Shed - extractor fan, or passive vents

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porridgebear

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Hey everyone,

Shed project is progressing. Onto the external cladding at the minute. A friend asked if I was adding vents and out of all the many You Tube videos of garden room/shed builds I've seen, I don't recall anyone ever fitting ventilation.

My build is an over-engineering shed. 4x2 stud work, 6x2 rafters, 50mm PIR insulation throughout, but it's a cold roof design with soffit venting. I plan on adding a vapour barrier internally before the ply walls go onto the stud work. Also there is no OSB sheath, but there is a breathable membrane wrapped around, air gap and then cladding. Rigidity provided by noggins and internally using ply walls.

But venting. What do people do with these garden room/shed builds? I have read placing 2 passive vents opposite ends of the building one high one low facing prevailing wind works well. I have read others fitting extractor fan + 1 passive vent (to draw air in) with humidity control.

I would have though vents defeat the object of all my insulation and vapour control efforts, so I am wondering whether vents are needed to keep things cooler in summer. I think this is probably me not understanding heat/cold and how insulation and cold roof venting all works.

Thanks!
 

woodfarmer

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What is your shed for ?
If you are keeping any kind of livestock much ventilation is crucial to their health. Most animals can survive a great deal of cold but not being wet or badly ventilated.

I fitted adjustable vents opposite sides, high up so things (chicks) on the ground were out of the draft. The inlet can be much smaller than the exit.
 

eribaMotters

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I added trickle/slide vents to windows and doors and a BIG Ventaxia diagonally opposite to bench/door area. I could get some through flow of air on a hot day.
Also, despite ceiling mounted air filter and good dust extraction it was surprising how grubby the fan blades got on the fan

Colin
 

porridgebear

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What is your shed for ?
If you are keeping any kind of livestock much ventilation is crucial to their health. Most animals can survive a great deal of cold but not being wet or badly ventilated.

I fitted adjustable vents opposite sides, high up so things (chicks) on the ground were out of the draft. The inlet can be much smaller than the exit.

Can't tell if you're being serious lol. It's a 4m x 2m over engineered tool / potting shed. No animals!
 

Fitzroy

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I’m assuming your shed is actually a workshop/ garden room where you are wanting to be comfortable during your hobby. A vent is potentially required to maintain good air quality, low relative humidity

A useful reference is to look at air changes per hour for passive houses, these are about as well sealed as any house comes. Most of these have mechanical ventilation with a warm/cold air heat exchanger/air conditioner to avoid loosing energy. I think these run at about 0.75 changes per hour. At these rates there should be no problem with condensation, which is what will get you in a poorly ventilated space way before your worried about low oxygen concentration. It’s probably possible to get a shed sealed up tighter than a passive house, especially if you have limited openings, doors and windows.

At 0.75 ACH a 3.5x6x2m space and air warming by 20degC will take 175W to maintain the temperature, with no other heat losses. At 5 ACH this will be 1.16kw, so yes a draughty shed will negate your insulation. Yes no one works in a 20degC workshop but it gives a picture. Basic extractor fans are c. 80m3/hr which would be 2ACH for my shed, more than required.

I’d install some adjustable vents and a humidity meter, and use these to find how much (if any) ventilation you need on cold days. On warm days open your windows/door.

Fitz
 

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