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

[Simon Robinson 4Participant @simonrobinson4]

[Simon Robinson 4Participant @simonrobinson4]

I have a 6 ft by 4 ft wooden tool shed with apex roof which gets very damp in winter. I’ve re-sealed the roof but there’s always dampness inside the shed despite being placed on a concrete patio on sleepers with reasonable space around it.

Should the vents be on both ends of the shed just under the apex point or should there also be vents low down as well? Any other ideas?

[DutchDanParticipant @dutchdan]

Advise I've read suggests placing one vent high and one low, the moist air is heavier and should exit from the lower vents. Not sure how much of a difference it'll make, but probably cheap enough to be worth a shot.

[BazParticipant @baz89810]

If it were me I would invest in a decent dehumidifier, dessicant one for preference and one that has a continuous drain facility.

[BazyleParticipant @bazyle]

Surprise ! the UK is damp in winter (and most of summer).

If you make holes in a shed it will be damp. Best thing is to seal it up and just open the door after very frosty nights when the air will be dry and really hot days in summer. You especially don't want to open the door on a day when it has changed from cold to feeling warm because a wet warm front has moved over the country.

[Dave HalfordParticipant @davehalford22513]

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.

[Ady1Participant @ady1]

Posted by DutchDan on 16/03/2022 16:26:08:

Advise I've read suggests placing one vent high and one low, the moist air is heavier and should exit from the lower vents. Not sure how much of a difference it'll make, but probably cheap enough to be worth a shot.

Works for prairie dogs, so their burrows get ventilated

eek! eek!

[John HaineParticipant @johnhaine32865]

A condensing dehumidifier would be good. Moist air is not heavier.

WHY IS MOIST AIR LESS DENSE THAN DRY AIR AT SAME TEMPERATURE

METEOROLOGIST JEFF HABY

The units of density are mass divided by volume (m/V). Density will increase if either mass increases while the volume remains constant or if volume decreases while mass remains constant.

Density of air will vary as the temperature and moisture content in the air varies. When the temperature increases, the higher molecular motion results in an expansion of volume and thus a decrease in density.

The amount of water vapor in the air also effects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.

The two most abundant elements in the troposphere are Oxygen and Nitrogen. Oxygen has an 16 atomic unit mass while Nitrogen has a 14 atomic units mass. Since both these elements are diatomic in the troposphere (O2 and N2), the atomic mass of diatomic Oxygen is 32 and the diatomic mass of Nitrogen is 28.

Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become.

You may be familiar with the concept that moist air is less dense than dry air. This is true when both have the same temperature or when the moist air is warmer. Said in another way, air with a greater percentage of water vapor will be less dense than air with a lesser percentage of water vapor at the same temperature. Often people erroneously believe that moist air is denser than dry air because very moist air is more difficult to breathe than dry air.

[David George 1Participant @davidgeorge1]

I have a dessicant dehumidifier in my garage and it works great and provides a bit of background heat as well.

David

[Jon LawesParticipant @jonlawes51698]

I use a condensing dehumidifier which actually warms and defrosts itself if it gets too chilly. This is quite useful for providing water for steaming as well.

[Howard LewisParticipant @howardlewis46836]

Firstly, when the felt on the roof eventually failed, I replaced it with rubber. (Mine has a flat roof, faliing from 8' at the to to 7'6" at the back, over 6'9" ) It is 10' 9" long external, so not too big.

The materials, and trims came from Rubber for Roofs. I gave the dimensions and the site calculates the material needed Order placed in the afternoon, everything, (Rubber sheet, adhesive, trims, ring shank nails, arrived 0800 next morning ).

make sure that you position the nails correctly before hammering them in. They do not come out easily!

INSULATE! Line the inner walls, but put glassfibre or mineral wool insulation between the inner and outer skins.

If you can, insulate the roof in similar fashion.

My shop has no windows, and inner lining to walls and roof is 12 mm ply, on 50 mm framing, with glassfibre between.

On the back wall,there are two small fixed vents close to floor level, and a fixed vent near to roof level. This has an external hood to prevent any rain ingress, when the small, rarely used, intake fan is running.

The moist air will exit at floor level and fresh, hopefully, drier, air enter through the high level vent.

SECURITY

The door is a fire door, with a 6 lever lock. I wanted a 5 lever, but the door came with the 6 lever. The weight necessitated a 100 mm wall for the door end.

I made hinge bolts by turning the heads off three fairly hefty woodscrewss. they were screwed into place by using a drill chuck, until only about 18 mm protruded.

Partially closing the door marked the post to show where to drill the holes.

So, the lock will be difficult to pick, and the hinge bolts will prevent the door being lifted out if the hinge pins have been driven out.

