# Insulating Walls (Block Shed)



## Mark-J (1 Dec 2019)

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.


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## Rorschach (2 Dec 2019)

No and no.


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## Mark-J (2 Dec 2019)

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?


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## Rorschach (2 Dec 2019)

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.


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## Stigmorgan (2 Dec 2019)

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.


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## Mark-J (2 Dec 2019)

Rorschach":39320buy said:


> 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


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## Mark-J (2 Dec 2019)

Stigmorgan":1fjylz16 said:


> 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


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## Woody2Shoes (2 Dec 2019)

Stigmorgan":2hlyicop said:


> 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


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## Woody2Shoes (2 Dec 2019)

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....


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## Mark-J (2 Dec 2019)

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


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## Woody2Shoes (2 Dec 2019)

Mark-J":2u0tp38e said:


> 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.
> 
> ...



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


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## Mark-J (2 Dec 2019)

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.


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## Woody2Shoes (2 Dec 2019)

Mark-J":ong21knx said:


> 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'd use Sandtex it's good stuff.


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## Mark-J (2 Dec 2019)

Woody2Shoes":izpwmrbl said:


> Mark-J":izpwmrbl said:
> 
> 
> > Thanks Woody for the reply
> ...


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


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## Mark-J (3 Dec 2019)

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?


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## Mark-J (8 Dec 2019)

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?


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## graduate_owner (10 Dec 2019)

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


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## will1983 (10 Dec 2019)

Mark-J":3r9u8p4k said:


> 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.


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## MikeG. (10 Dec 2019)

graduate_owner":1umlarfx said:


> 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.


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## Mark-J (10 Dec 2019)

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.


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## Mark-J (20 Dec 2019)

I have a problem with mould growing on the inside shed roof. Weirdly, it isn't all over, and is only growing in two spots; the opposite corners of the roof. The pic attachment shows the worst of the mould and the area it covers. The other mould spot is fist-sized growth and isn't growing. 

The roof is fibreglass on 18mm OSB, then a 25mm gap for airflow, then Kingspan 75mm insulation panels, and finally, plywood as can be seen in the pic. 

The shed walls are single skin breeze block and ventilation is via two small windows above a larger window. Unless I leave the door open, that's the only ventilation the shed gets. 

In January I'll be buying a Dehumidifier (Desiccant) and in spring I'll be insulating the walls (as mentioned earlier in this post). 

For heating, I have a calor gas fire and the temperature in the shed is always between 9 degrees and 14 degrees (I'd say it levels out at around 12 degrees). 

Is there anything else I can do to combat the mould? It's starting to worry me.


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## Woody2Shoes (20 Dec 2019)

Mark-J":2kkv2bqp said:


> I have a problem with mould growing on the inside shed roof. Weirdly, it isn't all over, and is only growing in two spots; the opposite corners of the roof. The pic attachment shows the worst of the mould and the area it covers. The other mould spot is fist-sized growth and isn't growing.
> 
> The roof is fibreglass on 18mm OSB, then a 25mm gap for airflow, then Kingspan 75mm insulation panels, and finally, plywood as can be seen in the pic.
> 
> ...



Be aware that the gas fire will be adding significant amounts of moisture - and this may in itself explain the mould growth (is the mould near where the heater(s) is/are?). https://sciencing.com/burning-propane-w ... 35338.html

Cheers, W2S

PS ventilation is always good!


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## Mark-J (20 Dec 2019)

That's another oddity, the gas fire is nowhere near the larger mould patches. 

I've got two humidifier monitors and on average, the humidity levels are around 85% to 95%. That's way too much (I think it needs to be around 55%). I'm hoping the Dehumidifier will sort that out. 

Would having a large fan on when I'm not in the shed help with the humidity? I'm thinking more air circulation. I could buy a timer so the fan isn't on all the time.


