Air flow vs. heat flow
August 21, 2020 8:52 PM Subscribe
Even with the windows and curtains closed, my house warms up during the day and cools off at night. How much of this is due to air exchange with the outside and how much is due to heat conduction? How could I go about estimating this?
Hi, I'm in California (how did you guess?). We've just had a heat wave and now we are trying to keep our home sealed against smoke from wildfires, which got me wondering about this question.
Obviously some air is getting exchanged with the outside, or else we'd suffocate. But airflow can't be responsible for all the indoor temperature variation, or else it would be as smoky inside as outside after a few days (a state of affairs which has mercifully not come to pass). I'm wondering if there is any simple experiment I can do to estimate the relative effect size of air flow vs. heat flow in regulating our indoor temp. I have multiple thermometers if that helps. If an experiment involves opening the windows, I'll wait to do it till the smoke passes, but I'm still curious to learn something for next time.
This question is pretty much driven by pure scientific curiosity, though I suppose it might have applications -- if opening the curtains at night will help move heat without moving more air, that would be great to know! (No A/C here, sadly.)
Hi, I'm in California (how did you guess?). We've just had a heat wave and now we are trying to keep our home sealed against smoke from wildfires, which got me wondering about this question.
Obviously some air is getting exchanged with the outside, or else we'd suffocate. But airflow can't be responsible for all the indoor temperature variation, or else it would be as smoky inside as outside after a few days (a state of affairs which has mercifully not come to pass). I'm wondering if there is any simple experiment I can do to estimate the relative effect size of air flow vs. heat flow in regulating our indoor temp. I have multiple thermometers if that helps. If an experiment involves opening the windows, I'll wait to do it till the smoke passes, but I'm still curious to learn something for next time.
This question is pretty much driven by pure scientific curiosity, though I suppose it might have applications -- if opening the curtains at night will help move heat without moving more air, that would be great to know! (No A/C here, sadly.)
I was also wondering if for some paths of air flow the smoke is getting caught - in the filter or in the insulation or other materials that act as filters. You know just like you can breathe through a dusk mask but the smoke mostly gets excluded.
posted by metahawk at 9:55 PM on August 21, 2020
posted by metahawk at 9:55 PM on August 21, 2020
The heat transfer is largely via radiation, and then internally via convection. I would imagine the primary air flow occuring is between areas of the room - radiation through the windows allows surfaces to heat up, resulting in convection currents between the air closest to those warmed up surfaces and cooler air in the room. The heat transferred via the exterior due to air flow would be quite minimal if the windows are closed relative to hear transfer from radiation.
posted by unid41 at 10:30 PM on August 21, 2020 [1 favorite]
posted by unid41 at 10:30 PM on August 21, 2020 [1 favorite]
Edit: sorry, reading this late at night and missed the "curtains closed" part. I still would think the radiation heats the curtains, which then act as the "warmed surfaces" I described, but with the curtains closed the relative effect would be reduced..
posted by unid41 at 10:34 PM on August 21, 2020
posted by unid41 at 10:34 PM on August 21, 2020
I think the search term to start with is 'solar gain'?
The way a greenhouse works is to let solar radiation IN, but not OUT.
In the winter, that's what you want - let all the infrared heat in and trap it there. The more solar gain, the better. But glass is pretty porous to heat transfer, so most of it escapes overnight.
In the summer it does no good, or overheats everything. So you leave all the vents open or actively ventilate it in the summer - you don't want solar gain.
So the windows on the sun facing south side. They need to be open during the day, to let any accumulated warmed air out. A No-Go during wildfire smoke. So is there any other way to exhaust warm air, like from the ceiling layer, outdoors? Positive pressure exhaust will keep the smoke out.
Or you could try shading the widows themselves. Not necessarily with curtains, since you still warm a layer of air between the glass and the curtain, which chimneys up into the room, drawing cooler air from the floor up behind the curtains to be warmed, etc.
Since you can't go back in time and plant a big leafy tree on the south side of your house...
