Shed Build - Vapour Barrier and Insulation Help

[Molynoox]

You will need a gap (service void) for electrics and the best place to put it is on the warm side such that moist air will not condense on cold surfaces.

Note: if you put your OSB on the inside, as opposed to just plasterboard like on my diagram, then it does change the wall design principles, it can actually reverse the logic somewhat, provided your outside wall is fully breathable (i.e. no OSB). In those cases it could make sense to have the PIR tight against the inside wall. Then you would put electrics either surface mounted in conduits on inside wall (industrial look) or you would have them on the outside surface in the cavity created by the battens (that method is not preferred by most electricians)

Martin

 

[Brdy]

You will need a gap (service void) for electrics and the best place to put it is on the warm side such that moist air will not condense on cold surfaces.

Note: if you put your OSB on the inside, as opposed to just plasterboard like on my diagram, then it does change the wall design principles, it can actually reverse the logic somewhat, provided your outside wall is fully breathable (i.e. no OSB). In those cases it could make sense to have the PIR tight against the inside wall. Then you would put electrics either surface mounted in conduits on inside wall (industrial look) or you would have them on the outside surface in the cavity created by the battens (that method is not preferred by most electricians)

Martin

Ah that’s it then I was going to go with conduit electric incase I want to add stuff in the future, plus it’s easier to hang stuff on the walls if I can see where the wires are.

Thanks again really appreciate it

 

[Fergie 307]

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.

is there any need to have a solid sheet roof under onduline, just seems like a waste of material. I can see it might help with noise in that it would enable you to pack the space more densely, but can't see the point otherwise. I have never done this I just put the breathable membrane over the joists, then battens and onduline. Insulation on top of the inner ceiling, and was always told that ideally the insulation should not be in contact with the underside of the membrane. And for cladding interior walls I have never understood people's liking for osb, nasty stuff and no longer even particularly cheap. I always use 18mm water resistant t&g 8x2 chipboard flooring sheets. Nice surface, neat joints and cheap. Have numerous buildings over the years built this way without any issues. If you have not used onduline before then my advice would be to make sure your horizontal battens are no more than 18 inches/450mm between centres. It has a nasty habit of sagging over time if not adequately supported, particularly if it is at a shallow pitch or in a location where it gets the full force of the sun.

 

[Fergie 307]

Ah that’s it then I was going to go with conduit electric incase I want to add stuff in the future, plus it’s easier to hang stuff on the walls if I can see where the wires are.

Thanks again really appreciate it

Great tip I was shown for getting a pull cord through a long run of conduit with a number of bends. Find a piece of soft foam about an inch square and tie a brick line, or similar strong cord to it. Shove the foam into one end of the conduit, and use a shop vac on the other end, pulls it through in seconds.

 

[Brdy]

Great tip I was shown for getting a pull cord through a long run of conduit with a number of bends. Find a piece of soft foam about an inch square and tie a brick line, or similar strong cord to it. Shove the foam into one end of the conduit, and use a shop vac on the other end, pulls it through in seconds.

is there any need to have a solid sheet roof under onduline, just seems like a waste of material. I can see it might help with noise in that it would enable you to pack the space more densely, but can't see the point otherwise. I have never done this I just put the breathable membrane over the joists, then battens and onduline. Insulation on top of the inner ceiling, and was always told that ideally the insulation should not be in contact with the underside of the membrane. And for cladding interior walls I have never understood people's liking for osb, nasty stuff and no longer even particularly cheap. I always use 18mm water resistant t&g 8x2 chipboard flooring sheets. Nice surface, neat joints and cheap. Have numerous buildings over the years built this way without any issues. If you have not used onduline before then my advice would be to make sure your horizontal battens are no more than 18 inches/450mm between centres. It has a nasty habit of sagging over time if not adequately supported, particularly if it is at a shallow pitch or in a location where it gets the full force of the sun.

Thanks guys, regarding the Onduline, it will be south facing and shallow pitch so will be sure to go max 450mm centres thanks, I’ll also look into the chipboard flooring, I’m assuming hanging vertical not horizontal.

Thanks for the shop Vac tip!

 

[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.

Apologies for my previous posting with incorrect info (the crossed out bit) and also VP instead of VB.
Was trying to write 2 things at the same time and should have reread before posting. I think I had heat transfer on my mind and not air transfer.

