Image Credit: Martin Holladay
More Musings of an Energy Nerd
During the winter, the air near your floor is cold, while the air near your ceiling is hot. Similarly, during the summer, the air conditioner keeps your first floor comfortable, while the rooms on the second floor are unbearably hot. What’s going on?
The usual answer is, “Heat rises.” But that explanation isn’t quite accurate. (It’s true that hot air rises by convection. But heat travels in all directions, including sideways and downward, by conduction and radiation.)
The scenarios I’ve described are variations on the theme of temperature stratification — problems caused when indoor air temperatures are layered horizontally, with cold air down low and hot air up high, like layers in a parfait.
Let’s start with a simple example: a single-story slab-on-grade house with temperature stratification problems. The thermostat is 48 inches above the floor, and it’s set at 72°F. On a winter day, when you are sitting at your desk, the air near your feet is at 65°F. Up near the ceiling, the air is at 79°F. Why?
There are two main causes of this phenomenon.
Air leaks coupled with the stack effect. The most important factor in this type of stratification is air leakage. Holes near a home’s ceiling allow warm interior air to escape into the attic. As this air escapes, it pulls in cold exterior air through cracks near the floor — for example, cracks near the threshold of an exterior door. The engine for this type of air movement is the stack effect. Since the warm interior air near the ceiling is at a positive pressure with respect to the outdoors, it wants to escape. Since the cool interior air near the floor is at a negative pressure…
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10 Comments
Case Study
I finished a townhome in 2010 started by others during the housing crunch in the mid to late 2000's. The insulation was done basically to code for Minneapolis and the ACH50 tested at 1.5. We did include R-10 subslab insulation and zoned the heating for both floors. The ventilation is mechanical (Venmar) and we did go with an R-60 cellulose attic insulation. The temperatures are very even on both floors throughout the year and this is one of the most comfortable homes I know of. Isolation of the foundation and the basement slab with rigid insulation is very important in keeping lower levels cozy.
Response to Doug McEvers
Doug,
Thanks for the testimonial.
Readers, you heard it from Doug: make your thermal envelopes tight, and include adequate insulation all around (including under your slabs), and you'll find that the risk of room-to-room temperature imbalances and temperature stratification problems will be greatly reduced.
Cold Near Windows
We are getting ready to build a home in the Pacific Northwest. Most of the windows will face Northwest (300 degrees) to take advantage of the water view. In addition to the problems we anticipate with heat gain due to the mostly westerly facing windows, I'm also concerned that the space near the windows is going to be relatively cold. We have that situation in the house we are living in now that has a wall of double pane windows. In addition to buying well sealed triple pane windows, what else can we do to keep the area by the windows closer to the temperature of the rest of the room? We haven't picked the heating system yet. We are considering radiant and leaning towards mini-split. Would installing radiant (hydronic or electric) in the wall below the windows be an effective solution? I'm thinking of a low output system designed to offset the cold air spilling down the windows versus being the only heating source for the room. The idea being that the wall heat would stay on most of the time in the winter versus the cycling that happens with registers placed near windows. I'm obviously brainstorming here and would appreciate additional thoughts.
Radiant walls can be lossy
When the radiant heat is mounted on a exterior wall it can substantially increase the heat load due to the much higher interior temperature. If you are looking to balance the mean radiant temperature and reduce the draft of cool air falling down the face of the glass it's better to use low-temperature convecting panel radiators (the standard in much of Europe). The rising column of air out of the panel radiator counters the cold air flow, and the temperature of the surface of the panel radiator is higher than a section of radiant wall, and the air space between the back side of the radiator and the wall keeps the temperature that the insulation is working against much lower too.
At 20-25F west-of-the-mountains PNW outdoor design temps <U0.20 triple panes alone would do a lot for countering the cold window effect, and the amount of panel radiator needed to interrupt the weak draft generated by the window much less that it would be in places where the design temps are in negative double-digits.
Before you can really pick a heating system a careful but aggressive heat & cooling load calculation is in order. With a lot of west facing glass your peak cooling loads may be quite a bit higher than your peak heating loads, despite the modest 1% outside design temps of the region. When the sun doesn't set until 9PM (well north of due west) after the house has spent a day of soaking up the sun with it's thermal mass already heated up the evening blast of solar gain from a house with most of it's glass facing west can be double the peak load compared a similar house with almost no west facing glass. Heating and cooling with mini-splits may indeed be the better option, but be sure it's sized to manage both heating & cooling loads.
Response to Kevin Camfield
Kevin,
You didn't mention the area of your windows -- but if you are worried about overheating in summer and feeling cold in winter, you may be basing your worries on the fact that your design includes a very large area of glazing.
If that's the basis of your worry, remember that less is often more. Maybe you need less glazing area. The view can be spectacular, even when viewed through a normal-sized window. (This is the Japanese approach -- a small window framing a spectacular view allows the viewer to exclaim, "aha!")
Reply to Dana on lossy walls
Very helpful comments. Thank you. I think the low temperature radiators is a good idea, especially if there is a way to control them with a reduction in heat output versus just on and off. I'm not sure if this is practical, but controlling the water flow to the radiator versus just shutting the pump on and off would give a more consistent temperature and comfort level. If we end up using hydronic radiant for the bathroom floors we might be able to utilize the same water source for the radiators. I'll look into what is available in the U.S.
Response to Martin
Martin,
It is a big expanse of glazing that we are planning for facing NW. The overall level of glazing in the house is very reasonable as there is limited glass on the other exposures. The view wall is where the majority of the windows are. You are right of course, the most sensible thing to do would be to reduce the amount of glazing. I'll have another look to see if we can do that without compromising the design of the house. Thank you for the comments.
Our problem is Radon in winter caused by the stack effect. We have a 1950s ranch in Pittsburgh. The basement level is above grade at the back of the house as we sit on a sloping lot. The home has a conventional radon system with functioning fan. When we purchased the home 7 years ago it passed the Radon test in September . However, last winter I installed an Airthings Monitor and discover the levels get as high as 23 in winter and are less than 1 all summer. We use the basement as an office, artist studio and workout area.
These are the actions that I am considering;
1. Install a HRV system to dilute the Radon & reduce pressure differential between floors.
2. Sealing the attic
3. Might add a sealed basement door to reduce airflow. Seems like this should help, but not sure.
Question should the HRV have its intake and discharge directed only to the basement or be tied into the homes ducts?
Please let me know your thoughts?
Thanks, Art
I'll be building my own pretty good house later this year or next. I'm glad to hear that a tight, well insulated house is the best measure to fight stratification. However I'm wondering if a wood stove as the main heat source changes things? The open concept living/ kitchen and all the bedrooms will be located on the second floor. There is a walkout basement below that with the foyer. The wood stove will be located in the living room, which will have vaulted ceilings about 16' high at the peak. Would installing a bath fan at the peak, blowing down to the basement, help with distributing heat to the basement? I've already read some GBA articles about how a bath fan isn't super useful for pushing heat into bedrooms. However, in my situation will the delta T between the cathedral peak and the basement be greater and therefore make a bath fan worth it?
Adam,
If you're building a tight house, wood stoves are a poor choice -- they often have problems getting enough combustion air. But if you insist on a wood stove, it belongs in your lowest floor -- in the walkout basement. But you need to think about combustion air.
If you ignore my advice and put the wood stove upstairs, your basement will always be cold.
More information in these three articles:
"All About Wood Stoves"
"How to Provide Makeup Air for a Wood Stove"
"Why Is This Wood Stove Misbehaving?"
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