There are low-transfer coatings / films you can put on window glass. Basically turning your windows into sunglasses, but for infrared instead of ultraviolet. Or even just try colored paper. Tape some sheets or make a post-it mosaic on the inside of the windows during the day, and see if the room stays cooler by a few degrees.
If you're getting into high tech home comfort, there's a thing called a heat recovery ventilation system. In your case, you could set it to take the hot, dry air from the 2nd floor, and swap it with the cooler, moister air from your basement. Eventually, they'd even out, of course. But it's a way to move air temperature around within a space, rather than by exchanging outside air and bringing i smoke. Or you _can_ hook it into an outside air conditioning system, but the high energy heating/cooling only kicks in after the internal exchange differential option has run out.
posted by bartleby at 12:13 AM on August 22, 2020 [2 favorites]
The way a greenhouse works is to let solar radiation IN, but not OUT.
In the winter, that's what you want - let all the infrared heat in and trap it there. The more solar gain, the better. But glass is pretty porous to heat transfer, so most of it escapes overnight.
In the summer it does no good, or overheats everything. So you leave all the vents open or actively ventilate it in the summer - you don't want solar gain.
So the windows on the sun facing south side. They need to be open during the day, to let any accumulated warmed air out. A No-Go during wildfire smoke. So is there any other way to exhaust warm air, like from the ceiling layer, outdoors? Positive pressure exhaust will keep the smoke out.
Or you could try shading the widows themselves. Not necessarily with curtains, since you still warm a layer of air between the glass and the curtain, which chimneys up into the room, drawing cooler air from the floor up behind the curtains to be warmed, etc.
Since you can't go back in time and plant a big leafy tree on the south side of your house...
There are low-transfer coatings / films you can put on window glass. Basically turning your windows into sunglasses, but for infrared instead of ultraviolet. Or even just try colored paper. Tape some sheets or make a post-it mosaic on the inside of the windows during the day, and see if the room stays cooler by a few degrees.
If you're getting into high tech home comfort, there's a thing called a heat recovery ventilation system. In your case, you could set it to take the hot, dry air from the 2nd floor, and swap it with the cooler, moister air from your basement. Eventually, they'd even out, of course. But it's a way to move air temperature around within a space, rather than by exchanging outside air and bringing i smoke. Or you _can_ hook it into an outside air conditioning system, but the high energy heating/cooling only kicks in after the internal exchange differential option has run out.
posted by bartleby at 12:13 AM on August 22, 2020 [2 favorites]
I spoke at an energy efficiency conference recently and one of the other speakers does remedial work on existing homes. She claimed that the some of biggest gains (especially vs outlay) come from sealing air gaps around doors, windows and anywhere else that leaks air, such as vents if you have them. Internal wall vents were common where I live when everyone burned coal or wood for heating and needed protection against carbon monoxide poisoning. New energy-efficient homes here are often airtight or close to it. Some are so well sealed that they require specialised systems to exchange air with the outside.
My house has the same problem as yours - I lose heat overnight - and I get a strong smoke smell coming in in summer when there are fires. My evaporative cooling requires open windows to work though.
I have had some success tracing the worst gaps with an infrared camera attachment for my phone. I go outside when it's cold and the heating is on and search the external walls and windows for signs of heat. It was not cheap to buy, but seems to work well and has helped me improve things. The one I got was made by FLIR, though there may well be others.
Curtains make a big difference too and I'm sure you would shed useful heat at night by leaving them open if it's cooler outside. To be most effective in stopping heat transfer though, they need to be insulated and have pelmets.
posted by mewsic at 1:25 AM on August 22, 2020 [1 favorite]
My house has the same problem as yours - I lose heat overnight - and I get a strong smoke smell coming in in summer when there are fires. My evaporative cooling requires open windows to work though.
I have had some success tracing the worst gaps with an infrared camera attachment for my phone. I go outside when it's cold and the heating is on and search the external walls and windows for signs of heat. It was not cheap to buy, but seems to work well and has helped me improve things. The one I got was made by FLIR, though there may well be others.