Its all about interstitial condensation and dew points.

 

[Goesbysteve]

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

Does that recommended air gap design change when dealing with solid concrete block walls?

Could you share the link to that original source if the images please?

 

[Molynoox]

Does that recommended air gap design change when dealing with solid concrete block walls?

Could you share the link to that original source if the images please?

Hi Steve

the images I supplied are 'debranded' versions of the content from my own website. I am not sure on the rules / ethos about providing links here - hence why I used generic versions when posting originally.

I will provide the link below because you have asked for it - in parallel I will investigate / ask moderators for guidance on doing this.

LINK

with regards to the block walls I have no experience so won't even try and advise. I am sure the principles are exactly the same, but the materials and wall construction isn't. I just haven't given it any thought to be honest so you probably know more than me.

Martin

 

[Jones]

Regarding Martin's diagrams above I think they are wrong.
Vapour barrier, a thin plastic impermeable sheet goes on the warm (internal) side of the insulation. If there's a service void it goes behind the void, the void is internal and so heated and not subject to condensation risk ,also any electrical boxes will penetrate the barrier which is almost impossible to then seal effectively. A breather membrane aka vapour permeable membrane goes on the outside to prevent water ingress and allow any that has entered to evaporate through. If you want fitting and use diagrams try the manufacturer sites or the robust details site for reliable up to date information.

 

[Goesbysteve]

I will provide the link below because you have asked for it - in parallel I will investigate / ask moderators for guidance on doing this.

I understand and I appreciate you sharing it.

with regards to the block walls I have no experience so won't even try and advise. I am sure the principles are exactly the same, but the materials and wall construction isn't. I just haven't given it any thought to be honest so you probably know more than me.

Martin

Understood. thank you

 

[Molynoox]

Regarding Martin's diagrams above I think they are wrong.
Vapour barrier, a thin plastic impermeable sheet goes on the warm (internal) side of the insulation. If there's a service void it goes behind the void, the void is internal and so heated and not subject to condensation risk ,also any electrical boxes will penetrate the barrier which is almost impossible to then seal effectively. A breather membrane aka vapour permeable membrane goes on the outside to prevent water ingress and allow any that has entered to evaporate through. If you want fitting and use diagrams try the manufacturer sites or the robust details site for reliable up to date information.

Having the VB behind the service void and adjacent to the PIR could mean you didn't have to penetrate it with the electrical back boxes and could be something of an ultimate setup. My thoughts below:

• The service void would need to be big enough to accommodate the back boxes, as opposed to only big enough for the electrical wiring
• This would mean either less insulation (say 60mm instead of 100mm) or it would mean using thicker studs all round (lets say 6 x 2 instead of 4 x 2)
• Having a bigger air gap than needed for the wiring alone may not be desirable
• Another option would be to embed the back boxes into the PIR as per usual (excavating rectangular recesses) and then run all your VB sheeting inside the little rectangular recesses also
• With it being an ultimate setup, you would also need to create an air tight seal around the electrical cables coming through the VB - how is that achieved?
• I have had a little look into Passivhaus standards in the past, pretty interesting, and this is the sort of detail that's very important in those applications, so it's interesting for me to think about it, but in reality that attention to detail doesn't have a place in on sheds and garden rooms (or even houses) in the real world from what I've seen.

All in my humble opinion

Martin

 

[Fergie 307]

Thanks guys, regarding the Onduline, it will be south facing and shallow pitch so will be sure to go max 450mm centres thanks, I’ll also look into the chipboard flooring, I’m assuming hanging vertical not horizontal.

Thanks for the shop Vac tip!

