I’ve recently constructed a new dedicated 24′ x 18′ workshop in my backyard. It has a cement slab floor with a vapor barrier underneath the slab. I’ve screwed down 2×4 pressure treated sleepers over foam sill seal and planned to lay the subfloor (3/4″ T&G Advantech) today which will serve as my permanent floor. However, when I entered the shop, tools I had laying on the sleepers showed signs of condensation and rust and even the craft paper on the wall insulation was damp as were any papers and cardboard laying around the shop. So I’m hesitant to lay the subfloor and move my all my tools into this rust crib until I address this problem.
Here in eastern Maryland, the temperature has recently increased from the 70’s to the low 90’s and humidity is around 80%. The humidity in my shop is about the same. The inspector said I need to close off the interior walls with drywall or sheathing but now I’m worried about creating a condensation and mold problem in the walls not to mention all my tools rusting. The walls are 6″ thick with R-19 insulation and Tyvek housewrap (no the exterior).
Questions: should I apply a poly vapor barrier over the sleepers before laying the subfloor? Do I need a special type of drywall for the walls or other interior vapor barrier that is mold resistant or will regular drywall work? I hate the idea of running a dehumidifier 24/7 so are there alternative structural solutions to minimize condensation and rust?
– Lyptus
Replies
breaktime
I think if you post this over at Breaktime you may get more knowlegeable responses.
New buildings particularly new concrete will be giving off moisture for awhile. Humid conditions slow the process. Open the windows and run some fans 24/7. Never leave your tools lying on construction lumber and never leave a piece of it on one of your iron tops. It has a fairly high moisture content. The construction related moisture will leave in time and you don't have to worry about rotting walls. You can expect some nail pops and other shrinking come this winter when the heat goes on, that's life.
Don't put poly sheating over your sleepers, it will trap them in a sandwich between the concrete and plastic causing premature rot and degradation of the sill foam. If you have a good vapor barrier under the concrete and manage the drainage around the building, you should have very little moisture wicking up after the concrete is completely dry. That can take quite some time.
Use a vapor barrier on the walls under the sheetrock. Unless you are going to be washing cars or splashing water around, you don't need special sheetrock. Taping and painting the sheetrock will add more moisture to the building but that will pass. Warm air (heated air) will carry water vapor until it hits cooler temperatures, then the water vapor condensates. You don't want this to happen in the wall cavity during the winter, freezing on the insulation. That's why you use a vapor barrier in the walls, to prevent the moisture laden air from getting into the insulation. Sheetrock is permeable and won't stop the moisture from passing through.
The attic should be vented by continuous ridge and soffit vents. Moist air getting in the ceiling will disipate with the movement of air, unlike in the walls which are not vented. Don't use a vapor barrier on the ceiling. If you don't have complete ventilation, the insulation will get wet from condensation. The more insulation in the ceiling, the better to stop heat loss and associated issues. Insulation and ventilation are key in climates where you heat for extended periods, helps with cooling, too.
It would have been nice if you insulated under the slab with 2" foam, makes the floor a bit warmer since the ground temp will be close to 40 degrees in winter. How are you going to heat the shop? Some sources, like propane, can cause a lot of moisture. In humid summer months, air conditioners dehumidify for you, if you will have one. Otherwise, you'll have to run fans and maybe a dehumidifier, unless you can change the weather.
vapor barrier in wall
The building has Tyvek house wrap over the sheething on the outside to serve as a moisture barrier. Won't adding another vapor barrier behind the sheetrock trap moisture in the insulation which will be between the Tyvek and the interior vapor barrier?
- Lyptus
It is okay to have the Tyvec moisture barrier on the outside. That moisture will eventually get out. It is imperative to have a good moisture barrier on the inside of the exterior walls where the moisture tends to penetrate during the heating season.
Tyvek is not a vapor barrier, it's a moisture and air infiltration barrier, only liquid water will not pass through it but water vapor will. You need to stop the warm interior air, which contains water vapor, from getting into the insulation or the insulation won't work. It will get wet from the condensation and you can get rotting within the wall cavity.
Humidity
I live on the west shore in Aberdeen. I have a 24 x 24 shop, concrete slab, 6" walls and R60 in the ceiling. Once the slab dried out, I haven't had a problem with moisture. My slab is sealed with epoxy paint but no floor over it.
type of vapor barrier for walls
Thank you all for your feedback! Can you also provide some product suggestions for a vapor barrier to place behind the sheetrock on the wall? Should the vapor barrier also be placed underneath the sheetrock on the attic walls and ceiling?
My new shop does have a continuous ridge vent, soffits, and space between the insulation and exterior sheathing.
Thanks again,
Lyptus
I recently taped a plastic sheet to my wall studs and to the cement floor to see where moisture might be coming from. The plastic sheet on the cement floor was dry after two days and so was the plastic sheet taped to the wall studs. However, I did notice condensation on the top of plastic laying loose on the floor leading me to believe that the moisture is coming from condensation of the inside air near the cool cement floor. So I'm now thinking that my raised wooden subfloor and cabinets might solve this problem by keeping the cool cement floor away from my tools.
On a related note, I've been reading elsewhere that painted drwall should serve as a sufficient vapor barrier in temperate climates without needing poly sheeting or other additional vapor barriers behind the drywall. Any thoughts on painted drywall as a suitable vapor barrier to reduce humidity in a workshop?
- Lyptus
Wet
I just now, (really), came in from my shop. I was sanding rust off of my table saw top. I made two gates for some new fence we were putting up last weekend. That pressure treated lumber is soaking wet. I left one gate on my saw over night and it rusted the top badly.
Somewhere in the back of my mind I knew it was a mistake but, oh well.
I think you'll have to ventilate your shop for a while to get the wood and concrete dried out. Good luck.
Best regards,
Fred
I would recommend heading off to the closest Drug Store and buy a $25 advanced hygrometer so you can, in any event, have an idea what your humidity level is.
I know that you're trying to maintain 40-50% humidity in your workshop. Mine was between 60 and 65%.
The greater part of us manages this issue as an individual room directs. For example, when I shifted my 1500sq. ft. shop in the basement of a building, I realized I was in it for the whole deal, so I made some temporary arrangements. It was constrained air warming. I fixed/obstructed the admission vent, at that point fixed up the two rooms with caulking, doing every one of the roofs and floors, introducing entryway seals/weatherproofing , (*inside entryways I am discussing) , purchased my own radiators (two Kenmore compact oil filled rads) and did all that I could to seal this shop so I could control the stickiness. I at that point purchased a dehumidifier and Now it keeps the humidity at 45%.
Your circumstance may simply require purchasing a dehumidifier.
I trust this makes a difference.
Treated lumber wet....
Just a small note...whenever I've bought treated lumber, it seemed quite high in moisture content. Just another part of the overall high moisture situation in your new shop? IMHO, not a good idea to leave tools laying on new PT lumber.
Oh, I forgot to mention it that I got to know about the humidity and dehumidifiers from a website named "https://dehumidifierslist.com/".
I searched my problem on google and landed on this site.
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