In two previous blog posts (here and here), I explored some of the ways our cottages lose heat. Sitting or sleeping near a wall or window that is losing heat can be uncomfortable, even if the cottage is maintaining the proper temperature overall. And cold walls and windows aren’t the only factors involved in achieving a comfortable winter temperature.

If there is a significant difference in temperature between your floor and your ceiling, you can be quite uncomfortable even if the temperature at intermediate levels is fine. That problem is called “temperature stratification” and some cottages have a serious problem with it. In extreme cases, the floor can be under 70 degrees while the ceiling is over 100.

Charles Robertson has been taking temperature measurements at three cottages (including his own), and he provided me with the data. I picked a particularly cold day for each cottage and used the data Charles provided to graph the inside temperatures.

Stratification in the Robertson cottage. Charles had mounted three temperature sensors on a tripod: one at about the 1-foot level, one at about 4 feet, and one near the ceiling. Here’s a photo of the setup he used for the measurements.

This is the setup Charles Robertson assembled for measuring temperatures at different heights. A small data logger captured the temperature information for subsequent transfer to his computer.

Here’s a graph of the ceiling, floor, and mid-level temperature at the Robertson’s cottage, #172, on December 12, 2020 (overnight low: 37°; afternoon high 62°).

Temperature at three levels. In this and the following graphs, the upper (yellow) line is the temperature near the ceiling, the middle (blue) line is at about four feet, and the bottom (brown) line is near the floor. The same scale is used on the following two graphs for ease of comparison.

As the graph shows, on the morning of December 12, after Charles and Pat initially turned thermostat up to 72, the ceiling temperature went up to 101 degrees. After that the ceiling temperature settled down somewhat, with peaks in the upper 70s and low 80s and valleys in the mid 70s. Meanwhile, the floor was around 68 degrees most of the day. At the 4-foot level, where the thermostat is, the temperature stayed in the 71-72 range almost the whole day.

Reaching 101 degrees was an extreme case, I’ll grant you. But the ceiling in the Robertson’s cottage went above 80 degrees at some point every day during the six-week period for which he provided data (November 23 to January 7), and it went above 90 degrees on most of those days. That meant Charles and Pat were routinely dealing with differences of 15-20 degrees between floor and ceiling, and more on some days.

Two other cottages didn’t have these stratification problems. In early 2021, Charles set up his monitoring system at the Sterrett’s cottage, #157. The graph below shows the temperature in the Sterrett’s cottage using the same measurement setup. These measurements were taken on February 9, 2021 (overnight low: 27°; afternoon high 41°).

As you can see from the graph below, it took several hours for the Sterrett’s cottage to warm up, but it did not show anything like the variability that the Robertsons experienced. The ceiling temperature stayed in the range of 75-80, and temperature near the floor (once it warmed up) was steady at about 71.

Charles also measured temperatures Jim Craig’s cottage, #102. Jim experienced even less temperature variability than the Sterretts. As you see in the chart below, using data from February 21 (overnight low: 19°; afternoon high 37°), Jim’s ceiling temperature only varied between 72 and 76, and his floor temperature was steady at just above 70. Jim did not change the temperature at night.

Why the differences? The most obvious difference among the three cottages was the source of heat. The Robertsons have one of the new heat-pump systems, which deliver their heat via forced air. The other two cottages use traditional baseboard resistance heating. The Robertson’s cottage, like many here, utilizes ductwork that was installed to handle air conditioning. (Baseboard heating, which needs no ducts, was originally used, but was removed when the heat pump was installed.) The incoming air, whether it is hot or cold, is delivered from fixtures in the ceiling.

That makes sense for air conditioning: cool air entering at the ceiling tends to sink, promoting mixing. But hot air entering at the ceiling tends to stay at the ceiling, leading to stratification, with a layer of warm air at the top that doesn’t mix very much with colder air below. It takes a long time for the heat to penetrate to lower levels. Since the thermostat is not at the level where the hot air accumulates, it continues to signal the need for more heat, and the ceiling area becomes very hot.

With baseboard heat, the hot air originates at the floor and rises, setting up a natural circulation that keeps the air mixed and the temperature relatively uniform. The thermostat is triggered only when the overall air temperature drops below the set point.

In a cottage like the Robertson’s, it is not even possible to say whether the new heat pumps are saving electricity in the winter, compared with baseboard heating. The overheated ceiling area is undoubtedly losing heat to the attic and the outside air, negating at least part of the efficiency that the heat pump provides. (On the other hand, the heat pump is clearly better than a standard air conditioner in the summer.) More detailed study would be needed to determine which system uses less electricity in the winter.

Temperature and comfort. If the floor is cold and the ceiling is hot, it is possible to feel uncomfortable even when the thermostat says the temperature is OK. Cold floors in the winter are a common problem for houses built on concrete slabs (as our cottages are), and the problem is made worse by stratification: you can end up with a situation where you have to choose between an overheated, stuffy cottage with comfortable floors, or a generally comfortable air temperature but cold feet. If your feet are at 65 and your head is at 80, you may be uncomfortable even though your thermostat says the temperature is a comfortable 72.

Some possible solutions. Let me be clear: I am not advocating a return to baseboard heat for the cottages that have heat pumps. There are a number of solutions to the problem of temperature stratification.

The easy fixes:

  • The Robertsons have experimented with a “membrane heater” which can go under a rug, providing a gentle heat like that of an electric blanket set on low. That seems to do a good job of rectifying the cold floor and doesn’t consume a lot of electricity. Similarly, Charles has a warming pad for his feet under his desk. Charles wears socks around the house, so the cold floors would be a problem without those warming devices.
  • Don Overton and Kathleen Gordon have placed small, inexpensive fans facing upwards in the rooms of their cottage. This has the effect of sending the cold, floor-level air upwards so that it mixes with the hotter air above. They report that this is working well, but it wouldn’t work for everyone because of the space required and the tripping hazard.
  • Ben James, at Crosslands, has ceiling fans in his cottage. He runs them at their lowest speed to provide temperature mixing. The slow speed means that occupants can’t feel the air movement, but it is sufficient to mix the air. Ben’s success with this approach has caused me to consider ceiling fans for our cottage. As with many of the cottages that have recently had major renovations, ours has the wiring and the switches for ceiling fans, but they haven’t been installed. Like most people, I tend to think of ceiling fans for cooling in the summer, but I now see the benefit in the winter too. Even in cottages without preinstalled wiring, a ceiling fan is not a huge project, and the cost to purchase one (under $200 for a basic model) is not great.

The major projects:

  • A more ambitious way to address stratification would be ductwork to remove cold air at floor level (and potentially additional ductwork at other locations to introduce heat at floor level). Installation of these ducts would be a significant project, probably only practical during major renovation. These ducts could only be used in the winter—you would still want the existing ceiling vents in the summer.
  • Another option during major reconstruction would be installing a different type of heating and cooling approach, called a “mini-split system”. These systems still use a heat pump to provide heating and cooling, but instead of circulating hot or cold air, they circulate a liquid at the desired temperature to blowers in each room. The tubing required for these systems takes up far less space than ducts do.

A working group from the Energy Committee has been looking into these options and discussing them with Joe Deckman, who is in charge of construction. For the time being, we have to work with the remnants of earlier choices (such as existing ductwork and baseboard units) and will have to figure out reasonable solutions for comfort that don’t involve too much construction. When the time comes to begin totally replacing cottages, we’ll have the opportunity to come up with highly efficient approaches that provide comfort and avoid temperature stratification.