Curtains make a big difference too and I'm sure you would shed useful heat at night by leaving them open if it's cooler outside. To be most effective in stopping heat transfer though, they need to be insulated and have pelmets.
posted by mewsic at 1:25 AM on August 22, 2020 [1 favorite]
It's not just windows - your house will gain heat just from the sun hitting it, although the transfer through a "solid" wall (probably stucco over plywood over wood studs with insulation between them if you're in California) will take longer than sunlight coming directly through a window and heating up objects inside your home. I live in Michigan now, but I can tell when the sun starts hitting the one exterior wall of my apartment even though my windows are in full shade at that point of the day. Shading your windows will certainly help, and even better if your home is well insulated, but your walls would have to be quite thick and well insulated to stop solar heat gain completely.
posted by LionIndex at 5:57 AM on August 22, 2020 [1 favorite]
posted by LionIndex at 5:57 AM on August 22, 2020 [1 favorite]
This is sort of a complicated question to answer with any sort of accuracy, but if you're looking for a qualitative answer then you can run a simple experiment to see whether direct sun exposure or outside air temperature is really impacting your home more.
On a day with full sun forecast and no wind, seal the entire house up as you normally would at, say, 7:00 am. Take regular temperature readings (either the same spot in one room, or pick multiple rooms to sample) and log them once an hour or thereabouts. At the end of the day, you now have a basic line graph of how your house responds to that kind of outside environment.
Now find a day where the forecast is overcast and no wind, but the high and low temperatures are as close to your first sample day as possible. Do the same logging at the same times in the same places. Compare your results. The difference is (broadly oversimplified) the effect of the sun on your house.
A more hands off way to do this if you're technically savvy and like gadgets would be to build a weather station which logs all these parameters for you - cloud cover, hourly temperature, wind speed and direction, etc. - in addition to some sensors in your home to log that information and then normalize the weather data on various axes to see how each one impacts your home. This might be overkill.
One of the big flaws in this experiment is that it's going to be very difficult to find two days, one sunny and one overcast, that have the same temperature swings. Clouds act to block solar radiation, obviously, but they also act as an insulator against the ground and will prevent the radiative cooling that happens on clear nights. So you'll probably see a flatter temperature gradient throughout the day (higher lows and lower highs), which means an overcast day may not allow your home to cool overnight as fully as it would on a clear night.
Also take into account the heat sources inside your home while collecting data. Your kitchen is a big one (and if you have an open floor plan that's going to impact the rest of your home), as well as computers, televisions, and people.
posted by backseatpilot at 6:38 AM on August 22, 2020 [4 favorites]
On a day with full sun forecast and no wind, seal the entire house up as you normally would at, say, 7:00 am. Take regular temperature readings (either the same spot in one room, or pick multiple rooms to sample) and log them once an hour or thereabouts. At the end of the day, you now have a basic line graph of how your house responds to that kind of outside environment.
Now find a day where the forecast is overcast and no wind, but the high and low temperatures are as close to your first sample day as possible. Do the same logging at the same times in the same places. Compare your results. The difference is (broadly oversimplified) the effect of the sun on your house.
A more hands off way to do this if you're technically savvy and like gadgets would be to build a weather station which logs all these parameters for you - cloud cover, hourly temperature, wind speed and direction, etc. - in addition to some sensors in your home to log that information and then normalize the weather data on various axes to see how each one impacts your home. This might be overkill.
One of the big flaws in this experiment is that it's going to be very difficult to find two days, one sunny and one overcast, that have the same temperature swings. Clouds act to block solar radiation, obviously, but they also act as an insulator against the ground and will prevent the radiative cooling that happens on clear nights. So you'll probably see a flatter temperature gradient throughout the day (higher lows and lower highs), which means an overcast day may not allow your home to cool overnight as fully as it would on a clear night.
