Damp proofing shed floor

by Ronda
(McHenry, IL. USA)

We have an existing shed. It was built using wood timbers cemented into place and has a wood floor. There was no moisture barrier put into place below and there is really no way of getting under the shed. It is the size of a 1 car garage with a full attic. we insulated the celing of the first floor so the attic is good but the main floor everything gets moldy in the damp seasons. What can we do to solve this problem and prolong the life of our shed?

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Apr 15, 2009
Thank you
by: Ronda

Thank you for your help, We will get started on insulating and working out a heating system this summer and hopefully be ready for next winter. But one more question... If we do heat it like you said. What is the minimum temp do you think we can leave it set at to avoid condensation?

Apr 12, 2009
Ventilating the shed floor
by: John - Admin

Hi Ronda,

Thanks for the extra information. I think that you need to check that there is plenty of ventilation beneath the floor so that moisture can't build up beneath the floor. This would take the form of holes in the siding on each side of the shed below floor level to allow cross ventilation. The holes would be trimmed and covered in mesh to keep out pests and critters.

The other two factors to pay attention to are then heating and insulation. The purpose of this is to keep the inside of the shed warm and dry. Most likely some form of low power electric heating, just enough to stop the mildew growing.

How does this sound?



Apr 04, 2009
Answer to Questions
by: Ronda

Sorry for the delay in these answers.
Yes, it is a pole barn. The floor is plywood. The shed is on a slight incline, so the front of the shed the support under floor touches ground but the rear of the shed is above. But not enough for us to get under.

Mar 30, 2009
A couple more questions to help get to the root of the problem.
by: John - Admin

Hi Ronda,
Thanks for your question and thankyou Tim for your input, I like the analogy about "the good hat and good boots".

Ronda, could you tell me a bit more?

You say that the shed is formed from timbers cemented into the ground, does this mean that this is a pole barn where the main vertical posts are concreted into holes in the ground and the rest of the structure is supported off this frame?

Also could you say a little more about the floor construction? Is it boards or plywood? Are the timbers that the support the floor in contact with the ground?

What I am trying to do here is identify the source of moisture as I believe that is one of the main reasons that the mold/mildew is enabled to grow. Reducing the moisture content of the air and increasing ventilation are the main two prongs of attack. Some form of heating, as Tim mentions, could help to reduce the moisture content of the air once we have stopped the moisture getting in to the building at source.

Knowing a little more about the problem may help to identify an appropriate solution.



Mar 30, 2009
Shed mould
by: Tim baber

I once read a really good book about building when I was living for 6 years in a shed made of hardboard. It said there are three things that you must do. INSULATE, VENTILATE and HEAT.

It worked for me, so without offering details try these three things.

I would do them in that order of importance, looking for no more than 1 air change per hour with the ventilation (possibly) to save on any heating you find is necessary.

An old saying from the English countryside is that a building needs a good hat and boots. Sounds and looks like you have a good hat, but you are going to need the boots now!

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