Possible Project - Seeking Some Feedback

[Deleted member 133]

I've been thinking of undertaking a small project but am having some doubts. (Ok, Gen's having most of the doubts and I'm now looking to address them.)

We had considered putting in a skylight above the main staircase, up through the second floor ceiling, through the attic, and opening onto the sloped roof. Not for the light (we get plenty from the upper hall window, and all the bedrooms that open to that area.) Rather for cooling / airflow.

On a day like today, where it's cool overnight but very hot during the day, we open the house up at night and air it out. Temp drops pretty good. But by mid-morning we close everything back up and put on the A/C, only to repeat the cycle just after sundown when it again cools down outside.

An open skylight would help create a "stack effect" - exhausting hot air up through the roof and drawing in the cooler outside air through open windows. All passive. The skylight we've looked at has a solar powered / battery backed-up motor with sensors that closes it if it senses any rain. My guess is that it will cost a pretty penny for the window and the contractor to install it - along with the mods to the house.

So I got to thinking. Instead of passive, why not go active? And instead of exhausting the hot(ter) air at the ceiling of the second floor right up through the roof, why not just blow it into the attic itself - said attic having great soffit venting and three high capacity roof vents?

And instead of having to cut a hole in the ceiling (and roof), why not just use one of the two existing attic "hatches" found in two of the guest bedrooms?

The plan would be to build a movable structure that would be kept in the attic. When summer comes I'd remove the attic hatch and replace it with a screen (to keep bugs and anything else from coming through.) On top of that would be fitted the "box structure" over the 18" x 27" opening. (That structure would remain in the attic. And the screen could easily be part of the box itself.) On one of the two 27" sides of the box would be fitted an industrial fan. See here for an example: http://www.industrialfansdirect.com/JD-VPES16.html.

This unit is pre-wired and I'd find a way to plug the cord into a socket in the bedroom. (The bedroom is unused almost all of the time. The design would allow me to change everything back to "normal" in the case we had guests and needed the room.)

The fan comes with metal louvres - only open when the fan is on. Another layer of pest control.

Even though the fan unit is billed as "smooth and quiet," and it would be mounted at 90 degrees to the opening through the ceiling, the box structure could be big enough to be lined with some sound absorbing material if need be.

I estimate a total cost in the range of $250 - versus upwards of $5,000 for the skylight. It is active (fan) and will therefore move much more air. This will in turn reduce our A/C costs during hot spells when it will be used.

It won't be pretty like a skylight; but it will be very functional (I hope).

I've not progressed to detailed drawings/ design. Just noodling the concept.

Any thoughts / feedbacks?

Much appreciated.

Jeff

 

[Wardsweb]

Your concept does work. When I was young, we lived in a house, on the coast, that did not have air conditioning. In the hallway ceiling, there was a huge fan covered with louvers that blew into the attic. A switch on the wall turned the fan on, the louvers opened and created an updraft in the house. With a few windows open and or a door or two with screens, the house was comfortable. Granted it wasn't over a 100 outside.

 

[Towen7]

I assume that the air in the attic is significantly hotter than the house during the day. Wouldn't that require the fan to run continuously? Otherwise heat from the attic would migrate to the house.

 

[PaulyT]

That's what the louvers are for, so that when the fan is off there's no passive airflow.

This sort of fan isn't so common around here, but I remember when we were in Augusta, GA doing habitat for humanity work while in grad school, we saw a bunch of them. That and walls painted with bright colors rather than boring white/beige; all the houses I've owned since then have been painted similarly (by me).

 

[CMonster]

Yeah, attic fans are pretty common here in the Midwest. The house I grew up in had one and our first house here in KC was framed/wired for one when it was built and I ended up installing one - it didn't work quite as well as I had hoped and we rarely used it. My memory is fading but I think part of the problem was it allowed cold in the winter to seep down so I had to climb into the attic to throw insulation over it, then climb back up to remove it in the spring; it just wasn't worth the effort so we didn't bother with one when we built our second house.

 

[lulimet]

Jeff, my brother has a roofing company in Toronto and deals with skylights on sloped and flat roofs on a daily basis. If you need an estimate or even just advice I can give you his contact info.

 

[Deleted member 133]

Thanks for the tips / feedback so far folks.

To address the points you raise...

During the summer, with the sun beating down on it, the air in the attic is considerably hotter. However our houses have "open" soffits and big vents mounted at the roof peak. Hot air rises out the vents and draws relatively cooler air in through the soffits. This keeps the attic temperature down (relative to what it would be otherwise) and greatly extends the life of the roof's asphalt shingles (which curl when their underside gets way too hot for way too long.) Even without a fan, if I open the attic "hatch" from the bedroom ceiling, it tends to draw air up into the attic. Hotter attic air cannot come down.

With the hatch closed the attic is sealed from the interior of the house. The attic is super insulated so if it is very hot up there, or very cold, the second floor ceiling changes very little in temperature throughout the year.

Theoretically, if the attic air is cooler than house air then the colder air would "fall" through the open hatch. But even as the house cools off at night during the summer, the attic air will almost certainly still be warmer than inside the house (with cooler air coming in through open windows.) That last bit raises a point: the fan would not be in operation when the A/C is running which usually runs when the outside air temp is quite a bit hotter than inside. If I actively exhaust inside air to the outside, that air must be replaced somehow. So 3000 cfm of relatively cool inside air would be replaced with 3000 cfm of relatively hotter air from the outside - making the A/C work even harder.

Since this would be a seasonal installation (ie. during hot summer months only) and perhaps even a monthly or weekly thing, it would be designed for quick set-up / use / storage - perhaps with the hatch being on or off (depending on design) being the only thing that needs to move.

In addition, it's use depends on there being cooler evening / nights. Right now it's stinking hot day and night and the A/C is turned on all of the time. The attic fan would not be used in this case. However last week it was hot during the day (A/C on) and cooler at night (A/C off). At night we'd open all of the house windows (assuming no rain in the forecast) and by morning the house was nice and cool - much cooler than the A/C's temp set point. But that was because there was a pretty good breeze blowing lots of air through the open windows. The attic fan would augment this and be the breeze when there's none outside.

In my case the louvres would be there more to prevent animal / bugs from coming down into the house through the open hatch. A screen where the hatch opening is will provided a second layer of defence.



Keep the feedback coming folks. Would much rather have someone alert me to a potential problem that I did not consider, sooner rather than later!

Jeff

 

[AndySTL]

We have an attic fan but we rarely use it. I have seasonal allergies that make me hesitant to use it when the outside temps make sense. About the only time we use it is if we burn something in the kitchen and need to clear the smoke.