Basic airflow/heat question

[DavidP]

A message was recently posted asking for advice about attic cooling. This got me thinking; in every house I’ve ever lived in, the upstairs has been noticeably hotter than the downstairs in summer. That just seems to be the way the world works (in the northern US, at least) but I never actually understood why.

My current house (two-story Victorian) has a large, tall attic, with a slate roof over it. The slates obviously get hot from the sun, and I suppose the heat is transmitted into the attic. No insulation under the roof. But why is the upstairs so much hotter than the downstairs?

Does heat actually come down through the attic floor/upstairs ceiling? We usually think about hot air rising, but perhaps in this situation heat is transmitted down to the lower part of the house? If so, insulation would help. I don’t think there is much, if any, insulation between the upstairs ceiling and the attic floor.

During the heating season, I leave the upstairs radiators turned off except during the coldest weather; otherwise, the upstairs is very uncomfortable. The heated air seems to be rising from downstairs to upstairs, but how? There is a medium-sized stairwell in the front hall and a small enclosed staircase in the back. Is that enough to account for the difference, or does hot air also go up through the ceiling/floor between the two storeys? If I insulated between the upstairs and the attic floor, would that help with the heat (since less hot air would escape from the upstairs, would more stay downstairs?) or make it worse (it would keep rising from downstairs and not escape, making the upstairs even warmer??). My drafty windows come into play here, too, I’m sure (they are on the list of things to work on this fall).

Sorry if these seem like silly questions, but I just never learned how this stuff actually works.

[shazapple]

Heat is basically transmitted 3 ways
- Conduction: the heat is transferred through a solid material (such as the heat you feel when you touch a radiator)
- Convection: gas or liquid (air or water) rise when heated because they are less dense. When they conduct their heat to another material they cool and fall, which creates air movement. An example would be the hot air that rises off a radiator.
- Radiation: This is the heat you feel from the sun or those 'infrared heaters' you can buy at Costco (and the reason why your attic is so hot).

We deal with these types of heat in various ways in buildings:
- Air sealing: convection causes hot air to rise and if it is not contained then it will rise right out of the building (this is also called the 'chimney effect'). Other forces such as wind or unbalanced HVAC ductwork will also pull your interior air out of the building. A lot of focus on energy efficient buildings is how well the air movement can be stopped.
- insulation: The whole point of insulation is basically to stop conduction and convection. Air actually transfers heat very poorly, so insulations like fiberglass or cellulose are methods of using air to limit conduction and holding that air still so convection cannot occur.
- ventilation: Walls and attic spaces are heated up due to radiation. Ventilation helps to deal with the heat build-up. Lighter colours are also used to deal with radiation as they will reflect it.

So to answer your questions insulation will definitely help, but you have to seal any areas where air can escape (windows, light fixtures, other 'holes' in the building). If your attic space is hotter than the interior and there is no insulation then that attic heat will conduct into the upstairs. If you have poor air sealing then the hot air will rise to the upstairs (and beyond) while pulling cold air into the downstairs.

[phil]

the open stairwell will allow a lot of warm air to rise even so much as a bed-sheet hung up can make a surprising different in the air flow.

most of the heat in your attic is because the roof is hot. If you put lots of insulation against the roof then that heat wont transfer and the heat will cause the roofing material to have a shorter lifespan. If you can have some heat coming in your eaves and some corrugated material between your roof sheeting and the insulation then a ridge vent you can get some cooling happening without it going down into the house.

if you have non living space in the attic you can put a fan in with a backward thermostat so the fan runs whenever it is over say 100 degrees or so.

In my house I have non living space on the lower ends of the attic and living space in the middle. I have a blower that exhausts some of that air from one side of the attic space so what I like to do is open up the attic windows about dinertime and put my blower on, and then if I need to I close them and put the AC on up there an hour before bed.

I put the AC unit in a box so the blower that exhausts the non living space also cools the back of the AC unit sich as it would if it were in a window.

