Insulating Walls (Block Shed)

[Mark-J]

Hi all

I have a 6m x 5m breeze block single skin shed on a concrete base (the base has DPC) and Kingspan in the roof. I'm about to insulate the walls with timber studs, Kingspan, and OSB.

Do I need a vapour barrier and does it go where I've placed it in the pic (the green strip) below?

Also, the only reason for the 25mm gap between the block walls and Kingspan is because I assumed I'd need it - not even sure if I do though.

So, do I need a vapour barrier, if so, where exactly should it be placed, and do I need that 25mm gap?

Thanks all

https://photos.app.goo.gl/rVW2cRMUtmHsTcy87

https://imgur.com/slSmSnm

I'll be using the shed for woodworking so I'll be in there about two to four hours a day.

 

[Rorschach]

No and no.

 

[Mark-J]

Thanks for the reply.

Sorry Rorschach, just to clarify, I don't need a vapour barrier at all and I don't need that 25mm gap?

 

[Rorschach]

Air gaps are to allow drying of insulation usually behind wooden outer surfaces. You have block and you are using a closed cell insulation that won't absorb moisture anyway. As well as being closed cell it also has a foil vapour barrier on both sides so you don't need one on the inside but you do want to make sure you foam the gaps and tape up the joints, I should have added that sorry.

 

[Stigmorgan]

I would have the 25mm gap between your studwork and the brickwork, this will prevent the cold bridging through the brick and into the timber and consequently into the room, the vapour barrier goes on the back of your studwork to prevent moisture reaching from the brick to your stud wall, then the OSB to close it up.

 

[Mark-J]

Air gaps are to allow drying of insulation usually behind wooden outer surfaces. You have block and you are using a closed cell insulation that won't absorb moisture anyway. As well as being closed cell it also has a foil vapour barrier on both sides so you don't need one on the inside but you do want to make sure you foam the gaps and tape up the joints, I should have added that sorry.

Ok thanks for explaining the whys and whatnot. I appreciate that. So make sure there are no gaps. I did very similar with the Kingspan I installed in the roof, only I had about a 75mm gap between the Kingspan and the actual roof itself (with ply under the Kingspan covering it all). I ran aluminium tape everywhere! What a tedious job that was

 

[Mark-J]

I would have the 25mm gap between your studwork and the brickwork, this will prevent the cold bridging through the brick and into the timber and consequently into the room, the vapour barrier goes on the back of your studwork to prevent moisture reaching from the brick to your stud wall, then the OSB to close it up.

So, more like this?
(green strip is the vapour barrier)

https://photos.app.goo.gl/PuZDj1nAWNaUdx7FA

 

[Woody2Shoes]

I would have the 25mm gap between your studwork and the brickwork, this will prevent the cold bridging through the brick and into the timber and consequently into the room, the vapour barrier goes on the back of your studwork to prevent moisture reaching from the brick to your stud wall, then the OSB to close it up.

Any vapour barrier needs to be on the "warm" side (ie inside the insulation) - always. A vertical DPC (not DPM) separating timber battens/studs from the inner face of the masonry would be wise if you can't guarantee that this face of the masonry will stay dry. Timber (again, when dry) is a relatively good insulator, so I wouldn't be too worried about cold bridging - especially as we're only talking about 25mm or so of insulation anyway. The OP does not say how the masonry wall is protected from rain on the outside (eg cladding or painted render or something), but if it's well protected then I wouldn't bother to put anything between (treated) timber battens and masonry.

Anyone who thinks that PU-based celotex/kingspan-type wall boards cannot absorb moisture (vastly affecting their thermal proerties apart from anything else), has not observed them in real life (it's partly why you'll struggle to find it used much for full-cavity-fill).

I've read on the Sterling OSB installation guide (which seems to have recently been removed from their website for some reason) that a 3mm gap should be left between OSB board edges, to allow for expansion. There seem to be conflicting approaches to this across the internet - I've done both in the past (I've found that most expansion of OSB is across the thickness, rather than the other dimensions). If you do leave gaps between the OSB sheet edges, then some kind of caulk (acrylic is usually paintable) would be good.

I think that the vapour-impermeability of the OSB would be further enhanced by a coat or two of paint - which is something I'd do to brighten a workshop anyway.

