What is the minimum temperature wood floors are safe at, that we can have our thermostat set at (when away)?

Q: We have a home in NC that we do not live in during the winter months. There are hardwood floors throughout the home. We do not want to pay high heating bills since the house is empty, but we also do not want to compromise the floors. What is the lowest safe temperature that we can have our thermostat set at / minimum temperature wood floors can take?

A: This is a difficult question, and one I have not yet come to terms with completely. I don’t think temperature is the main issue. I don’t think cold temperatures by themselves will damage your floor. I took a gamble when I lived in Toronto by installing quarter sawn white oak in an uninsulated but closed veranda. It was all windows with a dirt crawl space below. I left a small heater running to keep the temps above freezing. At night it likely dropped to mid to low 40’s. In the day, with the sun shining temps could go to the mid 70’s even on bitter cold days. However, the space did get an air exchange from coming and going through the door to the outside. Here is an explanation of vacant house syndrome.

CAUSE:
Security -conscious vacationers, a homebuilder’s unsold inventory, whenever a wood floor is deprived of an air flow in the environment, it can and will misbehave. Sunlight through windows generates heat, lowers humidity, moisture vapor enters to balance, nights cool off, humidity builds and wood floors cup. Thermostats set at 60 degrees and outside, winter howls, heating system runs constantly with no moisture added, and floors shrink.

CURE:
Avoid problems by leaving windows “ajar”, have neighbor air the house out occasionally. Treat floors as discussed under cupped, tented, or shrinkage cracks and only after environment returns to normal. Owner to pay.

The above commonly ask questions will help you and your wood floor contractor resolve some of the everyday concerns about wood floors. By no means is this a sure method or procedure. If in doubt, get a second opinion…

Lets just say you have a 5 gallon pail of humidity (moisture). In a cold house that bucket represents a higher level because cold air cannot hold moisture that well, so it will tend to condense on surfaces. Warmer air can hold a lot more moisture in suspension so this doesn’t happen. What are the sources of moisture inside your house? How well is the house insulated and sealed so outside moisture doesn’t get inside? How much sun exposure is there? Best I can say is keep the heat no colder than 50. what about leaving a dehumidifier running? You can have it run directly into a drain.

Follow-up Q: Thank you so much for your reply. We currently have the thermostat set at 65 degrees and we are being told that it is cold there right now with high temperatures in the 20’s. We have never lived in the home since we bought it last year with the hopes of renting it, which has not worked out. We will finally be moving there this July. So, I don’t know definite answers to some of your questions. I think the house is insulated fairly well. And, I think that it does get some sunlight during the day. I’m not sure what you mean by the sources of moisture in the home. We do have a dehumidifier in the basement that runs directly into a drain, but, I think it mainly runs during the summer months. I’m not sure how we would have the dehumidifier drain on the main floor, though.

Anyway, I’m still very confused, so any further input you can pass along would be greatly appreciated. I’m not understanding if it is better to have it warmer or cooler for the humidity levels associated with the floors. The floors are beautiful and one of the reasons we purchased the home and we would hate to have problems surface with them before we even move in. Thank you again for your time and expertise.

A: Sources of moisture? Well, for example a toilet! That is a source of moisture. As soon as people start living in a house, sources of moisture increase. We exhale moisture. We use the sink. We cook. I think I would empty the toilet for sure and shut off the water supply. I’m still consulting a few others on this. I don’t want to give you bad advice and it is a tricky question. Eliminate the possibility for condensation to form on windows, walls etc. and I don’t care how cold it is in the house. If it is as cold inside as outside and there is very low RH in the house, you should be fine with no issues other than perhaps some shrinkage in the wood. Of course, if it is below freezing, you couldn’t run the dehumidifier.

Here is something I noticed the other night. I have hot water rads and vertical blinds which were shut the other night across the front window. It is cold outside. I happened to pull on of the slats back to look outside and noticed water running down the window. Why? I decided to open the blinds part way to allow air movement and the moisture dissipated. Moisture laden air was trapped between the blinds and the cold window so I got condensation. Opening the blinds corrected the issue. Ventilation. Having nobody living in the house should greatly reduce the amount of moisture in the air. The lower the moisture the less condensing on other surfaces. Ventilation and dehumidification I think are more important issue than how cold it is in the house.

I will be back as soon as I have more information.

Follow-up Q: Thank you Craig for helping to educate us regarding this issue. As I mentioned previously, we are concerned about the house being vacant for so long and just don’t want problems with the beautiful wood flooring throughout. You are being so helpful and we will look forward to any other information you can pass along. I think I’m understanding now that it is best to have as low humidity as possible in the house from what you are saying.

A: I’m a member of a flooring forum, so I presented your issue and concerns there. Here is one comment I received:

Facelift-

I recently did an inspection on a home in Eugene, OR. The flooring contractor had done his due diligence and the 4″ BC was not installed before it had acclimated to the home and the dry subfloor. The $1.4 mil home was completed in July ’07 but was not sold and occupied until Aug ’08. during that 14 month period the builder, trying to economize, cut power to the heat source.

Within 60 days of occupation and consistent heating the floor, which looked “pretty nice” at the time of sale, began to shrink and cracks began to open. When I inspected the floor one year after occupation, in spite of the extensive shrinkage and gapping, the average board still measured .015″ wider than the original milled size.

My investigation showed that, during the year of vacancy and no heat, the average outside temp was 54.9 degrees and the average outside rh was 82 %. For the sake of argument, I figured conservatively that if the indoor temp for that period averaged 60 degrees and the rh only 65% each board (with an expansion co-efficient of .00300 for BC) could have expanded 3/64″.

The thing we are talking about is CLIMATE CONTROL. A consistent source of heat will counter-act the chaotic effect of winter sun coming through the window a few hours one or two days a week.

NWFA says that a controlled 30-50% rh and 60-80 F temp will allow for optimum dimensional stability.

A problem with the vacant house syndrome is not only the cold and damp but the unregulated temp swings and shock treatment of the wood that come with periodic bursts of sunshine through the windows. Unofficially, I kept my Willamette Valley (read moist) warehouse 2-4 degrees warmer than the outside temp and never had a problem with any wood I stored there.

I guess the question for your concerned party is how much are they willing to spend to protect their investment.

Does that help to clarify? NWFA feels the temps should not be lower than 60 with RH kept between 30 (which seems rather low to me) and 50% (which seems high to me for winter heating season). I think in winter we should try to keep the RH between 35-42% at about 72F.

Follow-up Q: Thank you so much for all of this information! We currently have the heat temperature set at 62 degrees. So, I’m assuming from what I just read here that we are within a “safe” range. I really don’t know what to do as far as figuring the humidity level.

A: You can buy a hygrometer from Radio Shack etc. or Sears for about $35 which gives you the temperature and RH. Just set it on the floor for half an hour and see what the conditions in the room are. This is a sticky issue for sure. I think I mentioned I installed quarter sawn white oak plank in an enclosed, non insulated porch but left a tiny heater running all winter, just to keep in in the 40’s F range. It would get into the 70’s on the coldest day with the morning sun. So, I had swings in temperature and never had an issue with the floor, but that cut of oak is more stable than flat cut. I think you are fine at the temperature setting you have.

Related Q: We have factory finished hardwood floors. If we leave the house for four months in the winter, what temperature should we set the thermostat on to protect the floors and wood cabinets?

A: I wouldn’t drop the temperature below 65F, and in winter try to maintain a RH level between 35-40%. I would have someone come in once a week to check that everything is OK and let some fresh air in.