I made baffles in this box to separate the front and back of the AC unit. I have a separate blower that sucks cool air from under the eves in the shade, that air goes through the AC section and then into the room upstairs. so the unit serves dula purpose it exhausts the hot air and it brings chilled fresh air in . Because it is in a box and behind the wall it is very quiet. Its only 20,000BTU so not enough for the whole house but it keeps the bedroom cool when it is hot. most of the year I get by just by running the blower and exhausting that heat.

another simple thing I do that helps is I cut some 2 inch dense foam sheets to fit in the windows. If I stick that in it stops the hot sun from coming in the windows and I will use that if traffic noise bugs me or if I need to sleep in such as working shiftwork.

I kid of doubt you will see huge changes by insulating your attic floor, I think a blower or some vents might do a lot more, some way to get the heat out of the attic. It wants to leave it just needs a hole to get out in a lot of cases.
even pull down solid blinds in the windows will likely do a lot.

I think lot of houses like mine were not designed to use the attic as living space which would keep the main floor more livable. By now I think a lot of homes have utilized this space as living space, and often little thought was put into the fact that it gets so hot up there. I added a couple of decorative vents to let air into the non living space, of course you can't just blow air out of a house or a room or a box unless you let the equivalent in somewhere else.

you will probably see a huge chimney effect if you just open an attic window and a lower floor window, and go stand on the stairwell it will be windy there. that will cool the attic. You may find that doing this actually warms the main floor because the cool air in the house gets replaced by other air to feed the flow of air that is going up your staircase and out the attic window.

one easy way to get more flow in the attic is by installing skylights. the problem with skylights is they are so non original so they take away from the original look. I won't install them for this reason but they sure can make a hole for that hot air to just escape easily from.

[DavidP]

I want to apologize to shazapple and phil for my very slow reply. I retired in June and am in the process of cleaning out a house etc etc. I just lost track of this thread I started but remembered it as I now am thinking again about heat (summer weather plus the fact that I usually prebuy my fuel oil in August).

Thank you both for your thoughtful replies. I have always done as phil suggests by keeping the attic windows open and running an attic fan. It's not one of those huge whole-house fans, but just a large fan I bought and placed in the west window. I run it in the afternoon and evening; before I go to bed I open the door to the attic (along with the second floor windows), and this also helps. But still the upstairs is often uncomfortable in summer.

The attic is large and tall enough that one could construct a room in the center and still have storage around the edges, but I have no plans to do so. I'm thinking that leaving the attic with limited natural cooling in summer and no heat in winter is the best course, but I would like to use less fuel in the winter and have the upstairs more comfortable in summer. shazapple thinks that insulation between upstairs ceilings and attic (along with sealing other openings) would help, but phil is not convinced. Anybody else want to comment? The plaster ceilings upstairs are not in great shape and I will have to deal with them sooner or later (yuck, double yuck, plaster dust). But if adding insulation would help then that's an incentive to tackle the project.

[phil]

hot air rises, if the stairwell is open it won't matter how much you insulate the floor the hot air will just go up the open stairwell. If you install a door in the stairwell it might help some. By the reverse, My basement gets nice and cool when I run my window air conditioners as that cool air finds its way downstairs.

You might find you have a pipe that gets heat from your furnace in the basement up to the attic space, if so maybe you can put a fan on that or even just try running the furnace blower to see if that helps any.

I find the same it is bloody hot in the attic and I just open the window up there and open a door on the main and you can feel the chimney effect. My problem is that I need screens or my cat goes on the roof.

just remember you can't push air unless it can go somewhere , if you could the house would inflate like a baloon. so if you blew cool air up to the attic you'd also need a way for the same volume of air to leave the attic, either outside or back down or as said our old houses are often leaky enough that it actually can go somewhere. you couldn't blow out of your basement without replacing that air. but there might be enough leakage that it can come in somewhere.

fans work better to move cool air as it is more dense. as the air gets hot it increases in volume and the same fan will actually then move less air.

[Don M]

We have 4 gable windows in the attic with wood slat grills that fit in the lower sash opening. There is a whole house fan mounted on a sheet of plywood that drops down over the attic stairwell. I open the attic door turn on the fan which blows all the hot attic air out thru the attic window grills & sucks cool air thru open doors & windows. It works very well for good air circulation through out the house. I only run the fan when the outside temperature is as cool or cooler than the interior of the house!

