Shed ventilation

I have a new shed that is 5m x 3m x 2m2 (LxWxH).

It is of timber frame construction with a shiplap exterior and lined with OSB and sits on a concrete slab. It has a butyl-covered almost-flat roof.

What does it require in the way of ventilation? Currently it has none.

I'm wondering about a number of metal slot-vents - perhaps like these:

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at the top and bottom of the outer wall with corresponding holes in the OSB lining.

TIA

Richard

Reply to
RJS
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Usually none.

Or have you insulated the inside?

Reply to
Tim Watts

B and sits on a concrete slab. It has a butyl-covered almost-flat roof.

ww.screwfix.com/p/map-vent-vent-louvre-vent-silver-76-x-229mm/68100 at the top and b

ottom of the outer wall with corresponding holes in the OSB lining.

I built a similar shed ten years ago, immediately noticed a damp problem, fitted four of those louvres, top/bottom on opposite sides. Total damp cure.

Bill

Reply to
Bill Wright

If the slab's damp sealed, probably none. If you get mould you may need some.

NT

Reply to
meow2222

Mine has "natural ventilation" consisting of deliberate gaps of roughly 2 cm between the ground & the bottom edge of the cladding on all sides. I also used corrugated plastic roofing sheets (Wickes own-brand, I think) with a good overhang on the high side but nothing to plug the gaps. The door is the least ventilated part.

But this shed was homemade from a homebrewed design.

Reply to
Adam Funk

Whirly bird in the roof?

Reply to
F Murtz

As you won't be cooking or showering in there (or indeed spending much time in there) no moisture will be generated. So the answer is none. There will be enough air leakage.

Reply to
harryagain

Hi Harry

There are signs of mould already - it's been up for just under a year.

Richard

Reply to
RJS

No insulation - other than that created by a double skin.

Reply to
RJS

And plants that grow outside the shed have an annoying tendency to find any hole big enough to grow inside as well.Close spaced mess can help of course. Brian

Reply to
Brian Gaff

Don't want to pierce the butyl roof.

Reply to
RJS

And are the skins moisture permeable?

Reply to
Tim Watts

In message , harryagain writes

Shiplap is not likely to be airtight, particularly as the boards dry out and shrink. You may find condensation an issue on stored items which do not follow air temp. changes quickly. (large lumps of cast iron) WD 40 type products can help.

Reply to
Tim Lamb

Didn't you know, what ever Harry says, the opposite is the truth/correct way to do things :)

Reply to
Gazz

In my quest to understand why things get rusty and damp in a shed, I have been monitoring the temperature and humidity inside a shed and outside.

One thing I have learned is that it's not a simple issue!

In winter, mostly a shed stays a little warmer and slightly lower humidity than outside, but there are times when this inverts. Also, in winter, the relative humidity is mostly very high (spending most of each day in the

90%'s, dipping below for a couple of hours around midday sometimes). Of course, the absolute humidity (measured as the dew point) is low at winter tempertures, but there's often only 1 degree between the temperature and the dew point, so a very small temperature drop will cause condensation. It's even possible for the temperature to drop below the dewpoint, causing super saturated air (over 100% RH) until the moisture drops out as mist (which holds off forming if there isn't enough dust or pollutants to start the condensing process) or contacts a surface to condense on.

The shed that I'm monitoring (not mine) has been partly insulated now and has a heater keeping the inside RH no higher than 80%. I only recorded a small amount of data before the heater and insulation were installed, although I have data since then when the heater is not been triggered.

Reply to
Andrew Gabriel

The timber in a shed should be treated to inhibit mould/fungus. I assume the shed rests on bricks to lift it off of the ground benath. Under the shed is the most important place for ventilation in order to lengthen it's life.

The main thing to watch out for is rust on your tools. I put mine in a heap and spray WD40 over the pile. I don't leave electric stuff in there for security reasons. Without a little heat you will never totally eliminate damp in your shed, it will be the same relative humidity inside as it is outside. So, unless you are generating moisture vapour inside the shed, excessive ventilation is pointless.

You can generate a little heat if you have south facing windows which helps.

Reply to
harryagain

The mould is appearing on the underside of the lowest part of the roof between the roof joists. I painted the underside with PBW emulsion.

Shed is on a concrete slab - I think I mentioned that in the OP. :-)

I had wondered about fitting a tubular greenhouse heater or humidity-switched Expelair (or similar).

Rgds

Richard

Reply to
RJS

Hi Andrew

Are you suggesting that insulation - under the roof, perhaps - might help?

Rgds

Richard

Reply to
RJS

In message , Andrew Gabriel writes

Worst case is fog after a long cold snap. Lumps of metal are slow to warm and the high humidity leads to condensation and rust.

All my machine tools get a coat of de-watering spray. Not actually WD40 but certainly a first cousin:-)

Reply to
Tim Lamb

I do not have evidence to show that either way. It might prevent some of the cooling at night, particularly radiating out to a clear sky, but it would also prevent solar gain heating it during the day. I'm not sure how much you can improve things with a purely passive approach.

This shed has been lined with 50mm celotex under the roof and on 3.5 of the walls (the person doing it is currently busy with something else but intends to finish the insulating). A raspberry pi programmed by me is doing the monitoring and also controlling a heater in the shed to keep it below 80%RH and above 5C. The data shows that whilst the absolute humidity is usually higher outside, that can change quickly when the weather changes, and the shed could do with a fan to forcibly change the air when the outside air becomes drier and the shed is lagging behind still with more moist air inside.

One thing that surprised me was how little heat was required to maintain the RH and temperature within limits. I'm using a large (by today's standards) 1kW oil filled radiator, reduced to 500W by using a series diode. However, the thing operates so little that it doesn't even feel warm when you go and feel it, but that's still enough.

Reply to
Andrew Gabriel

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