Creative Solution For Hot Attic Room?

[jyl]

I want to get your thoughts on a way to cool my 3rd floor family room.

For background, it is a 100 y/o house. Basement, main floor, second floor bedrooms, third floor attic finished to be family room. Central a/c was installed about 10 yrs ago, it works well on main floor and okay on second floor, doesn't do much on third floor because the heat load is higher (roof, lots of skylights) but even more because there is only one little a/c duct there. (Retrofitting central air into an old foursquare house is hard, very little place for ducting.)

It only gets to be a problem when it is >85F outside, and that's only a few weeks or a month in the summer, so I've basically put up with it. During those periods, my third floor family room is just a hot box. Tant pis.

I've looked at various options, of course, and they all have big problems. Reroute and expand the duct? Not gonna rip the interior of the house apart. Better insulate and vent the roof? Sure, when its time for a new roof, which is not even close. Install a big window unit just for the third floor? Ugh, I hate seeing those things hanging out of otherwise pretty houses. Install a ductless split system just for the third floor? Too expensive for what's not a major problem, and I don't want even more of my minuscule yard to be occupied by the compressor. Put a free-standing a/c unit in the family room, venting out one of the windows? Maybe, maybe, but it is a 600 sq ft room, I'm skeptical that those units have enough oomph . . .

So, here is a possible creative solution I thought of. Maybe its not a great solution, maybe it is more of a temporary fix until it is more convenient to do a more major project, maybe it is a good and permanent solution, maybe it is a totally stupid idea . . .

The idea is to run a duct up the exterior of the house, like a 5" x 12" rectangular duct, painted house color, from basement to third floor, and use an inline fan to move cool basement air to the hot attic.

My logic is, the central a/c has enough power. *On the hottest day (>100F) I can easily keep the 1st floor 70F and the basement is <70F. *(The basement is cool due to earth insulation and also due to the uninsulated a/c ducts in the joists, there are a couple of registers in the basement too but I keep them closed.) *The problem is simply that there is not enough airflow to the 3rd floor/attic. *Only about a 21 square inch duct (3" x 7").

The attic is roughly 600 square feet and average 8 feet ceiling so say 4800 cubic feet volume. *Suppose I want to replace hot attic air with cool basement air 1 time per hour. *Means I need to move 1.3 cubic feet/second. *Suppose max acceptable air velocity in duct is 3 ft/sec (limited by friction, noise). *Means I can use a duct with 0.43 sq ft cross section. *(Compare to 0.16 sq ft of current a/c duct). *That is about 8" diameter round duct, equivalent rectangular duct is 5" x 12".

I would need that duct to run from basement to attic, on exterior of house, preferably north side (cooler), with a fan at the basement end (less noise in attic). *Ideally it would turn on when temperature of attic exceeds some level AND it also exceeds temperature of basement by some specified amount - or I could just control it manually.

What do you think?

[look 171]

John,

In the past, we have done this for our clients. We install a bath type fan in the ceiling or attic to move the stagnate air near the highest point of the ceiling on the second floor. We use an inline remotely mounted fan up in the attic for this job. I use only Fantech or Panasonic because they make a really good product. It hooks up to a 4 or 5 inch vent. Paint it to match the ceiling. Depend on the size of the fan, it can draw air from three or four locations. What you need to do is to remove the dead air into the attic. The air in the attic is much hotter then the air trapped under the ceiling. So you kill two birds with one stone. I would also consider having an attic fan install just to get the hot air out of there. If you to do this, make sure you install timers so the motor will shut off so it wouldn't burn itself out. You WILL forget to shut them off. They are very, very quiet. You will not hear them. I install lots of them in people's bathrooms. In my own home, I have a big Fantec drawing air through two small 4" vents neat the ridge in my family room. It works pretty well. Heck of a lot cheaper then a mini split system.

[mepstein]

There are also booster fans that fit into the a/c duct so more of the cool air gets sent up to the attic. Push more cool air in and get rid of the hot air surounding the room and you should feel better.

[john70t]

If the house is meant to be a sealed system, then adding a return air vent will pull the hot air from the attic.
Even running the recirculatory fan without the a/c will equalize the house.

You can insulate the roof better without too much mess. There is slow-rise open-cell spray foam that can be squirted through 1" holes in the drywall to insulate the interior.
It will push aside or seal the existing fiberglass batt.

Be careful the soffit->peak ventilation is adequite, or the shingles will fry and/or the plywood rot.
When re-roofing, use a lighter color shingle also.

[Jim Bremner]

I had a 1,600 CFM fan that was set on a thermostat in my garage. The garage roof was in the sun pretty much all of summer and the roof peaked at 19'

on a 100* day the garage temp at 6' off of the floor was 120* and at the peak was 135*

The fan was set to turn on at 110* at the peak. it would remove the hot air up top and draw air from ground level vents.