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## Woody2Shoes (20 Dec 2019)

Mark-J":c4qoyo6n said:


> That's another oddity, the gas fire is nowhere near the larger mould patches.
> 
> I've got two humidifier monitors and on average, the humidity levels are around 85% to 95%. That's way too much (I think it needs to be around 55%). I'm hoping the Dehumidifier will sort that out.
> 
> Would having a large fan on when I'm not in the shed help with the humidity? I'm thinking more air circulation. I could buy a timer so the fan isn't on all the time.



I think you'll find that the propane heater is putting excess moisture into the space. There would seem to be little point simply circulating air with a fan - the moisture is still carried in the air until it finds somewhere cooler to condense. You would be better with an electric heater than the propane heater, and possibly also instead of a dehumidifier e.g. a couple of these, maybe: https://www.dimplex.co.uk/product/ecot- ... thermostat


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## Woody2Shoes (20 Dec 2019)

Woody2Shoes":2ygrb2lu said:


> Mark-J":2ygrb2lu said:
> 
> 
> > That's another oddity, the gas fire is nowhere near the larger mould patches.
> ...



PS you might even solve the problem with no additional heating, but simply (better) cross-ventilation


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## Woody2Shoes (20 Dec 2019)

Mark-J":3t2zpdns said:


> That's another oddity, the gas fire is nowhere near the larger mould patches.



The mould patches are simply telling you where the condesation is worst. The mould is showing you where the excess moisture from the propane fire is ending up and it is also effectively showing you the coldest surface - relative to the warmest and dampest air (which will nearly always be high up, since heat rises). So - fire creates warm wet air, which rises and finds the coolest surface on which to give up some of its moisture.


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## Mark-J (20 Dec 2019)

Thanks for the advice Woody. 

The calor gas heater cost me almost £80 a couple of months ago, but if it's causing moisture build-up, I'll gladly get rid of it. 

Do those lights actually work? I did read they are only good for small greenhouses and the inside of my shed measures 6m x 5m x 2.2m in ceiling height. 

Ideal if they do work because I'd imagine they'd be cheaper to run.


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## Mark-J (20 Dec 2019)

Woody2Shoes":3elm370i said:


> Mark-J":3elm370i said:
> 
> 
> > That's another oddity, the gas fire is nowhere near the larger mould patches.
> ...


This is the best explanation I've had concerning the mould patches. 

Thanks Woody!


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## Mark-J (20 Dec 2019)

Woody2Shoes":12zw69zg said:


> PS you might even solve the problem with no additional heating, but simply (better) cross-ventilation


Not sure if I have the know-how to fit vents into the breeze block walls (for cross ventilation) because I'll be fitting a timber frame and OSB over insulation panels on the interior walls later on. 

Would I just have to cut out an opening in the OSB to access the exterior vents?


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## SammyQ (21 Dec 2019)

W2S got it spot on re propane heater/ condensing on cold spots, but you also need circulation. I recently installed a PIV system and banished previously sodden (with condensate) surfaces. 
I suggest, ventilation and a dryer heat source. You can pick up oil- filled rads second hand and also small (1kW or so) air heaters for £20 'ornearenuff' from the big sheds. I have used both and they are a good combo; oil-filled as frost protection, air heater to boost when you are present..
Alternatively, diesel caravan/boat heaters are gaining popularity- they vent externally, but heat internally. Someone on here has one and published details. 

Sam


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## Mark-J (21 Dec 2019)

I may be overthinking this. I've found that after about 5 to 10mins working at the bench, I am warm to the point I have to remove my hoody. If remove the gas fire heater and that slows down (stops) the mould growth, I'd be happy with just _me_ being warm. For the rest of the workspace, I'll be timber framing and insulating the walls in spring. 

Perhaps then, a standard 1.5kw halogen heater placed close by will be enough for me. Or one of those infrared heaters. Seeing as the temperature has never dropped below 4 degrees in winter, I can't really say it gets very cold - just, cold.


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