Also take into account the heat sources inside your home while collecting data. Your kitchen is a big one (and if you have an open floor plan that's going to impact the rest of your home), as well as computers, televisions, and people.
posted by backseatpilot at 6:38 AM on August 22, 2020 [4 favorites]
The power of the sun to warm your house is pretty amazing. Something white or reflective in the windows will help, esp. south-facing. You can get mylar space blankets, mount to cut-to-fit posterboard or foamboard with glue, though blocking windows is gloom-inducing.
posted by theora55 at 7:53 AM on August 22, 2020
posted by theora55 at 7:53 AM on August 22, 2020
Shading the outside of your house is great for avoiding heat gain from sunshine. Could you put up a Coolaroo shade or something like it on the south or west? Ideally perpendicular to the summer sunlight, so more like a pergola extension of your house than like fabric shutters. (When built in, this kind of shade is called a break-sun, usually in French.)
We coddle along west side street trees that are officially too big for the tree lawn because they are finally taller than the house, they are maples that leaf out late, and we can feel the stop in heat gain in the afternoon when the sun starts hitting the trees instead of the roof. But a shade sail would be easier and a lot faster.
posted by clew at 8:46 AM on August 22, 2020
We coddle along west side street trees that are officially too big for the tree lawn because they are finally taller than the house, they are maples that leaf out late, and we can feel the stop in heat gain in the afternoon when the sun starts hitting the trees instead of the roof. But a shade sail would be easier and a lot faster.
posted by clew at 8:46 AM on August 22, 2020
If you are in California and your house was built a while ago you almost certainly have no insulation in your walls and only minimal insulation in your attic space. This is a big, big contributor. If you have the time, open up an outlet or switch on an outside wall, pull out the electrical hardware, and peer inside.
The two best things I did for my 1959 California house were to replace all of the windows, and have insulation blown into the wall cavities (along with a bunch more into the attic). Made a huge difference.
...having said that, if you wanna really dig into this I like these temperature probes. I've got them scattered around (north outside wall, inside the garage, inside the garage attic space, etc.) and they've been hugely reliable for me -- and I can dump all of their records into Excel for analysis. Really handy for learning all kinds of things about household temperature/humidity gradients and behaviors (note: I do not use their wifi gateway, just the bluetooth connections).
posted by aramaic at 10:11 AM on August 22, 2020 [1 favorite]
The two best things I did for my 1959 California house were to replace all of the windows, and have insulation blown into the wall cavities (along with a bunch more into the attic). Made a huge difference.
...having said that, if you wanna really dig into this I like these temperature probes. I've got them scattered around (north outside wall, inside the garage, inside the garage attic space, etc.) and they've been hugely reliable for me -- and I can dump all of their records into Excel for analysis. Really handy for learning all kinds of things about household temperature/humidity gradients and behaviors (note: I do not use their wifi gateway, just the bluetooth connections).
posted by aramaic at 10:11 AM on August 22, 2020 [1 favorite]
I have a mid century Californian house. In addition to the other points, consider subfloor vents (air bricks). To keep the crawlspace dry, there are vents around the outside walls deliberately to allow outside airflow. And under the floor there is no insulation and no effort at air sealing between the crawlspace and the living space. So it's an unconditioned space that will try to come with equilibrium with the outside, admittedly fairly slowly, and will leak heat and/or air into your living space as it does that.
There are ways this might have been addressed or could be addressed in the future. You can insulate the underneath of the floor; you can also seal the crawlspace by putting plastic over the ground and blocking up the vents, making it a part of the conditioned space. Both are easy things to spot if you can peer into your crawlspace.
posted by How much is that froggie in the window at 7:52 PM on August 22, 2020
There are ways this might have been addressed or could be addressed in the future. You can insulate the underneath of the floor; you can also seal the crawlspace by putting plastic over the ground and blocking up the vents, making it a part of the conditioned space. Both are easy things to spot if you can peer into your crawlspace.
posted by How much is that froggie in the window at 7:52 PM on August 22, 2020
Get yourself an instant read infra-red thermometer thingy. Amazon.com: Infrared Thermometers - Temperature & Humidity: Industrial & Scientific, (also great for checking the temperature of a skillet on the stove).
Point it at your walls and floor and ceiling and that random cup sitting out on the table all day long. Know the rough temperature of your home.
Find somewhere outside in the shade that you can point the thing at to find the temperature outside.
