Vapour barrier vs air vents - at odds?

[RichyB2024]

Hello,

 

 

I am about to embark on framing, boarding and insulating my workshop (double garage, single skin) and I have a question about ventilation. 

 

I have done a lot of research on cold vs warm insulation and I am going for a cold insulation with PIR boards / vapour barrier inside the insulation. 

 

One thing I can’t get my head around is venting - I understand the basics of stopping moisture escaping from within the insulated area to the cavity, and the purpose of the vapour barrier, but I also know the moisture needs to escape somehow and ventilate. The two seem at odds to me - is installing a view vents just a way of controlling where the air gets in/ out? 

 

Also, how come vents don’t compromise / negate all the insulation?

 

Finally, the garage door seems a bit of a conundrum - do I just insulate it as best as possible and accept some heat loss? (I will need to use it now and again, perhaps once a month)

 

Thank you in advance

 

Rich

Edited August 19 by RichyB2024

[saveasteading]

21 minutes ago, RichyB2024 said:

, the garage door seems a bit of a conundrum

If it is an up and over spring one then there is usually a gap over the top and draughts all round. You won't need any other ventilation. 

Insulation will still moderate  the temerature , but what about floor and roof?

[RichyB2024]

That’s a good point. Will be PIR boarding / vapour barrier the floor and roof too. 

[saveasteading]

Ok so the roof and walls provide short term insulation against  extreme hot or cold, and keeps you comfortable enougn for  a workshop. Until the draught returns it towards the external temperature.

[RichyB2024]

And if I wanted to block off the garage door and seal that preventing a draught - how would the moisture (in the air I exhale) escape? This is what I can’t get my head around. Thanks 

[saveasteading]

Then you need some ventilation.  Insulate and deaughtproof first. Provide controlled  ventilation after that.

If you infill the gap with a window in it you are sorted.    Assuming it is to be a workshop and not a living space.

 

[RichyB2024]

Thanks for the quick reply. Yes it isn’t ever going to be a living space. What counts as controlled ventilation? Thanks 

[Redbeard]

Controlled ventilation  = known cross-sectional area of air in and out, as against uncontrolled - the aggregate of all the gaps in the structure. Seal up the gaps and rule out the uncontrolled, at the same time bringing in controlled ventilation. It may be overkill for a garage but I am a great fan (sorry!) of small fixed vents and slow-running decentralised mechanical extract fans (or one, in your case, probably). The slow-speed extract of the fan is used to pull fresh air *in* via trickle vents or similar. Just relying on the wind blowing in and out of the right holes is a bit haphazard.

[RichyB2024]

Right that makes sense. Thanks for this. I assumed an extractor fan would be the way forward but have no experience with ventilation to say my ideas were correct. 

[LowCarbonLiving]

Hi There,

 

Just signed up to the forums so hello to all. I concur with what Redbeard said. I work in the Retrofit Industry. The aim is to have an air tight space, but, with controlled ventilation. The easiest way to have some degree of controlled ventilation is trickle vents in the windows. It's impossible to know if this is going to be adequate however without an Air Quality Monitoring system, on any given day, as wind speed (and building exposure) will be the main determiner of ventilation . In my experience, once you reach a level of air tightness considered decent for energy efficiency, you absolutely need an active ventilation system to keep CO2 and humidity levels in check. The Retrofit Academy (Retrofit Training Provider) have a Ventilation Guide on their website. Among other things, it tells you how many liters of fresh air are required per person indoors per unit of time. Get an exhaust fan that can handle the ventilation you expect to need based on this guide. An extra consideration would be that a "workshop" might need even higher ventilation rates if hazardous substances are assumed to be introduced to the air (burnt petrol, paint fumes). Then you may need to refer to basically industrial workplace guides for ventilation, I believe the Health And Safety Executive publish such guides.

 

[scottishjohn]

as siad before the draughts around the door will give ventilation 

 

 and unless its for your super car  etc

 

  then just put a jumper on and some portable heating when needed 

 

I doubt you will be in there long when its freezing ouutside 

 or if big enough    you could make an insulted sub workshop area at the back  as i did when younger and doing aero modelling I had a big widow inthe back of the garage 

 Idid insulate the flat roof area with some thick stuff from  https://www.secondsandco.co.uk/

 

Edited August 24 by scottishjohn

[Dave Jones]

non-habitable rooms should not get damp enough to worry about as its us that generate most of the moisture. Insulation job is to prevent condensation by moving the dew point inside the insulation where condensation cant form.

[JohnMo]

If you read building regs for a workshop or garage the ventilation required is huge. Because it assumes you will be driving a vehicle in there and it needs to get rid of fumes etc.

 

But moving on

You are mixing two different reasons for ventilation and stopping ventilation.

 

Insulation and building fabric - you are trying to keep moisture away from the building fabric, the moisture as it passes from inside to outside can condense back to water within the building structure and then over time lead to failure of the building structure.

 

Ventilation, is there to keep humidity and co2 at below a threshold where is not good for human health or the building. Ventilation is a necessary eval, you heat the air and bring in cold - the more you seal the more you have to replace with mechanical means. Old building leaked like sieves so didn't need additional ventilation, air was lost at a huge rate you just didn't notice it.

[JohnMo]

8 minutes ago, scottishjohn said:

put a jumper on and some portable heating when needed

There's your best but of advice.