Cheers, W2S

 

[Woody2Shoes]

PS Don't forget that timber sizes are nominal and celotex/kingspan thicknesses are much closer to the truth - for example, fitting 50mm celotex between two-by-two battens (actually 47 X 47 or worse) leaves the celotex proud, which can be a pain....

 

[Mark-J]

I've learned quite a bit just on this post! Thanks all for the feedback.

The exterior block walls have been rendered but when it pours heavily I am still seeing dark damp patches on the interior walls, so I am also going to apply a water seal over them when the weather gets a little warmer/drier. I think the addition of DPC behind the studwork is a great idea.

The water seal (Thompson's) will hopefully counter the damp but I feel safer knowing there's the added gap of 25mm gap between the blockwork and interior framing/walls.

Ok, so right now I'm favouring this set up (I've tried getting the images to display but for some reason they won't):

Green line = vapour barrier
Blue line = DPC

Interior Wall Setup

 

[Woody2Shoes]

I've learned quite a bit just on this post! Thanks all for the feedback.

The exterior block walls have been rendered but when it pours heavily I am still seeing dark damp patches on the interior walls, so I am also going to apply a water seal over them when the weather gets a little warmer/drier. I think the addition of DPC behind the studwork is a great idea.

The water seal (Thompson's) will hopefully counter the damp but I feel safer knowing there's the added gap of 25mm gap between the blockwork and interior framing/walls.

Ok, so right now I'm favouring this set up (I've tried getting the images to display but for some reason they won't):

Green line = vapour barrier
Blue line = DPC

Interior Wall Setup

What you've drawn would probably work fine - some/many people would omit the vapourcheck barrier (as the OSB is supposed to be sufficiently vapour-impermeable, especially if you've sealed around the edges of the insulation boards with PU foam, which I much prefer to tape). Given that the wall is not completely rain-screened, I'd think about the top and (especially) bottom of the new wall boarding - maybe ventilate the cavity (of course some heat may then be lost by convection, but OTOH drier materials insulate better)?

I'd want to understand more about why your wall gets damp on the inside. You are going to put any water seal on the outside, not inside, yes? Could you paint the render with masonry paint? Could you add extra "rainscreen" eg cladding? Are the gutters/roof working properly to shed water away from the walls?

Cheers, W2S

 

[Mark-J]

Thanks Woody for the reply

Yes, the water seal will go on the exterior of the blockwork. Not sure why it's still getting damp; the builder is very experienced and has done this sort of thing for decades (he's past retirement but still going strong) so I assume he used some waterproofing in the render. It's a bit of a worry, which is why I want to make sure I get all this right.

I've got masonry paint, which I bought months ago (just haven't gotten around to using it yet). I was going to use that - but in the back of my mind were those damp patches, so, I decided to go with the water seal. Trouble is, you cannot use the water seal over paintwork, and you cannot paint over it either. So apparently, it's masonry paint or water seal but not both.

The roof is fibreglass with a raised lip around three of the four edges, with a run-off toward the rear of the shed; that's the only place there is guttering, which seems to be working just fine.

The wall that suffers from dampness suffers the most from the weather.

EDIT: I was just reading the info on one of the Sandtex masonry paint tubs and it says it has a waterproofing agent in it that protects against rain/moisture (beading or something). Perhaps I don't need that Thompson's water seal.

https://photos.app.goo.gl/yX9MZrBYre8nNZzn7

How good is it though. It would be great if I could use the paint because it would look a lot nicer; although the render is new (summer of this year), it's a bit drab in colour.

 

[Woody2Shoes]

Thanks Woody for the reply

Yes, the water seal will go on the exterior of the blockwork. Not sure why it's still getting damp; the builder is very experienced and has done this sort of thing for decades (he's past retirement but still going strong) so I assume he used some waterproofing in the render. It's a bit of a worry, which is why I want to make sure I get all this right.

I've got masonry paint, which I bought months ago (just haven't gotten around to using it yet). I was going to use that - but in the back of my mind were those damp patches, so, I decided to go with the water seal. Trouble is, you cannot use the water seal over paintwork, and you cannot paint over it either. So apparently, it's masonry paint or water seal but not both.

The roof is fibreglass with a raised lip around three of the four edges, with a run-off toward the rear of the shed; that's the only place there is guttering, which seems to be working just fine.

