More Building Science
Back in October, I wrote a blog post called “Can Buildings Take Vacation Too?” about the potential need to be concerned with moisture issues when a building is shut down for an extended period of time. I had just completed a building investigation of a home that was left unoccupied for about four months each winter.
At the end of that post, I mentioned that I had been given permission from the homeowners to monitor the home when they shut it down from December 2019 through April 2020. I had four HOBO data loggers recording temperature and relative humidity: one outdoors, one in the basement, one on the first floor, and the last on the second floor.
Here’s a look at the charts that show temperature (blue) and relative humidity (orange) in the basement, on both floors, and outside. Note that both floors of this home have a lot of south-facing glass and during shutdown all the shades are left up to allow the sun into the home, with doors between all rooms wide open.
Some generalizations from the data
Not surprisingly:
- The basement data show the least variation in temperature and relative humidity.
- The outdoor data show the most variation in temperature and relative humidity.
- All spaces show big changes in temperature and relative humidity, much of the time as expected—when temperature goes down, RH goes up and vice versa, but there are times when both temperature and RH rise together, and vice versa.
- All four loggers show drastic changes on December 19/20—the day the house was shut down—and March 16/17—the day the couple returned and turned everything back on.
The table below shows the high/low for each space monitored during the shutdown:
| Space | Temp – high (°F) | Temp – low (°F) | RH – high (%) | RH – low (%) |
| Outdoors | 79.4* | -5.8 | 100 | 23.5 |
| Basement | 43.2 | 32.5 | 68 | 46.5 |
| 1st floor | 51.8 | 20.6 | 63.8 | 30.6 |
| 2nd floor | 56.7 | 19.7 | 58.8 | 27.2 |
*The outdoor datalogger was tucked up into a small, open-frame, gable roof over an entryway (clad with dark green metal); on the day this logger read 79.4° F, the closest weather station recorded a high temperature of 66°F.
No space (except the outdoors) reached a relative humidity of 70%.
The table below shows averages for each space.
| Space | Average temp (°F) | Average RH (%) |
| Outdoors | 33.7 | 60.2 |
| Basement | 45.5 | 52.9 |
| 1st floor | 46.1 | 37.9 |
| 2nd floor | 46.8 | 32.2 |
Is precipitation driving swings
There are a lot of big swings in both relative humidity and temperature in the three interior spaces in this shutdown home. Take a look, for example at the image below for the morning of January 12.
Here’s the data for the morning of January 12 (7 am) from all four loggers, adding in the absolute moisture content in grains of water per pound of dry air.
| Space | Temperature (F) | RH (%) | Absolute Humidity* (g/lb dry air) |
| basement | 41.7 | 57.8 | 23.7 |
| 1st floor | 44.6 | 63.8 | 29.3 |
| 2nd floor | 49.0 | 58.8 | 31.9 |
| Outdoors | 56 | 98.4 | 69.7 |
*using an altitude of 1500 feet above sea level.
Remember from Part I of this project, I identified three drivers of moisture content in the home: bulk water (including capillarity/wicking), air leakage, and diffusion.
I looked first for a connection between interior moisture and precipitation, with the assumption that if connected, precipitation is getting into the basement (perhaps primarily up through the basement slab and/or splash-back onto the above-grade portion of the foundation (which is mostly several courses of brick).
I used daily weather data from weather underground; the closest reporting station with daily data is North Clarendon, about 40 miles from Saxton’s River. I simply could not find a consistent relationship between precipitation and interior moisture. And for January 12, the absolute moisture content of the air in the basement is less than the floors above.
What about air leakage?
Incidents like the morning of January 12 are interesting because both temperature and interior relative humidity increased a lot and quickly. And on the same day, the image below shows air temperature, precipitation, and wind.
Sure looks as though after precipitation (in this case, rain)—while the outdoor moisture content is quite high (at least in relationship to interior moisture content)—it’s wind that drives moisture into the home by way of air leakage. Checking similar conditions throughout the home shutdown period supports this.
Similarly, air leakage driven by wind can rapidly dry out interior spaces; see the table and weather data screenshot below for January 17. When it is dry and windy outside, air leakage is drying out the interior spaces (with the above-ground floors responding more quickly than the basement).
| Space | Temperature (°F) | RH (%) | Absolute Humidity* (g/lb dry air) |
| basement | 38.7 | 51.8 | 18.9 |
| 1st floor | 36.4 | 31.0 | 10.3 |
| 2nd floor | 34.1 | 33.9 | 10.3 |
| Outdoors | 15.0 | 38.4 | 4.2 |
NOTE: While I did not conduct a blower door test on this home, I do know that no air sealing has been done and the home seems quite leaky.
Conclusions
I don’t want to draw too many conclusions based on the data from just one building, but it sure seems as though unless buildings are getting wet from bulk water, interior spaces with no other source of moisture respond to outdoor air moisture content, with the response deepened and quickened in air-leaky buildings.