HTH

Howard

[James AlfordParticipant @jamesalford67616]

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

[Jon LawesParticipant @jonlawes51698]

I think it's crucial to have free flow of air through that gap. Maybe that is what you've achieved.

[James AlfordParticipant @jamesalford67616]

Posted by Jon Lawes on 18/03/2022 12:00:03:

I think it's crucial to have free flow of air through that gap. Maybe that is what you've achieved.

I think that you are right. I do have a small computer fan over the vent holes in the apex, intended for use if I a using any noxious materials in there, but I have yet to turn it on, as much as anything else because the whole building acts like a resonating box and it is surprisingly loud.

James.

[Neil WyattModerator @neilwyatt]

Interesting to read how damp air actually rises. The condensation problem on my metal roof was terrible, even with a dehumidifier. I used the metallic bubble film. It has a reputation for not being hugely effective compared to other insulation, but I wanted something that wouldn't lose me headroom and could go between beams and the roof as much as possible. It's been really effective, I used a version with self-adhesive strips on it, but ended up using a roll and a half of double-sided tape as well for about 20m^2.

I'm hoping it will help keep things cool in summer as well.

Neil

[Peter Cook 6Participant @petercook6]

My little 6 x 4 shed (used to store the mower, shredder and other potentially useful junk) used to get damp. I installed a couple of passive vents, one low at the back, one High at the front and a cheap solar powered vent in the door. That has kept it dry ever since.

[Simon Robinson 4Participant @simonrobinson4]

Posted by Peter Cook 6 on 19/03/2022 14:16:53:

My little 6 x 4 shed (used to store the mower, shredder and other potentially useful junk) used to get damp. I installed a couple of passive vents, one low at the back, one High at the front and a cheap solar powered vent in the door. That has kept it dry ever since.

What sized vents did you use?

[Simon Robinson 4Participant @simonrobinson4]

Posted by Neil Wyatt on 19/03/2022 10:15:09:

Interesting to read how damp air actually rises. The condensation problem on my metal roof was terrible, even with a dehumidifier. I used the metallic bubble film. It has a reputation for not being hugely effective compared to other insulation, but I wanted something that wouldn't lose me headroom and could go between beams and the roof as much as possible. It's been really effective, I used a version with self-adhesive strips on it, but ended up using a roll and a half of double-sided tape as well for about 20m^2.

I'm hoping it will help keep things cool in summer as well.

Neil

Never though about it but car roofs must get a lot of condensation between the upholstery and metal roof. I’m not sure how they prevent condensation in vehicles otherwise the ceiling would soaking wet most of the time.

[Simon Robinson 4Participant @simonrobinson4]

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.

[Simon Robinson 4Participant @simonrobinson4]

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.

[Roger BestParticipant @rogerbest89007]

Maybe, vents will help, small 6" ones will make all the difference.

Condensation will form on any cold metal object, cold being relative to the relative humidity. So you won't remove the problem entirely but if you wrap up your equipment it will help a lot.

Best option is the proper insulation with dehumidifier. A little heat will improve that "relatively cold" parameter.

Good luck.

[Roger QuaintanceParticipant @rogerquaintance14196]

Nature abhors a vacuum, if the air outside is damp it will be damp inside unless you have dry heating, and by that I mean not propane which only makes the damp problem worse. Dehumidifiers will collect the moisture from the air making space for the next lot to come in, in short without heating you are on a good hiding to nothing and forever fighting rust.

[Peter Cook 6Participant @petercook6]

Posted by Simon Robinson 4 on 19/03/2022 15:00:31:

What sized vents did you use?

I used a pair of Activents (Activent Ventilation Solutions for Sheds – Condensation Solutions). They needed a 60mm hole in the wall, but have a cowl to keep out rain and an insect screen to keep out bugs (the gap under the door is however still available to them!).

Not sure if its the passive vents or the solar extractor that does the most good. The solar unit drags fresh air in most enthusiastically when the sun is shining and the air is fairly warm and dry, less so on dull damp days and not at all at night when the air is coldest.

[BazyleParticipant @bazyle]

Posted by Peter Cook 6 on 19/03/2022 16:50:23:

when the sun is shining and the air is fairly warm and dry,

When the air is warm it is able to hold more moisture. Having drawn it into your workshop when it cools down it will drop it's excess moisture.

[Peter Cook 6Participant @petercook6]

Posted by Bazyle on 19/03/2022 19:13:40:

Posted by Peter Cook 6 on 19/03/2022 16:50:23:

when the sun is shining and the air is fairly warm and dry,

When the air is warm it is able to hold more moisture. Having drawn it into your workshop when it cools down it will drop it's excess moisture.

Agree on the physics, but the key factor is the sunshine which drives the ventilator. On sunny days the air is likely to be drier. Warm damp (rainy!) days tend to be dull and overcast – and the ventilator doesn't draw that air in nearly as enthusiastically.

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