[DavidP]

Don,

That's a great system you have! I am guessing from the picture of your house that the stairwell is centrally located, which would be good for pushing hot air out from both ends and pulling cooler air more or less evenly in throughout the house. My attic stairs are at the edge of the house (you go up 2/3 of the way, turn 180 degrees on a landing while bending to avoid hitting your head on the rafters, then go the rest of the way). A fan here wouldn't work as well, I think. I could put a whole-house fan in the center of the upstairs hall ceiling (approximately the center of the house), which would work as yours does. But that is the best storage space and also the area where one might finish off an attic room, so I am reluctant to do that. But I will continue to think about a better attic fan.

Also some of the whole-house fans I have heard cause a _lot_ of noise and vibration. How does one mitigate that? Choosing the right fan? Some sort of mounting technique?

[DavidP]

hot air rises, if the stairwell is open it won't matter how much you insulate the floor the hot air will just go up the open stairwell. If you install a door in the stairwell it might help some.

Right now there is little (if any) insulation below the attic as well as leaky windows etc. on the second floor. My thought (hope? prayer?) was that if I insulated and sealed leaks, less heated air would rise because it wouldn't have much of an outlet. Right now I'm sure the hot air goes up and out easily, making room for more. Note that I said "less"; some heated air would rise, no doubt, even with insulation. Am I just way off base with this? (I have been wrong before!) Or would the effect be so small that it wouldn't save any appreciable amount of fuel and so not be worth the expense and effort?

Installing a door in the stairwell is neither realistic nor attractive. My aunt and uncle had one in their farmhouse in Vt. But they did not use the upstairs, and the stairwell was a rectangular opening so a big piece of plywood, hinged on one side, did the job. I do use the upstairs and the stairwell is architecturally interesting as well as complex.

[jharkin]

Lots of good info above.

To add, if you are running air conditionsers in the summer the chimney effect indeed does reverse. The air conditioners are net removing heat out of the interior air mass, making it colder and less dense relative to the outside so it wants to fall into the basement and force its way out down there... in turn sucking warm air in at the upper floors.

This is the opposite of the winter chimney effect where your heating system is net adding heat to the interior air making it warmer less dense and prone to rise relative to outside and escape out the attic.


This is often unintuitive to folks because we all hear "warm air rises" and think of that warm air upstairs as wanting to continue rising right out of the roof... but for warm air to rise there has to be a corresponding amount of cold air sinking. Or think about it this way:say its 90 outside and inside your AC is keeping the upstairs maybe at 80 and the downstairs at 70... its all colder than outside and sinking

Making it even more complicated - the attic itself might be warmer than outside so there would be a positive stack effect there, fighting against the negative stack effect in the conditioned space. Thats where good soffit venting comes in.

Gets your brain working, doesn't it?

[wletson]

You're in the northern part of the US. Northern New York, you are pretty close to the same extremes in temperature as I am up in Canada. (even worse if you are close to Lake Erie)

If you don't have good insulation between the empty attic space... Get some. Don't worry about between the roof and attic. The open space of the attic though has to be ventilated. Gable vents in either side of the attic, or roof vents with soffit vents should be there. If not, get some. During the summer, you gotta get the heat out of the attic. If you get the right combination of vents, you won't need a fan system.
So, that addresses your summer issues.

Winter being still too hot upstairs... that baffles me, and also makes me think you are spending a ton on fuel oil.
Where does the furnace draw the air from to heat? Do you have cold air returns that suck the air back to the furnace to heat up again? It sounds like you are lacking in air circulation in your house. (during the summer too)
Caulk windows, insulate the floor of your attic. (oh, I'm guessing the thermostat for your furnace is on the bottom floor?) If you're planning on working on windows, start on the main floor, that sounds like it is the cold spot in the house. Eliminate the cold from coming in during the winter and that may help equalize things too.

All of the old houses I have lived in have had gas burning furnaces of some sort in them, and all have had the option of turning the fan on during the summer. If you have return ducts to the furnace, this will circulate the air through the house, hopefully equalizing the temperature throughout.