Keep track of these things. Get a feel for them. Experiment. When it's cooler outside than inside, open the windows. When it starts to get warmer outside than inside, shut the windows. Close the blinds and doors on the hot side.
There's likely (if oldish) no insulation to speak of and you're trying to find the thermal carrying capacity of your walls.
You can reverse the equation when it's cold vs when it's hot.
Your walls will bring in the temperature of what it was a few hours ago. When it's cool at night, they'll keep you a bit cooler through the morning. When it's cold out, they'll give you a few hours of warmth when the sun goes down.
Otherwise, use the simple window thing. When you want it cool, close up a bit before the outside is warmer than the inside. When you want it warm, close up a little bit before it gets cold. On the middle, open up when it's colder/hotter outside than inside or vice-versa. There's just a couple of hours worth of delay before the heat/cool makes it way through the uninsulated walls.
I can knock about 15℉ insid.e/outside just by closing windows and blinds and doors. It's not perfect, but it's that nice climate in general so no insulation and no real A/C or Heat vs the very basic it'll be fine for the handful of weeks that it's cold/hot.
If you want to actually check for "pure scientific curiosity" then you still get that instapoint IR thermometer and you check inside and outside and do an A/B test with opening windows or not to narrow things down to what works for your particular living situation.
I'm in SoCal for 20+ years and have this down to a science for my particular apartment. But it will take a bit of pointing that IR thermometer around until you get the hang of knowing how to optimize the cool/warm of your abode.
You can figure out the thermal transfer vs airflow thing in a few days of approximately the same outside temperature vs the inside temperature vs opening or closing windows.
No, seriously... I point an IR thermometer thing around and check whether it's time to open the windows or not. I also live in a drafty as f*ck apartment with slats for windows and no hope of any sort of efficiency. You might be different up North where it gets a bit colder.
posted by zengargoyle at 11:35 PM on August 22, 2020
Point it at your walls and floor and ceiling and that random cup sitting out on the table all day long. Know the rough temperature of your home.
Find somewhere outside in the shade that you can point the thing at to find the temperature outside.
Keep track of these things. Get a feel for them. Experiment. When it's cooler outside than inside, open the windows. When it starts to get warmer outside than inside, shut the windows. Close the blinds and doors on the hot side.
There's likely (if oldish) no insulation to speak of and you're trying to find the thermal carrying capacity of your walls.
You can reverse the equation when it's cold vs when it's hot.
Your walls will bring in the temperature of what it was a few hours ago. When it's cool at night, they'll keep you a bit cooler through the morning. When it's cold out, they'll give you a few hours of warmth when the sun goes down.
Otherwise, use the simple window thing. When you want it cool, close up a bit before the outside is warmer than the inside. When you want it warm, close up a little bit before it gets cold. On the middle, open up when it's colder/hotter outside than inside or vice-versa. There's just a couple of hours worth of delay before the heat/cool makes it way through the uninsulated walls.
I can knock about 15℉ insid.e/outside just by closing windows and blinds and doors. It's not perfect, but it's that nice climate in general so no insulation and no real A/C or Heat vs the very basic it'll be fine for the handful of weeks that it's cold/hot.
If you want to actually check for "pure scientific curiosity" then you still get that instapoint IR thermometer and you check inside and outside and do an A/B test with opening windows or not to narrow things down to what works for your particular living situation.
I'm in SoCal for 20+ years and have this down to a science for my particular apartment. But it will take a bit of pointing that IR thermometer around until you get the hang of knowing how to optimize the cool/warm of your abode.
You can figure out the thermal transfer vs airflow thing in a few days of approximately the same outside temperature vs the inside temperature vs opening or closing windows.
No, seriously... I point an IR thermometer thing around and check whether it's time to open the windows or not. I also live in a drafty as f*ck apartment with slats for windows and no hope of any sort of efficiency. You might be different up North where it gets a bit colder.
posted by zengargoyle at 11:35 PM on August 22, 2020
This thread is closed to new comments.
posted by aws17576 at 8:56 PM on August 21, 2020