Official Luthiers Forum!

I know it has been discussed ad nauseum but I thought it may be a good thing to bring up since I am setting up my new shop and this issue has reared it head in the process.

First the details.

My shop has been in an 12' X 17' unfinished room in my downstairs for a number of years now. About 15 years ago I built a garage and since my house is a walk out basement with full 8' ceilings I decided to build my garage as a 2 story garage with the lower half as my future shop. I had pre-stressed concrete installed as the floor of the garage (the upper part) with poured walls for the lower part. In essence the lower part is attached to my house on one side, 2 sides are underground or earth bermed and one side is the walkout with a 6 foot wide opening for a set of double doors as well as a regular 32" standard size door. I have another door that leads into my downstairs in the house. So what I have essentially is a 28' X 22" shop made from 8" cement including the ceiling. The reason I chose the pre-stressed concrete is because I do not need any support posts in the shop. It was a bit strange at first knowing there were 2 cars parked above my head with no visible supports.

I am in the process of just starting to stud in the walls and I needed to learn what process is the best way to do it. While researching it I found out that cement really acts like a sponge when it comes to humidity. I have talked to a couple of contractors and both had different ideas on how to do the finish work. But both talked about me having a wood shop and the issues of humidity control. Up until now I have only kept my table saw and radial arm saw out there and I have always kept a good coat of wax on them since I do not have heat out there. I have noticed over the past couple of years my table saw has had a bit of rust on it but I try to sand it down and keep wax on it to keep the rust down. I just assumed it was because it was cold in the winter.

I never thought that humidity would be an issue since it seems dry and my house is built on gravel so I have fantastic drainage. So the other day I got curious since I am putting my new band saw in there and I took my hygrometer down and let it sit for a day. To my surprize the humidity is 85%! The humidity in my house is usually well maintained at between 40% and 50% and the humidity outside now has been around 60% So I am in the process of learning about cement walls and how to control humidity. What I have learned is that a vapor barrier like plastic is NOT how to address the issue. That will only create more condensation between the plastic and the cement which will cause water to drip down the walls and create a wet floor and will lead to mold problems. The answer is to have about 1" between the studs and the cement. The way to do it is to install 1" of Styrofoam insulation (with out the plastic coating) against the cement and then install the studs against the foam with fiberglass insulation and drywall. This will allow the cement walls to "breath" while giving about 5" of air space to dissipate the humidity before it reaches the inside of the room. Basically it changes the dew point from directly on the cement and extends it out so it doesn't condense into water as easily.

I am still going to have to heat and de-humidify the shop when I get it finished but it seems my issue will not be low humidity but just the opposite.

I just thought I would share this with you since some of you may be thinking of doing something similar or if you are in an un-heated cement basement or similar you may want to consider this issue.

Thanks for reading, I hope it helps someone.

Cheers,
Bob

Perhaps I am not fully understanding what you are saying here...but as one who used to build houses the poly vapour barrier goes on the inside of the studs (towards the room). With the stud space insulated there is no chance of condensation as the poly remains the same temp as the room.

_________________
Doug

Don't let fear or common sense stop you from trying to build something

Yes that is exactly what I thought also. But my neighbor is in construction and I was asking him about the vapor barrier and he told me he use to do it that way but found out later than on poured concrete is is not the proper way. A vapor barrier will actually promote condensation between it and the cement which will lead to mold.

Here is the article he sent me about it. There is also another good one by the EPA that explains it as well (Sorry I didn't save that link).

http://www.buildingscience.com/documents/information-sheets/basement-insulation/?searchterm=insulation

This one explains the idea quite clearly.

When I went to Home Depot to pick up the foam I coldn't find any without the plastic facing on it so I asked if I needed to order it. He said that most of the contractors just pulled the plastic off. There was actually a contractor that was standing there and he confirmed that is what is code now (here in Michigan). He said he did a job and had about half of the framing done when the inspector came in and made him pull it all out to remove the plastic from both sides of the foam. That plastic acts as a vapor barrier.

I think the vapor barrier between the studs and drywall is still called for on a standard wood frame house but now they are saying not to use it on poured concrete walls.

Cheers,
Bob

I think you are on the right track. As a builder of basement homes in the Carolinas I would say humidity is always a concern.

The two big issues that lead to the problem are 1. Spaces, large and small, that don't have proper ventilation. 2. Any area below grade.

