I'm trying to avoid that scenario. I believe the article called for pulling off hot air from the attic and pushing it into the basement, evening out the temps between those floors. To reverse that air flow could be a problem - bringing damp basement air into the attic would add to the summertime humidity issues and it fights the stack effect - point well taken.

1. 
   Mongo | Jan 21, 2009 09:02pm | #6

   At the stage you're at, you could even consider a couple of inches of closed-cell spray foam applied right on the underside of your sheathing. Then you can fill the remaining rafter bay with whatever you want, or even pack it with a few more inches of foam.

   An alternative could also be to add mesh, strap the rafter faces, and dense pack the full depth of the rafter bays with cellulose.

   The worst thing I've found (my opinion) is just fiberglass batts. Cellulose and foam do a far superior job of preventing heat transfer through the roof structure, into the attic, and eventually into the living spaces below.

   "Also did you notice the loss of solar gain in the winter?"

   Well, with the rafter bays full of only fiberglass, my attic was pretty darn cold in the winter. I don't ever recall measuring the temp, but it was painfully cold on bare feet when we went up there. We could see our breath at times.

   After I did the polyiso and drywall, the attic sat unfinished for the first winter, which was a cold one. It stayed in the high 60s to low 70s with no supplemental heat.

   When I originally built the house I set aside two loops on my upstairs radiant floor heat manifold for attic heat. I even had the pex on hand. But after seeing how comfortable it was that first winter, I didn't bother adding the rfh.

   No regrets.

1. 
   JTBfromPA | Jan 22, 2009 03:04am | #10

   Mongo,

   "At the stage you're at, you could even consider a couple of inches of closed-cell spray foam applied right on the underside of your sheathing. Then you can fill the remaining rafter bay with whatever you want, or even pack it with a few more inches of foam."

   OK, but what about venting for the shingles?"The worst thing I've found (my opinion) is just fiberglass batts."

   Totally agree. Great product under lab-like conditions, but the real world of insulating around elec. boxes, etc. and filling odd sided bays makes it not so great in reality. Takes a meticulous installer to get good results from fiberglass.

   My overall question for you is:  Do you attribute your thermal improvements in your attic primarily to good insulation, or do you feel the foil facing (fighting radiant gain) really made the difference? Sure they both helped, but do you feel the foil facing really helped a lot, or it just improved things a little more?

    

    

   Edited 1/21/2009 7:05 pm ET by JTBfromPA

1. 
   Mongo | Jan 22, 2009 04:35am | #11

   Why vent? You can or you can't. Your choice though. If you want to vent, then use channels and foam over those. Definitely recommend a closed cell versus an open cell foam though.

   I attribute the improvement to the higher effective R-value of foam over fiberglass.

   Air infiltration was stopped. No loss from air washing like you get with FG. No convective movement in or around the batts. Foam pretty much holds its R-value as temps decrease. FG? Not as well.

   No radiant gain in the summer. Foam blocks it. Energy on one side of a FG batt travles though the batt much more easily than through foam.

   I think the foil face provides a lesser contribution. I strapped the ceiling so the foil does have an air space in front of it, but I think foil-less foam would be almost the same performance-wise.

    

    

    

2. 
   BillHartmann | Jan 21, 2009 10:38pm | #8

   "To reverse that air flow could be a problem - bringing damp basement air into the attic would add to the summertime humidity issues "Not really.If you blow the air down from the attic then it is going to force the moist air up to the first floor and that air up to the second.After a period of it will all be homogenized.But blow the attic air down is probably slightly easier as it works with the natural stack effect.Also, in either case, you need to make sure that there a good path for the air going the other direction.Often there is a door on the basement stairs. Maybe it will need a grill cut into it..
   William the Geezer, the sequel to Billy the Kid - Shoe

4. 
   JAlden | Jan 21, 2009 09:11pm | #7

   I believe it was in a "Houses" issue. The home was a two story w/ basement in Asheville.

   The author pulled air to the basement from the second floor.

   I was looking at that issue recently. I can post the date when I get home.

   Justin?

1. 
   JTBfromPA | Jan 22, 2009 02:53am | #9

   Eureka - FHB #195 / page 57.

   Perfect, simple, just what I was thinking. Thanks for the help locating it!