I put it horizontal, and stagger the vertical joints in each run like bricks. This has the advantage that your uprights dont have to be spaced in any exact relationship to the boards. If a board ends in between two uprights the T&G joint will hold it together, and the runs above and below will be spanning the same gap if that makes sense. You will inevitably have to trim some to length and a good tip is to use a cut off section to knock the joints in tight without damaging the T& G edge. So place your first sheet at the bottom of the wall and screw the bottom edge firmly to the uprights. I use the torx head flooring screws sold by screwfix and others. Put in further screws half way up but not fully tightened. Place the next board on top and use a cut off section slotted into the T&G to knock the joint tightly together if necessary. You can bash this sacrificial cut off section with a hammer or whatever and so avoid damage to the T&G edge. If you cant do this then just lay a piece of 4x2 on top of the board edge and hammer on that. You should find the joints close up really easily. Wheter you glue them as well is up to you. The advantage of leaving the centre screws only nipped up is that it allows the board to move away from the uprights a little, which makes it easier to close the joint with the next board. Once you have the second row in position tighten up the middle screws on the bottom row and add a further row of screws near the top of the board. Then repeat the process as you work up the wall.

 

[HOJ]

• The service void would need to be big enough to accommodate the back boxes, as opposed to only big enough for the electrical wiring
• This would mean either less insulation (say 60mm instead of 100mm) or it would mean using thicker studs all round (lets say 6 x 2 instead of 4 x 2)

The service void is created by counter battening, over the air tightness barrier (VB, VCL or what ever you wish to call it) typically using a minimum of a 25mm x 50mm batten fixed to face of stud, outlined by Timber frame industry specifications produced by the likes of Trada and the STA (Structural Timber Association)

Having a bigger air gap than needed for the wiring alone may not be desirable

Not really an option, if its to be done properly.

• Another option would be to embed the back boxes into the PIR as per usual (excavating rectangular recesses) and then run all your VB sheeting inside the little rectangular recesses also
• With it being an ultimate setup, you would also need to create an air tight seal around the electrical cables coming through the VB - how is that achieved?

Really bad idea, you have to maintain the integrity of the air tightness barrier, this way you can't guarantee air tightness if you poke holes in it, too late to find out when you do an air test.

I have had a little look into Passivhaus standards in the past, pretty interesting, and this is the sort of detail that's very important in those applications, so it's interesting for me to think about it, but in reality that attention to detail doesn't have a place in on sheds and garden rooms (or even houses) in the real world from what I've seen.

Agreed, all the above is absolutely pointless for a shed and to an extent Garden rooms, with low occupancy, however I would still be inclined to adopt the principals of best practice on Garden rooms, with regards houses, passive or not, it is critical, you wont get sign off by BC if it fails the air testing.

All in my humble opinion

And mine, but I have built a few Stick and Sips timber frame houses, including to Passive standard.

 

[Molynoox]

The service void is created by counter battening, over the air tightness barrier (VB, VCL or what ever you wish to call it) typically using a minimum of a 25mm x 50mm batten fixed to face of stud, outlined by Timber frame industry specifications produced by the likes of Trada and the STA (Structural Timber Association)


Not really an option, if its to be done properly.


Really bad idea, you have to maintain the integrity of the air tightness barrier, this way you can't guarantee air tightness if you poke holes in it, too late to find out when you do an air test.



Agreed, all the above is absolutely pointless for a shed and to an extent Garden rooms, with low occupancy, however I would still be inclined to adopt the principals of best practice on Garden rooms, with regards houses, passive or not, it is critical, you wont get sign off by BC if it fails the air testing.


And mine, but I have built a few Stick and Sips timber frame houses, including to Passive standard.

All good info

If going SIPS route, I would 100% be using the battens over the top of VB approach, nice and neat solution, keeps everything on the right side of the barrier, I like the idea a lot. I've honestly not seen it done that way on an insulated stud framed structure in the garden but there is absolutely nothing stopping you doing it that way if you need or just want it to be 'air tight'.

Its got me thinking actually - I used 5x2 for my walls, with 100mm PIR and 25mm service void, I could have instead done 4x2, VB on top, then battens and had a theoretically better design. But then I have basically built a home made SIPS panel Also, there is a risk the electrician comes along and hacks big holes in everything anyway, depending on the depth of back box they are using. Having said that, you can explain things to them so they know you have a vapour barrier you are trying to preserve - I certainly had that conversation multiple times with mine

Martin

 

[Fergie 307]

If the appearance doesnt bother you then surface conduit is by far the easiest method. Also has the advantage that it makes it very simple to extend or modify the installation afterwards. If you havent done it before just make sure you plan the installation carefully, and bundle up your cables for installation in one go. It can be very difficult to get a cable through a conduit that already has other cables in it! You can use tipex, or small pieces of shrink wrap to identify individual cables of the same colour.
For a building where I didnt want surface conduit i simply made a suitable sized cut out in a piece of board. This was then screwed to the wall and used to guide a router to cut an accurate hole through the internal board wall. I used the plastic boxes designed for platerboard. They clip in place and are finally tightened by the mounting screws of the faceplate. Very easy and neat, and gives minimal projection behind the wall.

