Venting heat with clerestory windows

dano605August 11, 2014

I am planning a home and I would like to vent excess heat that builds up through the clerestory windows at the top of the ceiling. If the windows open at the bottom the heat would not rise out the opening. If the windows open at the top the heat would rise out the opening. The problem is that if the windows open at the top, the roof overhang would block the heat flow up. I would like an overhang that would shade the summer sun. Any thoughts.

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The way the windows open will not diminish the amount of air that escapes because the warm rising air will create a higher pressure inside than outside. What will reduce the air movement will be wind pressure so locate the windows away from the prevailing wind.

    Bookmark   August 11, 2014 at 10:25PM
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Dano605 you raise a good point about natural ventilation and energy efficient design.

One of the keys to effective passive energy design and ventilation is to take advantage of the natural breezes of the site. A room that has a bank of windows on two (preferrrably opposite) sides will allow effective cross-ventilation, if one of the banks of windows is facing the direction of the prevailing summer wind flow.

Using low operable windows (awning windows for example) for the cool air to enter and high windows (hopper or awnings will both work) on the opposite side of the room for warm air to exit enables a constant flow of natural air, cooling and ventilating the space.

Ceiling fans may be used to accelerate the air flow.

When it comes to shading, effective overhangs can be designed for southern exposures, which block the sun in summer, while allowing the sun to penetrate in winter. To achieve the proper overhangs, one only needs to know the latitude of the building site and use readily available sun angle charts for the four seasonal solstice periods for the given latitude. There are many sources for this data; most architects simply use their Architectural Graphics Standards book. This reference book may be found in many libraries for use by non-architects.

Overhangs for solar control on north facing windows and walls are really unnecessary in the northern hemisphere. Eastern and western exposures are almost impossible to design effective overhangs for solar control due to the low sun angles that occur on these exposures.

This is why orienting the major glazed face of any building to the south, while minimizing eastern and western faces of the building, is always a desirable strategy. This is also why architects, given a choice, will always recommend property which allows for south-oriented buildings.

Hope this helps. Good luck on your project.

    Bookmark   August 11, 2014 at 10:28PM
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Great advice from both resident architects here. Clerestory windows are likely to contribute to LESS energy efficiency unless they face south. The reason being that they contribute to more house volume, surface area, air leakage and possibly unwanted solar gain.

There is also the issue of how to open the windows. On the home below we installed a motorized opener on the center window just to see how it worked before making the investment for all 3. It works great but I have not heard if the homeowner likes the performance of the venting enough to install the other two. I want to say the price of the motor was around 300$.

The decision to go with the clerestories here were more about aesthetics than performance but they do face south which helps. Still, unless the south facing windows are High SHGC and significant square footage, clerestory windows are more likely to hurt overall energy performance because of the building envelope interruptions.

Its common for designers to make recommendations on prevailing wind directions but I feel the path of the sun largely rules out those considerations most of the time. If natural ventilation is important, use casement windows. Locating them opposite interior doors can help ventilation and feelings of spaciousness when entering a room. Usually these measures are more than enough for ventilation and clerestories are unlikely to make a measurable difference other than the energy losses for the times of year one is not ventilating.

I usually recommend people scale back their plans on the amount of operable windows too. With casements, it really only takes one medium size operable window per room to provide adequate ventilation in favorable weather. Fixed windows have lower upfront costs and lower ongoing energy and maintenance costs.

Here is a link that might be useful: Passive solar design

    Bookmark   August 12, 2014 at 11:27AM
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The problem is that if the windows open at the top, the roof overhang would block the heat flow up.

No - the warm air will flow out and around the overhang and continue up.

    Bookmark   August 13, 2014 at 5:32PM
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Thanks for the info . I was trying to create a chimney effect by bringing in cool air from from the north vented crawl space, pulled in from the hot air escaping at the peak where the the windows are located on the south side. Just wondered if anyone has tried this.

    Bookmark   August 14, 2014 at 7:05PM
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Whoa there, did you say vented crawlspace.. ? That sounds like a bad idea. What climate are you in dano?

    Bookmark   August 14, 2014 at 8:44PM
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Use windows low down on the north side and windows high on the south

Running air out of the crawl space is NOT a good idea.

    Bookmark   August 14, 2014 at 8:48PM
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Vented crawlspaces are not a good idea in general!

    Bookmark   August 14, 2014 at 9:19PM
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What I was thinking of was building the cabin on piers and insulating and sealing the floor and pulling the cool air from under the home with an opening to the north side and skirting the rest of the opening. Crawl space was maybe not the right term.

    Bookmark   August 15, 2014 at 10:19AM
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There is probably little measurable difference between the outdoor air surrounding your home whether it be on the North, South or underneath (on piers with no crawl).

I feel your planned passive cooling strategy will make little difference compared to opening windows on opposite sides of the house and keeping interior doors open. This strategy can be improved by "night time flushing" where you open things up at night, possibly assisted by an inward blowing fan in the bedroom, and closing things up tight in the morning. This strategy works best with an airtight and continuously insulated home.

Tough to offer opinion/advice with no idea what climate youre in though. Unless are near a risky water body, I would reconsider the pier foundation. They are tough to airseal and insulate and you remove the most affordable form of thermal mass from your home, slab on grade/stemwall. Having some thermal mass would probably help with night time flushing strategies.

    Bookmark   August 15, 2014 at 11:27AM
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