Damp shed: what’s the best way to add ventilation?

[James AlfordParticipant @jamesalford67616]

Posted by Simon Robinson 4 on 19/03/2022 15:08:41:

Posted by James Alford on 18/03/2022 09:21:24:

I have a 17' by 19' wooden garage which is used as a workshop. Whether it is luck or not, I do not know, but it gets no damp or condensation.

The walls and doors have foil-backed bubble-wrap insulation stapled over them with flake board on top. The roof is unlined. There is a modest amount of ventilation under the eaves and around the doors which are a "comfortable" fit. There are also a couple of holes drilled in the apex.

The wooden floor is up off the ground, standing on beams. I put a sheet of plastic damp proof membrane over it and covered this with hardboard and those cheap inter-locking rubber mats.

Originally, I also lined the roof with polystyrene covered with hardboard. This was a major cause of damp and was soon removed.

It is not the warmest place on Earth, but it is dry and damp free.

Regards,

James.

Edited By James Alford on 18/03/2022 09:21:43

Edited By James Alford on 18/03/2022 09:22:19

Edited By James Alford on 18/03/2022 09:22:43

I wonder if my mistake was painting the inside walls in white emulsion? It’s stops the wood from breathing and probably increases condensation as dampness can’t soak into the wood and evaporate through to the outside due to the paint. Both inside and outside are painted.

I suspect not. My inside walls, admittedly lined with flake board, are painted with white emulsion and the outside is painted with Cuprinol garden paint .

[Howard LewisParticipant @howardlewis46836]

The vents do not need to be huge.

A LOT of air will ,pass through a 1" hole

Think how quickly a tyre goes down with a pin hole puncture..(My two floor level vents are only 1" diameter, and the high level vent is obstructed by the 6" square ventilation fan, which is rarely needed.)

One of my exhibition "crowd pullers" feeds air at 5 psi through a 1/16" drilling. A 3 cu ft air compressor cycles on about a 50% duty cycle keeping the reservoir between 80 and 120 psi to feed the pressure regulator, that reduces pressure to the 5 psi level. So you can imagine just how easily air at low pressure passes through a small hole!

Howard

[Dave HalfordParticipant @davehalford22513]

Posted by Simon Robinson 4 on 19/03/2022 15:12:46:

Posted by Dave Halford on 16/03/2022 18:41:57:

What did you 'seal' the roof with? It should have been heavy duty felt, the expensive stuff. 6×4 sheds tend to be made cheap with the thin felt and thin walls made from fencing panel timber.

Without gutters rain will run off the roof hit the patio and bounce up wetting the walls.

It’s tongue and groove panels but the felt is thin and poor and nailed on which I’ve had to paint tar over.

I think my big mistake was painting the inside of the shed in white emulsion because the moisture can’t absorb into the wood and evaporate to the outside due to the paint layer and so it just condenses. The outside is also painted but with Cuprinol.

Simon,

There's nothing wrong with nailed on felt

Spend the £42 at B&Q and get some decent 15year felt, rip the old felt off, I think you will find a wet roof under it which will need to dry in the sun for several days. There's so much movement in a wooden building + the thin felt moves as well, it's impossible for that felt paint to work for more than a week.

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