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Humidity from treated lumber

Boy I think I messed up!

I am finishing my walkout basement into a guest bedroom with a full bathroom, a small sitting area and an office. The space is 38 x 13. I used treated lumber to fur the perimeter walls and wall plates and regular lumber for all the interior walls. Well, the humidity in the space has gone through the roof. As soon as I walked in and felt moisture, I had that spine curling feeling that tells me I messed up.

I hope I will not have to take everything down. The cost and time will be very painful. Does anyone have any words of advise...consolation....hope?


Re: Humidity from treated lumber

All new lumber will have moisture even though it was kiln dried at the sawmill. By the time it goes from the mill to the supplier to your finished project the lumber will have absorbed some moisture either by the humidity in the air or being exposed to rain.

It's curious if the sudden rise in humidity you noticed isn't just from covering the basement walls.

Re: Humidity from treated lumber

A dehumidifier would help lower the humidity.

Re: Humidity from treated lumber

Thanks for the reply and comments. The walls have not been covered yet. All I have is the frame to fur the perimeter walls and the interior walls. I only used the treated lumber on the perimeter against the block walls. I am ready to do the electric next but I am worried about this humidity.

My main concern is that when I cover the walls I will trap that humidity coming from the treated lumber. I have purchased a dehumidifier and plan to keep it going during the rest of construction. I am wondering if I really made a mistake by using the treated lumber for furring the walls and what possible complications may come up from it at a later day.

Thanks again for commenting.

Re: Humidity from treated lumber
DwarfWytch wrote:

What is acceptable/best in Canada and Michigan is significantly different then best practices for NC and further south on the eastern coast.

What ??:confused:

As mentioned earlier all wood will accumulate moisture from the point of manufacture to when it's at the project also it will acclimate to the surroundings. This perhaps may only happen in Canada and Michigan and will never happen in " NC and further south on the eastern coast " according to DwarfWytch.

Re: Humidity from treated lumber


My spin is this.....don't worry and run that dehumidfier.

Treated lumber is wood that's kiln-dried to construction-grade specs/levels and then it gets sent to the treatment facility. Then that KD lumber is submerged in the treatment solution ...under pressure. As soon as the treatment chamber is drained, the soaking-wet lumber is tightly banded and hoisted onto a truck destined for your lumberyard......usually. (2KD is the exception to "usually")

The contained moisture/water from the treatment is "free" water/moisture as opposed to cell-bound moisture.
Consequently, that moisture will be able to leave the wood again in a rapid fashion. That's why you're experiencing this sudden & dramatic rise in relative humidity. (If it was the cell-bound water, it would leave at a much slower pace) Run the dehumdifier and you should be fine......but don't close up the walls until the interior RH returns to "normal" levels for your locale.

A potential alternative would have been to purchase 2KD treated lumber. 2KD is kiln-dried a second time...... after the treatment process to remove this excess absorbed free water from the treatment solution. 2KD also costs a bit more, but it's what we buy for our projects. Our yard stores all 2KD lumber under roof so it's close to EMC (equilibrium moisture content) when we pick it up (no absorbed rain water). You may or may not even have a yard nearby that stocks 2KD. Can't say from here. A few phone calls would likely answer that question for future projects.

Edit: Another alternative would be to buy the wet treated lumber and then sticker and stack it either inside a ventilated outbuilding...... or outside under a tarp.... to dry down before you use it. Stickers are pieces of wood (1x1,1x2 or similar) which you'd place between the boards approx. every 16" or so as you stack them up....... so that air can freely flow around each and every board. Stacking the boards directly on top of one another won't do much good in the short term. If you tarp the wood stack, don't wrap the tarp around the pile, but rather use the tarp more or less in a suspended shading/rain-shedding fashion above the stack. You need good air flow to allow the moisture to escape. Leaving the wood to air-dry for ten days to two weeks (or more) should render good results, provided that the ambient temps are above freezing. Warmer is usually better, of course, as would be lower relative humidity levels.

Re: Humidity from treated lumber



Eenie,weenie......chili beanie...........the spirits are about to speak. Oooops........they already have!

Re: Humidity from treated lumber

Goldhiller ... nice post well done with the details.


Eenie,weenie......chili beanie...........the spirits are about to speak. Oooops........they already have!

Oh yes, LOL.

Re: Humidity from treated lumber

The humidity level issue with respect to the PT lumber I think has been answered so I am not piling on there.

The question I have for the OP is how he/she has used vapor barriers and insulation since this is a basement project. :confused: I am no expert, but it seems to me this is a very tricky part of the project that does vary by location. I would check out the building science website for suggestions on the best materials and methods for your area. Especially before you close up the walls!


Someone else may have another site that provides some of this information as well.

Re: Humidity from treated lumber

Well, I feel a lot better now. Thanks Goldhiller et all. Nice to have expert’s advice.

For bp21901, I am not sure if OP refers to me but based on the question, I will assume so.

The walls are block and prior to start of construction I checked the basement for water, wetness, humidity, etc. The entire room (38 x 14) is VERY dry. For insulation, I am using 1” thick extruded foam insulation from DOW. I have the PT 2x4s flat on their backs 16” on center and the foam is cut to fix snug in between. I did not glue the foam to the block hoping to leave a gap of air between the two were possible. This leaves me with ¼” gap between the foam and the drywall once installed. I have not yet looked into the vapor barrier. I see there are plenty of options. Your comments on what I have done and your suggestions for the vapor barrier would be appreciated.

After reading the initial comments about the PT lumber, I was considering taking the lumber down and replacing it with regular lumber but, the work and the waste of $$ was not very attractive. This latest information has convinced me to keep going ahead.

So, this is the plan. The dehumidifier is up and running. It will stay that way the entire time I am under construction. I will leave the vapor barrier and the drywall to the very last so as to give as much time to the wood to dry.

I will keep an eye on the RH. I agree, it should return to normal for this area.

Last but not least, I will keep my fingers crossed that I will not have a major humidity / mold issue after I cover the walls.

Who wants to put it to a vote??????


No go?


Re: Humidity from treated lumber

Hi Flipper,
OP is short for original poster (at least in my version of net abbreviations!). From your description it sounds like you are on the right track with the foam insulation. It may have been better to use it on the entire wall and not just between the flat 2x4's, I don't know if that will make a significant difference.

I have attached a link discussing basement insulation systems. It covers vapor and air barriers, good and bad systems. There is other research there that will probably be helpful to review.

You know your temperature and humidity conditions better than anyone else posting here, so you are the best judge of which strategy fits your scenario.


Have Fun!


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