Shed Build - Vapour Barrier and Insulation Help

[Brdy]

Hi Guys,

Struggling to understand from all the different guides online and even Mikes guide in this forum which is brilliant, but I’d like to confirm with everyone how my new shed should be insulated.

I’m looking to build a 2.4 x 4.8m pent roof shed.

If I use 50mm rock wool insulation in the walls should it be laid out like this from outside in…

Cladding | Batten | Vapour Barrier? | Insulation | 11mm OSB 3



Also the roof, using kingspan or celotex 50mm with a 50mm air gap, from top to bottom…

Felt/Onduline | Vapour Barrier? | Batten | OSB 18mm | 50mm air gap | 50mm Insulation | 11mm plywood


I’m just slightly unsure about the vapour barrier locations really if anyone could help that would be brilliant.

Many thanks.

 

[Fitzroy]

Cladding - batten - breathable membrane - insulation - vapour barrier - osb. However OSB is so glue filled you can exclude the internal vapour barrier.

Roof
Onduline (no idea how you would use felt without a solid substrate) battens - roofing breather membrane - osb - air gap - insulation - plywood. With the cavity between insulation and osb vented at the ends.

However you are going to a lot of effort to put 50mm of insulation in place. 50mm of kinspan may be worth it, but 50mm of rockwool won’t do much for you.

 

[Fitzroy]

The cladding or roof covering is a rain screen. Any thing that gets past is is kept off the building structure by the breathable membrane.

The internal OSB or vapour barrier keeps the warm moist air from penetrating into the walls where at some point the temperature is low enough for condensation to occur. If this does happen then the breathable membrane lets this moisture evaporate overtime, the roof cavity behind the insulation needs to breath for the same reason.

 

[Brdy]

Cladding - batten - breathable membrane - insulation - vapour barrier - osb. However OSB is so glue filled you can exclude the internal vapour barrier.

Roof
Onduline (no idea how you would use felt without a solid substrate) battens - roofing breather membrane - osb - air gap - insulation - plywood. With the cavity between insulation and osb vented at the ends.

However you are going to a lot of effort to put 50mm of insulation in place. 50mm of kinspan may be worth it, but 50mm of rockwool won’t do much for you.

The cladding or roof covering is a rain screen. Any thing that gets past is is kept off the building structure by the breathable membrane.

The internal OSB or vapour barrier keeps the warm moist air from penetrating into the walls where at some point the temperature is low enough for condensation to occur. If this does happen then the breathable membrane lets this moisture evaporate overtime, the roof cavity behind the insulation needs to breath for the same reason.

Thank you for this, what would you suggest for wall insulation? I’d like to try and keep the sound down as the neighbours are fairly close by to me, so don’t want to run loud machines all day

 

[Molynoox]

Sounds insulation is achieved mainly though density, adding more or denser materials to the walls. You can add extra layers of plasterboard for example, and can get special acoustic versions. But also watch out for air gaps, they kill your sound insulation.

So your vented roof (if you follow Fitzroy advice above, which I agree with 100%) is not going to be doing you any favours in that respect.

I have a vented roof in mine and 100mm PIR all round - it does insulate the sound pretty well, but I think sound escapes a little through the tiny gaps in the light fittings, into the roof, and out of the air vents. So it's not optimised for sound, although like I say it is pretty good, and easily good enough for what I need (workshop machines and blasting loud movies)

But I wouldn't change it because I like having a vented roof (I've optimised it for condensation basically, not for sound)

Martin

 

[HOJ]

If you are proposing to use the space all day everyday with machines running, presuming you won't run into the issue of disturbance to neighbours, I'd suggest you build it bigger, put a pitched roof on and work towards sound reduction over vapour control, however I doubt you will keep the door shut on a nice day, so its going to be a case of finding the balance of purpose and function over the limited issue in an environment such as this with vapour control, sometimes the focus goes towards this more than is really necessary, your not going to be living in it, sleeping, cooking, bathing etc which is how the potential vapour/moisture problem propagates itself.

 

[Brdy]

Thanks for helping me understand. It will just be lighter hobby use as apposed to all day everyday, so probably a few hours on a sat and sun. I’ll opt for a moisture control but fill with some insulation on the walls as intended to help warmth.

 

[charles79130]

Thank you for this, what would you suggest for wall insulation? I’d like to try and keep the sound down as the neighbours are fairly close by to me, so don’t want to run loud machines all day

Because I was considering noise and not upsetting my neibours I built my shed 5x3 meters from 4x2” including double doors and insulated withe 4” wool for sound and heat loss

 

[Brdy]

Because I was considering noise and not upsetting my neibours I built my shed 5x3 meters from 4x2” including double doors and insulated withe 4” wool for sound and heat loss

Did it do the job?