 

[Fergie 307]

All good info

If going SIPS route, I would 100% be using the battens over the top of VB approach, nice and neat solution, keeps everything on the right side of the barrier, I like the idea a lot. I've honestly not seen it done that way on an insulated stud framed structure in the garden but there is absolutely nothing stopping you doing it that way if you need or just want it to be 'air tight'.

Its got me thinking actually - I used 5x2 for my walls, with 100mm PIR and 25mm service void, I could have instead done 4x2, VB on top, then battens and had a theoretically better design. But then I have basically built a home made SIPS panel Also, there is a risk the electrician comes along and hacks big holes in everything anyway, depending on the depth of back box they are using. Having said that, you can explain things to them so they know you have a vapour barrier you are trying to preserve - I certainly had that conversation multiple times with mine

Martin

5x2 uprights, thats certainly not going to be going anywhere! I have always used 4x2, seems a good compromise between strength and cost, and the wall built up as you describe. I have mostly used surface conduit so have no need of a large gap. My main workshop has all the walls built that way. Its 7.2x10m with a central girder spine supported on girders either end, inside the external walls. 8x2 joists coming off plates welded to the top of the beam onto the top of the walls (keeps the sides of the girder free for running a hoist up and down, very handy)

 

[Molynoox]

5x2 uprights, thats certainly not going to be going anywhere! I have always used 4x2, seems a good compromise between strength and cost, and the wall built up as you describe. I have mostly used surface conduit so have no need of a large gap. My main workshop has all the walls built that way. Its 7.2x10m with a central girder spine supported on girders either end, inside the external walls. 8x2 joists coming off plates welded to the top of the beam onto the top of the walls (keeps the sides of the girder free for running a hoist up and down, very handy)

haha yeah, 4x2 is absolutely the sweet spot for strength / cost - I just have a habit of over engineering.
that's a huge building, hence the steels I suppose, it must have been fun building it have you got a build thread on the forum?

 

[Fergie 307]

haha yeah, 4x2 is absolutely the sweet spot for strength / cost - I just have a habit of over engineering.
that's a huge building, hence the steels I suppose, it must have been fun building it have you got a build thread on the forum?

No, this was built about 15years ago, before I joined. Biggest fun was lifting the main beam without access for a crane. We fitted temporary side plates to the top of one upright so that one end could be placed on it with the other on the ground, then used a teleloader to lift the other end up, quite nerve wracking ! Was pretty chuffed that everything lined up perfectly and the bolts went straight in withoit any drama. Each wall frame was built on the concrete base and the internal T&G chipboard added, then lifted into place and bolted together before insulating, wrap, battens and featherboard on the outside. Base is 6" compacted MOT, heavy duty DPM and 6" reinforced concrete. Its used mainly for working on cars so good solid floor needed for jacks and so on. The bottom beams of the walls are bolted to the concrete slab using the self threading type bolts. DPM between the two and hanging over the sides so it extends below the floor DPM, and covered by the wall external membrane at the bottom of the wall. The walls have a 6x1 board at the bottom which overlaps the floor wall joint by about 2" and has its own dpm backing . Have never had any issues with damp, even on the end of the building which takes the full impact of any wind and rain.

 

[Molynoox]

sounds great... the beam lifting does sound nerve racking

 

[Seascaper]

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.

Hello,
As follows,
Exterior cladding
Vapour barrier - such as Tyvek Supro
Insulation between shed construction uprights - thickness will depend on available space but you want at least 50mm. Rolls of fibre are easy to cut or Celotex, all available in differ thicknesses.
Lining - you mention OSB, this is an absolute minimum but my advice would be to get some decent plywood, the thicker the better. If you use 19mm exterior shutter ply fro example you will have all the advantages of fixing anything to the walls. OSB can fall apart especially if it gets wet.
Regards