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Breezes, how do they work?
July 5, 2014 10:51 AM   Subscribe

Please explain to me how to maximize natural ventilation in my house.

Circumstances: Los Angeles, the hot part. Today's forecast is for 85 degrees in the next hour, high of 89 at 4pm. Humidity is currently 33%, though it is disappointingly still. Indoor temperature is about 79, but thanks to fans and the low humidity and a pretty good shade barrier all around the house, we're both very comfortable in shorts right now.

House faces North, there are double front doors* and a sliding glass door directly opposite them on the south side, across an almost open plan high-ceilinged common area. Bedrooms along the east side have windows, master bedroom only has south windows. West side of house only has one window and it's right outside the neighbor's window so we never open it. All windows are shaded more or less appropriately, all are side-sliders and generously-sized (though that mostly means they're taller than fans). We rent and are on a tight budget, so major modifications are not an option.

We have central air, but it's pretty lackluster especially right at the hot part of the afternoon and on real muggy days. We own at least one of every kind of fan (except the 20" box, but I can remedy that in 10 minutes) - stand, clip, window-fitting, small box, tiny usb. No ceiling fans. There is an attic fan that seems to be doing what it's supposed to.

Normally, we just go outside around 5-6pm where we have a television and can plug in a fan if it's especially humid and we're fine. The temperature starts to drop as soon as the sun is gone, though this is the time of year when it only drops to the upper 60s by 4am rather than cooling off very fast to an appreciably chilly temperature. By the time we come in to go to bed around 10-11 the bedroom, at least, is usually livable with a fan.

But yesterday was humid, and we had to come inside with a couple of fireworks-traumatized dogs and it was gross in the living room but it was also pretty damn miserable in the bedroom so that we brought in extra fans and wet washcloths, and it didn't really get sheet-on-feet cool until after 2 even though it was in the mid-60s outside. (All through the evening the thermostat said it was in the mid-upper 80s in the house.)

I kept futzing with the windows, but I can't figure out best practices for the later half of the day. Should I close all but one window on the north side and one on the south (but in the bedroom there's no same-hallway north window because the front northeast corner is the garage)? Open them all? Open half of them and put fans in strategic places? Put a bed in the yard?

Note: the advice about opening all the windows in the morning and then closing them works fine through the early afternoon, but 4ish pm really seems to be the breaking point no matter what I do. Which I can live with for a couple of hours, so long as I can cool it off after dark.

*Difficulty: dogs who run away. I can build a barricade, but I probably can't rely on one full-time.
posted by Lyn Never to Home & Garden (10 answers total) 8 users marked this as a favorite
 
[PS: I am not claiming that 89 is actually bad but we have 100-degree days to look forward to soon and I need to have a system by then.]
posted by Lyn Never at 11:03 AM on July 5


You asked about natural ventilation, but you haven't mentioned anything about prevailing winds and you listed the fans you own, so I'm going to answer as though you wanted to know about powered ventilation.

As soon as it's cooler outdoors than inside the house, put fans in the windows to blow air out. They will work more efficiently if you cover up any areas not occupied by fan with cardboard. All your fans sound kind of small so you might need more fans or different fans. The "in window" fans that are a few inches wide don't move very much air. I use the 20" box fans, they usually cost about half as much as the "in window" fans and work better. Open other windows or doors to let cool air in.

Think about how the air will flow through the house and what areas you especially want to cool off. You might need to open or close different windows for more or less airflow through your house.

If you can't figure out if air is coming in through a window or not, add more fans in windows.

I guess what the attic fan does is to blow hot air out of the attic? If you have a hatch into your attic, open it and let the house air provide replacement air into the attic.

Leave the fans on until the outdoor temperature is equal to the indoor temp. If it's colder than you would like all the better, it will help your house stay cool through the day and not be as hot when you start the cycle over again the next evening.

It also helps to keep the house as cool as possible by not using the oven, turning off lights, using LED bulbs in lights, etc.

we have 100-degree days to look forward to soon

If it still cools off a fair bit at night you can cool your house off, but if it's going to be 90 at night consider other plans.
posted by yohko at 12:32 PM on July 5


Is installing a supplemental, window-based a/c unit in the bedroom an option? My friends who live in your part of L.A. have basically designated one room of their home a Cool Room and throw all their weight into making it so. They bunker down in there with pets during the summer months. (The pets are of course secretly thrilled to be allowed in the Two-Legged Animal Sleeping Room for those four months).

I have no a/c, and though I live in a relatively chillier part of L.A., over the years I've developed a daily dance of moving a series of reflective window shades (the kind for car windows) around my windows to follow the sun's angles. I keep the windows open, but block the glass part with the shades (silver side facing outward). And my Vornado fans follow me from couch to bed to kitchen.

September is often the cruelest month in SoCal -- high temps and monsoonal humidity and fires fires fires -- so A/C units fly off the shelves around then, so a pre-emptive purchase might not be a bad thing.
posted by nacho fries at 12:50 PM on July 5


I open all of the windows wide overnight, with all of the curtains up and blinds open. The following morning, all windows are closed and all blinds drawn and curtains closed, especially on south facing aspects. As soon as it starts to cool down outside, windows and curtains/blinds are reopened.

Part of the trick is thinking that there are different kinds of heat. There's radiant, like radiators give off and what you feel from sunlight, and convected, like hot winds. Having something white, like a duvet, pressed tight against the window really helps, because it forms a wind-tight and light-tight seal. If there's light coming in, there's heat coming in too. If there's air coming in, there might be heat coming in, depending on the air temperature. A shaded area away from the sun is likely to be cooler. In the northern hemisphere, that's likely to be a north facing aspect.