 

[charles79130]

Yes no complaints from neighbours also I put 50 mm Kingspan in the roof and under the floor so cooler in summer and not freezing in winter I also used the latest rubber material for the roof guaranteed for 25 years but it is exspensive approx £400 available on EBay but it will see me out
hope this helps
Chas

 

[Jones]

I think you have breather membrane and vapour barrier mixed up. Vapour barrier is thin plastic sheet that is used on the warm side of the insulation to keep moisture from moving into the insulation. In your case the OSB can be considered a vapour barrier so you won't need an additional one. A breather membrane goes on the outside of the insulation and behind the roof or wall cladding. It prevents any water coming through the cladding entering the building and allows any moisture that has penetrated to evaporate through. There should be a clear air space to allow air circulation to the outside of the membrane this may need battens and counter battens depending on their orientation. On the insulation front solid insulation will need an air gap between it and the membrane, rockwool etc will not and can be full fill.

 

[Brdy]

Thanks for this, just in regards to the roof, would I put it over the rafters, then batten, then roofing sheets?

And should I use Osb under the roofing sheets? Or just on the inside of the building

EDIT: Actually this question is covered in Fitzroys first post.

Thanks!

 

[charles79130]

Thanks for this, just in regards to the roof, would I put it over the rafters, then batten, then roofing sheets?

And should I use Osb under the roofing sheets? Or just on the inside of the building

EDIT: Actually this question is covered in Fitzroys first post.

Thanks!

Osb kingspan osb then roof membrain

 

[Jones]

If you're using sheet roofing there's no need for OSB on the roof, you can drape the membrane between rafters to form gutters then batten across and nail sheets to that. If you pull the membrane tight you will need to counter batten as well. I would use a 3 or 4 X1 batten, it's stronger and you can't miss it. I have had mixed success with coroline and onduline, if they are in shade and subject to leaf litter collecting then moss can grow which degrades the sheets quickly, less than 10 years life. In the open it lasts much better

 

[Lard]

Not wishing to repeat anything already said but a vapour barrier (VP) does exactly that, ie it stops vapour from being able to pass through it. What it doesn't do is stop the passing of air molecules and so a VP stops moisture but allows ventilation. ......EDIT - sorry, thats wrong...my error!

Jones (above) hit the nail by saying that what you don't want is moisture getting into any insulation as some (moreso a lot) lose/reduce their insulationary properties when wet.....therefore always main VP on the warmer side of insulation. If you want to add a second (elsewhere for other reasons) then that's your choice

 

[Brdy]

Not wishing to repeat anything already said but a vapour barrier (VP) does exactly that, ie it stops vapour from being able to pass through it. What it doesn't do is stop the passing of air molecules and so a VP stops moisture but allows ventilation.

Jones (above) hit the nail by saying that what you don't want is moisture getting into any insulation as some (moreso a lot) lose/reduce their insulationary properties when wet.....therefore always main VP on the warmer side of insulation. If you want to add a second (elsewhere for other reasons) then that's your choice

Is the ‘warmer side’ the outside of the insulation?

Also, should I use different types of VP for roof and siding?

Is there a particular product or brand that is best or just the first VP I can find?

Thanks again.

EDIT: I think I misunderstood again, so what you are implying is I am better off with a breathable membrane as apposed to the Vapour Barrier

 

[HOJ]

What it doesn't do is stop the passing of air molecules and so a VP stops moisture but allows ventilation.

For clarity, a vapour barrier or vapour control layer (VCL) is basically plastic sheeting, it stops moisture from getting in to the timber frame, and is used to create an airtight seal on the inside (warm side) of a building as @Jones has clarified

I think you have breather membrane and vapour barrier mixed up. Vapour barrier is thin plastic sheet that is used on the warm side of the insulation to keep moisture from moving into the insulation. In your case the OSB can be considered a vapour barrier so you won't need an additional one. A breather membrane goes on the outside of the insulation and behind the roof or wall cladding. It prevents any water coming through the cladding entering the building and allows any moisture that has penetrated to evaporate through. There should be a clear air space to allow air circulation to the outside of the membrane this may need battens and counter battens depending on their orientation. On the insulation front solid insulation will need an air gap between it and the membrane, rockwool etc will not and can be full fill.

 

[Brdy]

For clarity, a vapour barrier or vapour control layer (VCL) is basically plastic sheeting, it stops moisture from getting in to the timber frame, and is used to create an airtight seal on the inside (warm side) of a building as @Jones has clarified

Oops! When Lard mentioned about Jones’ above post, I read his post directly above and not further up which you have quoted! Haha thanks for clearing the confusion up.

 

[Molynoox]

The information provided at the very start from Fitzroy is already spot on so maybe go back to that for reference.
Or maybe a diagram will help. I have one somewhere hang on...

Found them.... the attached diagrams show good and bad design examples plus the reasons. Note that OSB on outside is debatable as an optimised design but it's what 99% of people do (without issues) so I wouldn't get too hung up on that personally

Martin

 

[Brdy]

The information provided at the very start from Fitzroy is already spot on so maybe go back to that for reference.
Or maybe a diagram will help. I have one somewhere hang on...

Found them.... the attached diagrams show good and bad design examples plus the reasons. Note that OSB on outside is debatable as an optimised design but it's what 99% of people do (without issues) so I wouldn't get too hung up on that personally

Martin

That’s perfect thanks!

It’s interesting as some have said that there should be no gap between inside Osb and insulation, although your diagram shows a gap.