The wall that suffers from dampness suffers the most from the weather.

EDIT: I was just reading the info on one of the Sandtex masonry paint tubs and it says it has a waterproofing agent in it that protects against rain/moisture (beading or something). Perhaps I don't need that Thompson's water seal.

https://photos.app.goo.gl/yX9MZrBYre8nNZzn7

How good is it though. It would be great if I could use the paint because it would look a lot nicer; although the render is new (summer of this year), it's a bit drab in colour.

I'd use Sandtex it's good stuff.

 

[Mark-J]

Thanks Woody for the reply

Yes, the water seal will go on the exterior of the blockwork. Not sure why it's still getting damp; the builder is very experienced and has done this sort of thing for decades (he's past retirement but still going strong) so I assume he used some waterproofing in the render. It's a bit of a worry, which is why I want to make sure I get all this right.

I've got masonry paint, which I bought months ago (just haven't gotten around to using it yet). I was going to use that - but in the back of my mind were those damp patches, so, I decided to go with the water seal. Trouble is, you cannot use the water seal over paintwork, and you cannot paint over it either. So apparently, it's masonry paint or water seal but not both.

The roof is fibreglass with a raised lip around three of the four edges, with a run-off toward the rear of the shed; that's the only place there is guttering, which seems to be working just fine.

The wall that suffers from dampness suffers the most from the weather.

EDIT: I was just reading the info on one of the Sandtex masonry paint tubs and it says it has a waterproofing agent in it that protects against rain/moisture (beading or something). Perhaps I don't need that Thompson's water seal.

https://photos.app.goo.gl/yX9MZrBYre8nNZzn7

How good is it though. It would be great if I could use the paint because it would look a lot nicer; although the render is new (summer of this year), it's a bit drab in colour.

I'd use Sandtex it's good stuff.

That's what I was hoping for as it solves one problem for me.

 

[Mark-J]

My initial thoughts of putting the studwork up was like in the pic below (75mm x 50mm):

https://photos.app.goo.gl/BL5mvC3uA6ugaGms7

But having done a little more research, I notice the studwork is aligned more like in the pic below (50mm x 75mm):

https://photos.app.goo.gl/Sn6NeSJS2hVdUWMQ9

Does it matter if I go with 75mm x 50mm or 50mm x 75mm?

 

[Mark-J]

I've got hammer fixings for attaching the wooden sole to the concrete floor, but will I need a special type of screw for attaching the timber to the breeze block walls (I'll be using those Fischer Duopower wall plugs) or will ordinary wood screws work?

 

[graduate_owner]

Things might have changed in the last 40 years (!!!) but on my first house I put a silicon water repellent on the non- cavity rendered walls, followed by a few coats of Dulux Weathershield, and it was still looking good when I sold it about 5 years later. Certainly stopped moisture ingress.

K

 

[will1983]

I've got hammer fixings for attaching the wooden sole to the concrete floor, but will I need a special type of screw for attaching the timber to the breeze block walls (I'll be using those Fischer Duopower wall plugs) or will ordinary wood screws work?

Normal wood screws with either red or brown plugs will work fine. I usually just get one fixing in the middle of each vertical stud, screw it in tight and then plumb the stud up with my long level. Once this is right I drill straight through the stud and into the wall (make sure your into a block not a mortar line) hammer a plug through the timber into the wall using a screw to push it through and then tighten it down. I usually do one fixing for every 12-18" of stud length. It goes pretty quick once you get into a rhythm.

I also mark the centres of all my studs on the floor and ceiling so I know where they are when it comes time to fix my wall cladding in place.

 

[MikeG.]

Things might have changed in the last 40 years (!!!) but on my first house I put a silicon water repellent on the non- cavity rendered walls, followed by a few coats of Dulux Weathershield, and it was still looking good when I sold it about 5 years later. Certainly stopped moisture ingress.

K

The huge fundamental problem with that and all similar impervious coatings on the outside of solid walls is that the moisture is inside the house trying to get out, not outside the house trying to get in, for most of the time.

 

[Mark-J]

Thanks all for the advice, all much appreciated, and I'm now just about ready to order everything.

@MikeG Won't a vapour barrier stop any moisture from getting outside in and inside out? I'll be putting it directly behind the OSB3.