I've never really had much success with fans. Unless they're cooling the air somehow, like air conditioning, they're just blowing hot air around and the wind chill factor is pretty minimal. It feels a bit cooler, but any heat energy that gets blown off you is just going to end up in the room. Unless you're venting the energy somewhere, just blowing it around won't make much difference. For a fan to work properly, there needs to be a pressure difference. For air to be blown, there must be air to be blown. If you have a window fan at the back of the house blowing out, you need to have one at the front, preferably in line of sight, blowing air in.

With regards to windows, only open them if the air you're allowing in is cooler than the air in the house. I have a sheet of 3mm perspex over one south facing window. A thermometer on either side of the sheet has shown nearly a 10°C difference in temperatures, with the curtains closed. The room does get somewhat fuggy by doing this, as there's obviously no airflow. It's easily removed though - I use magnets to hold the perspex against the window surround, giving a 6" buffer space, like double glazing. No need for screws or anything damaging like that.

You could even make up one of these if you only want to cool a smaller area. You'll heat up the room your fridge is in a little, but you'll get some pretty cheap cooling. I tried it and had to put a drip tray underneath after a few minutes as humidity in the air was condensing onto the coil and dripping onto the furniture.

Can you control the areas that the central air is trying to cool? It might be better to have one well insulated room that's blasted with cold air that you can use as an escape, than to have lots of rooms where you're fighting an uphill battle.
posted by Solomon at 12:50 PM on July 5 [1 favorite]


They key to breezes is that there has to be a way in and a way out. You also want to make sure your way in isn't from the hotter side of the house (why bring in sunbaked air that has flowed over concrete and asphalt to the shade of the house?).

Also - fans actually increase the temperature - the mechanical heat plus the air friction - you feel cooler with a fan because of evaporative cooling - the air blown over you picks up sweat so you get skin surface cooling - they definitely don't lower the temperature. Too many fans can overwhelm the evaporative effect by adding to much heat (you only have so much sweat). The only time a fan will make things cooler is when they are moving air from a cooler location to a warmer location either directly or by moving out warm air so cool air can replace it.
posted by srboisvert at 12:50 PM on July 5


Blinds closed during the day to reflect the heat. Especially on the hot side. You are banking "cool" with the mass of your building.

Hot of course rises so if you use a fan have it blow in from the bottom and out from the top when the outside temperature is cooler than the inside temperature.

"Should I close all but one window on the north side and one on the south?" Yes. This will allow you to control ariflow.

I lived south of New Orleans for a year without AC or heat.
posted by vapidave at 3:12 PM on July 5


When the air outside is cooler than the air inside aim your fans out. You lose evaporative cooling, but you gain cooler outside air. Evaporative cooling only helps when you're sitting in front of the fan or feeling the air move.

The preferred homemade AC unit is the Home Depot bucket. That'll chill down a room until you can cut things with your nipples!
posted by 26.2 at 4:34 PM on July 5 [3 favorites]


BTW: AC serves two functions: cooling (duh!) and "wringing out the air." When you're letting in the cool morning air you're also letting in the higher morning humidity. Have you ever been in an office building that was uncomfortable in the morning and overly cold in the afternoon? Sometimes that's because the AC is set to use 100% outside air if it's at the correct temp, but doesn't adjust for humidity. So in the morning, the air is flowing into the building in all it's humid glory. (It also happens if the monitoring/control sits in the path of a window and gets blasted with morning sun.)
posted by 26.2 at 4:39 PM on July 5


This is probably a much more involved than you want, daytime-only solution but: solar chimney

A couple of my grade school years were spent, in Northern New England, in a 100-year-old school building whose designers appeared to have been either unaware of or overenthusiastically devoted to this effect. The central hallway on the top floor of the building would be like a wind tunnel during the final days of the school year in the summer. In the classroom at the end of the hallway, on the shade side of the building, the pressure differential could get high enough to blow the door open and thunderously slam it against the wall if it wasn't locked, on one occasion forcefully enough to shatter the door's half-pane of tempered safety glass.
posted by XMLicious at 5:43 PM on July 5


Thank you all for your answers, as collectively they helped guide me toward a procedure that is working pretty well. I actually did not come up with the final best configuration until a couple of nights ago because I'm dumb.

For posterity, this is how we do:

We shroud up the house around 10am and let the A/C come on at 75. This makes most of the workday pretty comfortable. As 26.2 pointed out, it's the humidity too, and that dries the house out for a while. (It also freezes up eventually and spends all night thawing out, which we are bugging the landlord about, but I've been a landlord and I know where my priorities were w/r/t HVAC.)

On very hot days, we just give up around 5 and go sit on the patio, which has been shaded all day, with a fan on. It's more comfortable out there than inside.

(And at the time I wrote this question, I was sure we were never going to use our sad weird little spa pool without the heat on. That assumption was extremely incorrect, and there's nothing like getting your core temperature down in armpit-depth water to make life bearable.)

At night, the show belongs to this window fan. It makes hours worth of difference in cooling off the bedroom. I'd been running it in another room and using a weaker fan in the bedroom, and that was dumb. It draws in that cool night air fast, which is all I wanted.

I'm sure a 20" box fan would do it too too, but our windows are weird. And they're side-sliders, which means the window fan goes in vertically and blows directly on me and also over me to my husband's airspace. I used this window fan 300 nights a year in our old house that had no air conditioning, and I don't know why I didn't realize I can't live without it. I'm going to get another for the guest bedroom.
posted by Lyn Never at 6:13 PM on September 9


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