So, yes, in general, homes can go on vacation too, at least hygrothermally…
-Peter Yost is GBA’s technical director. He is also the founder of a consulting company in Brattleboro, Vermont, called Building-Wright. He routinely consults on the design and construction of both new homes and retrofit projects. He has been building, researching, teaching, writing, and consulting on high-performance homes for more than twenty years, and he’s been recognized as NAHB Educator of the Year. Do you have a building science puzzle? Contact Pete here. Photos and illustration courtesy of the author.
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15 Comments
Very interesting. It would be great to see an extended version of your test showing the comparison with a tight building envelope house/cabin closed for winter. Part III?
I appreciate you providing this data. I'd like to share two comments about your conclusion "...in general, homes can go on vacation too...".
1) This data was only for a winter "vacation" in a cold climate. Cold climate winters often have very dry air. A house that is shut down over the winter in a milder, wetter winter climate like the Pacific northwest may experience much higher humidity levels.
2) A house that's on "vacation" during a hot, humid summer may also experience much higher humidity levels. The summer dew points here in the mountains of Virginia in recent years are now frequently 65-72 degrees. I've measured humidity levels as high as 70% inside homes without AC.
Without active AC in this and similar climates, bed sheets and paper become extremely damp - especially during the night. I've seen mold start to grow on susceptible items (such as books) in this situation.
So, although a house vacation during a cold winter may be fine, that may not be the case in other seasons or climates.
Yes I know exactly what your are talking about. Try having a house at the Jersey shore where we can have very cold winter days but very humid days in fall and spring before the season opens-with wind driven rain coming in sideways. As I mentioned in a previous response, we spend a fortune keeping dehumidifiers going through these shoulder seasons until it becomes too cold for them to work....
So you might look at it by asking how things stored in unheated, but well-built sheds in your climate fare? My observations mirror yours: In the PNW I get minor rusting on some tools, sandpaper and notepads are soft and unusable for much of the year, and any fabric (like rags) smell of mildew. However I don't find any mold or deterioration of the structure or finishes.
Good points Debra - would love to conduct/compare similar work on homes in other climates. It's really pretty easy; just needs a willing homeowner and 4 loggers...
Nice work! This is a really important question because there are hundreds of thousands of vacation homes in America that are only used part of the year. Many people leave heat/AC on all year to avoid freezing pipes and mold issues, but this wastes a lot of energy on unoccupied houses. So it should always be best practice to shut a house down (including hot water heater, "always on" appliances, heat and AC) when a house is going to be unoccupied for any period of time. And yet in places like the Jersey Shore (where my dad has a house used only 4 months a year), shutting a house down raises all sorts of risks, including those associated with variable humidity levels. We spend a fortune running dehumidifiers in the off season....
Now that you have some data, I would love to see an article on best practices for those who want to shut down their homes for much of the year (particularly winter). If this would encourage more folks to shut the heat off during the winter it would do a good service toward reducing CO-2 emissions...
I suggest air sealing the house very well so you aren't spending so much on dehumidification.
Note that air at only 60% humidity doesn't mean that some cool, high thermal mass surface (eg, floor or wall) isn't experiencing 100% humidity, condensation and retention of condensed moisture, leading to mold.
Indeed Jon -
I really should have included in the article that the homeowners report NO mold or mildew problems at all each time they return. I think the rather wild swings inside over these winter months mean drying that equals wetting.
Pete
The low temperatures probably also help. It's tough for mold to do much growing below 50F.
“[Deleted]”
For shore houses, I find that heating them to 45-50 degrees in winter keeps them dry enough to avoid mold, and with our average winter temps well above freezing, this amount of heating doesn't cost too much in well-built houses. Drywall and other modern finishes don't really like dampness and freezing conditions, so a little bit of heat works well. If using dehumidifiers, setting them to 60%-70% is also generally enough to avoid mold without costing too much in energy.
Jon is right - air sealing can help a lot. But this can be difficult in typical shore bungalows. Many of these are uninsulated and leaky as a sieve. Traditionally, they used solid wood for structural members , interior and exterior finishes. Wood doesn't care much about freezing or dampness. Drywall and wall paper hate it. High-mass items sweat a lot during our somewhat frequent warm, damp days after a cold spell.
As mentioned there is a bit of solar gain so perhaps adding a circulation fan into the mix to evenly distribute the heat gained may help. There is a PV array so even it you turn off the power when you are not there you can use that to power a fan or maybe even a small heater if necessary.
Great article, Peter.
Off topic: I love your new "UnBuildIt" pocast. You should spread the word on GBA!
I agree it would be great to see this in more climates and with less leaky houses! I have no idea what to leave my heating or Zehnder system set to when I leave for 1-2 months. I will be measuring it starting in 2022, and will be able to control things remotely. But I'm not even sure what the targets should be to prevent mold.
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