Workshed ventilation

[Windows]

My workshop is a shed 10’ x 10’, pent roof.

It’s this guy:


(Not my actual shed, but he looks like this guy)

He’s thin skinned and uninsulated.

In winter, he gets condensation on the roof internally. Sometimes the condensation freezes. Like today. Heavy frost outside on the roof. Light frost inside on the ceiling.

The moisture could come from the air, the ground, or my body. Not sure what proportion comes from where. I forgot to look at the ceiling when I first went in there this morning, but I looked fairly soon after, so the internal frost could have been from 3 minutes of me breathing or maybe it was there before I arrived.

Anyway, last year I drilled holes near the roof internally a couple of places front and back to increase ventilation in the hope of avoiding condensation. It might have improved things fractionally, but essentially it didn’t work.

Should I add more holes? Air bricks on all four sides?

Today I Karcher vacuumed the ceiling then worked in there with the doors open. By the end of the day, outside temps were just above zero and there was no condensation on the ceiling. This gives me hope that ventilation will fix it.

On the other hand, the climate. It always rains here and when it’s not raining there is still water on everything. The air is at 80% humidity even when the temp is below zero. There’s enough glass (it’s not glass) on the shed that when the sun hits, the internal temp rises rapidly and vice versa at night.

 

[Cabinetman]

Not an expert, I would run a dehumidifier in there on a time clock for an hour before you start work, I think the ordinary ones ( without a compressor) will also warm your shed up a little too.
Ian

 

[Mike G]

You may have to force air through that artificially. If you were to have have low level vents letting air in on one wall, and a high level extractor (as per a bathroom) high up on the opposite wall, that would certainly help. Make the path from inlet to extractor as long as possible, It wouldn't need to run continuously, so control it with a timer, or a humidistat. You might also benefit from replacing much of the glass with something more solid, because rapid temperature fluctuations really aren't your friend.

 

[Lurker]

I have a similar workshop. Never had any condensation issues.
Can get cold in the winter but I use one of those radiant (dry heat) heaters if I need to be in there for long.
My windows are double glazed glass, so I wonder if your problem is due to what I assume is polycarbonate windows.

 

[Windows]

Not an expert, I would run a dehumidifier in there on a time clock for an hour before you start work, I think the ordinary ones ( without a compressor) will also warm your shed up a little too.
Ian

Dessicant dehumidifiers put out quite a lot of heat (as my warm bathroom will attest) which could be useful in itself. I think it would have to be a dessicant one. (Compressor dehumidifiers don’t work well at low temps which they would definitely have to contend with here.) I might have to try this, but I’m going to explore the ventilation strategy first since the whole shed is so porous already.

 

[Windows]

You may have to force air through that artificially. If you were to have have low level vents letting air in on one wall, and a high level extractor (as per a bathroom) high up on the opposite wall, that would certainly help. Make the path from inlet to extractor as long as possible, It wouldn't need to run continuously, so control it with a timer, or a humidistat. You might also benefit from replacing much of the glass with something more solid, because rapid temperature fluctuations really aren't your friend.

Do you know of a good place to look for ventilation calculations? Or just a rule of thumb? I did a quick glance at roofing ventilation requirements, but there’s so much AI generated nonsense, I couldn’t find a reliable site that made sense to me. I saw one thing that said 1:150 vent area to roof area. I don’t have as much venting as that currently, so if that is a useful guide, would it be worth adding more vent area first?

 

[Mike G]

Do you know of a good place to look for ventilation calculations? Or just a rule of thumb? I did a quick glance at roofing ventilation requirements, but there’s so much AI generated nonsense, I couldn’t find a reliable site that made sense to me. I saw one thing that said 1:150 vent area to roof area. I don’t have as much venting as that currently, so if that is a useful guide, would it be worth adding more vent area first?

Sorry, I don't know of any such facility.

 

[Windows]

Sorry, I don't know of any such facility.

No worries. I gave up looking at roof ventilation requirements as a baseline and turned to underfloor ventilation. Seems like those calculators would suggest 2 air bricks front and back for an equivalent area of subfloor ventilation which doesn’t seem like a crazy amount for this situation and is more than I currently have, so I think I’ll see how that goes.

I’m also going to consistently wear a mask and work with the doors open when it’s cold. I’m not really in a position to heat the space, so might as well embrace controlling moisture at source and utilising air exchange as fully as possible.

 

[duke]

What floor system do you have for your shed? If it is concrete without a DPM below it possible moisture is wicking up through the slab.
Or if only a wood floor without DPM the same could happen.
As MikeG mentioned a low vent, high vent is a good idea to keep the air moving.

 

[Windows]

It’s a kit shed that had to be assembled very quickly (with the intention of upgrading later - but can’t see that happening in the near future). It came with minimal thickness tongue and groove boards (with open knot holes and gaps), which are attached to a few 2x2s below. It’s sitting on existing paving stones (which are on a slope) with a couple of extra rocks to level it. Despite, but also because of, the slope, the paving below is almost certainly slightly wet from November to May. The shed floor itself is nice and dry, but evaporation from below may come up through the gaps to exacerbate condensation problems.

 

[fuse]

If it were me I would have used a vpm in the construction.
Its not impossible to remove the roof ( next summer) and lay some on battens.
You would need only minimal ventilation, if any ,because the humidity will pass through it and cure the issue continually.

 

[Spectric]

Just need some airflow, condensation occurs when warm air meets a colder surface so keep it warm or prevent the warm air from reaching the colder surface. Any reason you could not use some of that thermal wrap on the ceiling, they even do it with a self adhesive backing.

 

[GaryR]

If the ceiling surface is colder than dewpoint of the incoming air, no amount of ventilation will stop condensation on the ceiling. You will need either to raise the temp of the ceiling above the dewpoint (a heater of some kind) lower the dewpoint of the incoming air (de-humidifier) or isolate the ceiling surface from the moist air (vapour barrier). Or a combination of the above.

 

[SamQ aka Ah! Q!]

Two years ago, I took delivery of a 16'x10' "prefab" shed for the heritage railway I volunteer at, to act as "The Woodworking Kabin"....
The dimensions, construction and siting were all decided and ordered 'above my head' and a camel duly arrived.
They insisted on a perimeter of grond-level-flush breeze blocks, filled in with road scalpings, and the ab0rtion was placed directly on same: "You can't have it on breeze blocks, with ventilation underneath Sam, 'cos the rats will move in and nest!" Bulwarks.
The sides are spit thin, timber obviously cut 20 minutes before being nailed into place. We struggled to fit anorexic Celotex between the dripping, barely adequate green uprights and the floor is mysteriously damp... Fixing thin ply to bolster the barely there Celotex and the vitamin-deficient 1" square urights has helped rigidity, but the doors mysteriously go in and out of fit...🫩.
Moral of the story? Never let a committee near a plan or specification.

 

[Windows]

If the ceiling surface is colder than dewpoint of the incoming air, no amount of ventilation will stop condensation on the ceiling. You will need either to raise the temp of the ceiling above the dewpoint (a heater of some kind) lower the dewpoint of the incoming air (de-humidifier) or isolate the ceiling surface from the moist air (vapour barrier). Or a combination of the above.

As evidence that ventilation is not sufficient on its own, my wood store - which has no wall on 1 and a half sides - has ice on its ceiling this morning. Ventilation will help clear that & dry it out eventually, but it didn’t stop the moisture gathering & the ice forming.