I would build it like this and I would work at getting ventilation (Windows, ducting from the house system, or whatever) :

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Joe Beaver
Maker of Sawdust

I think the code may be different here in Michigan about putting up a vapor barrier in a poured concrete structure. With Michigan winters getting well below freezing they say with the vapor barrier it will allow condensation to build up. I think the idea is to allow the humidity from the cement to slowly dissipate through the wall. Then the heat system or de-humidifyer/Air Conditioner to extract it from the air. Everything I have read including the Energy Code put out by Michigan.gov says not to use a vapor barrier when insulating poured concrete walls. I think they do recommend it on stick frame above ground structures though.

We will see how it works when I am finished... I just bought a de-humidifier today and it is already down by 10% and has extracted a few pints of water already.

Bob

It is true that if the concrete wall has been properly water proofed on the outside and a French drain has been installed, then a moisture barrier on the outside of the studs is not needed but I install a layer of tyvek, not plastic, anyway. In cold climates they do require installation under the slab to the frost line and between the dirt and the concrete wall. They specify zones in the international building code (which is used across the US), for installing that insulation. Again a properly waterproofed concrete wall will sweat very little and the barrier on the stud wall is probably not needed.

The main way moister will enter the room is most likely through the floor. That is why I use a 5mil plastic under the slab (required for inhabitable rooms)and install drains under the slab (not required by code). Unless the water can escape from under the slab it will become a problem.

Other than that, even with a good waterproofing job, mold will form in a below grade room unless it has ventilation.

I just looked at the link you posted. Some of the info is good but some not so good. For instance, most waterproofing companies won't warranty there work unless the slab is at least one foot above the footing. You need room to collect the water and drain it away from the joint between the slab and wall.

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Joe Beaver
Maker of Sawdust

Hi Joe,
Unfortunately This was built as a garage and it doesn't have a vapor barrier under the floor but the footing do go down 2 feet below the slab which I think is code here in Michigan. Fortunately I live on a lake which was an old gravel pit in the 1960's and I have about 8 feet of pure gravel which sits on a vein of pure sand below that. My basement floor is about 15 feet above the lake level and we have never had any issues of water not draining away from the foundation.

I wasn't actually planning on finishing the floor since it is going to be a workshop and that is one of my concerns with humidity coming through the floor. I did just install a dehumidifier yesterday and it has so far brought the humidity down to about 55% so far and that is still with bare concrete so I am hoping when I get the walls and ceiling done it will be good to go for my shop. I do plan on installing an overhead heater but as it is right now if it gets 20 degrees outside the room still maintains between 40 to 50 degrees. I won't have a need to air condition it since in the heat of summer with 90+ degree days it stays pretty cool as well. I am hoping the dehumidifier will be sufficient to keep the humidity levels around 45%.

I know this thread is probably way off topic for this forum but hopefully it may help someone in the future that wants to build a poured concrete addition for a shop or has a 2 story garage like I do.

Bob

Hey bob,

I did the floor and my walls like this in my basement a few years back. Also I sprayfoamed the rim joist area.
One thing I think I remember is that after I was done they had switched their recommendation to 3/4 and not 1 inch.
Water vapour will continue to move, but the isolation stops all the less desirable things like mold and cold. Dehumidifier is still required to regulate RH.
Ultimately, if you have a serious water problem it needs to be addressed at the foundation level.

I was very happy with the results from this building method in my scenario.

Gregor

_________________
Wake up and smell the rosewood!

Bob, sounds like you have it well under control. You may want to consider a two part epoxy finish on the floor. Sherwin Williams sells a good one.

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Joe Beaver
Maker of Sawdust

Thanks for the suggestion Joe, I thought I would finish the floor with something but was not sure with what. The bare cement is quite smooth but still collects dust. I have seen some really good epoxy floors and some pretty bad ones that don't hold up. I will certainly check out the S.W brand.

Thanks again for your input and suggestions. I really hope when I am done I don't still have humidity issues... that would not make me a happy camper.

Bob

Bob, if it is within your budget, I would seal the floor, and then put a ply or aspenite subfloor down. Rip 2x4 pressure boards, lay them down 16" OC with a dab of construction adhesive every couple of feet, and then screw the sub floor down.
The first time you drop your favourite chisel or a finely crafted bridge, you'll really be happy you did. Plus, your feet, ankles and knees will appreciate it as well!

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

Thanks for the suggestion Alex, That is something I have been thinking about since my current shop also has cement floors. I guess I am getting old because I have been working in there since we moved into this house in 1990 and it has been in the last couple of years my feet, ankles and knees have been hurting from the cold cement. This year I laid down some plywood and a piece of carpet in front of my bench and my work table which helped a lot.

I didn't actually think about dropping something on the cement floor so I will have to give that some serious consideration. I am sure it will make it warmer in there as well.

Cheers,
Bob