# Shed insulation



## JWF (22 Dec 2009)

I am in the process of insulating my new shed with polystyrene and hardboard.
My question is; do I also put a plastic membrane on and should it go between the polystyrene and outer wooden wall or between hardboard?
John


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## 9fingers (22 Dec 2009)

Vapour barrier always goes on the warm(er) side of insulation. No exceptions

Hardboard is pretty impervious to modest quantities of water vapour but poly sheeting is so cheap I would use some as belt and braces.

Remember that polystyrene can attack the electrical insulation of cables so route all wiring on top of the hardboard. If you need to put some on the polystyrene side, wrap it in polythene first.


Bob


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## Steve Blackdog (23 Dec 2009)

It is obviously the weather that is making so many of us think of a job we should've done in the summer  

I have an insulated shed floor. 10 x 12 shed.

I can't face the effort of cutting up polystyrene boards again to insulate the walls and roof and wondered if anyone has a view on using rockwool loft insulation for the walls and roof - it is as cheap as cut price chips at the moment - I could probably do the whole lot for under a tenner. 

Guidance please.

Steve


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## frugal (24 Dec 2009)

Steve with the Black Dog":2c1niu4k said:


> I can't face the effort of cutting up polystyrene boards again to insulate the walls and roof and wondered if anyone has a view on using rockwool loft insulation for the walls and roof - it is as cheap as cut price chips at the moment - I could probably do the whole lot for under a tenner.



One of the things that has been mentioned is that for the roof your should leave a 50mm air gap between the top of the insulation and the roof. 

That is the only reason that I have not put rockwool style insulation in the garage. The rafters are 100mm deep and I was thinking of putting 50mm Kingspan in leaving 50mm above it.


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## Shultzy (24 Dec 2009)

Steve, nothing wrong with rockwool as insulation except some people react to the fibres. Suit up with a soco type paper overall and wear a mask and gloves. There's a section in my workshop build if you want to look.


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## Steve Blackdog (17 Jan 2010)

Shultzy":2a73k358 said:


> Steve, nothing wrong with rockwool as insulation except some people react to the fibres. Suit up with a soco type paper overall and wear a mask and gloves. There's a section in my workshop build if you want to look.




Thanks Stefan

Your photos are very useful.

I have been meaning to ask whether you think the plastic membrane has made a difference. I already have waterproof paper between the noggins and the T&G boards.

Steve


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## MikeG. (18 Jan 2010)

post deleted as it contained nonsense!


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## MikeG. (18 Jan 2010)

JWF":u6qm06bc said:


> I am in the process of insulating my new shed with polystyrene and hardboard.John



Be very careful with polystyrene. It is a serious fire hazard. If you are to use it it really must be sealed in completely, and I would really suggest avoiding it's use over your head where it can drip nepalm-like globules. It exudes a deadly smoke.

Mike


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## fyall (18 Jan 2010)

If I can throw in my experience as I built my wife an insulated studio 2 years ago. Waterproof paper also known as building paper is not a vapour barrier and should be put on the outside of the building and if possible an air gap between it and whatever type of cladding you are sheathing the building in. Vapour barrier goes on the warm side behind your p/board or t&g cladding which sandwiches your rockwool/polystyrene in between a vapour barrier and the building paper. Difference is the bulding paper is more like a cloth (Roofshield or Protec are a couple of brand names) and it is essentially a water restant protectant layer. A vapour barrier is like polythene only it has microscopic holes to allow the building to breathe but blocks the vapour from getting in and wetting the insulation. This is the way I built the studio which is a substantial building which we heat with electric heaters in the winter when being used and it has always been warm and dry and have had no problems. I live in rural Perthshire so we get cold/wet winters which tests the membranes well!. Hope this helps.


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## MikeG. (18 Jan 2010)

Completely right, of course, and just a case of me mis-reading. The paragraph started off talking about plastic sheeting.......my mistake.

So, for the avoidance of doubt. From the outside.......the ideal is cladding (boards, render), 25mm ventilated void, building paper or timber-frame breathable membrane, studs fully filled with insulation, vapour barrier (optional), inner lining of ply or OSB.

Sorry for reacting in haste....

Mike


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## Steve Blackdog (18 Jan 2010)

Mike Garnham":142193u3 said:


> Completely right, of course, and just a case of me mis-reading. The paragraph started off talking about plastic sheeting.......my mistake.
> 
> So, for the avoidance of doubt. From the outside.......the ideal is cladding (boards, render), 25mm ventilated void, building paper or timber-frame breathable membrane, studs fully filled with insulation, vapour barrier (optional), inner lining of ply or OSB.
> 
> ...



Phew!

As my shed is a bought one (nice but not bespoke) it has 2.5" frame, which means I can either stuff the entire gap with rockwool (preferred solution) or go for a compromise 1/2" air gap and 2" kingspan or 1.5" air gap and 1" kingspan. I could stud out the wall to make more of a gap, but then my 12 x 8 shed gets even smaller!

Steve


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## PeterBassett (18 Jan 2010)

I'd favour a 1/2" air gap and 2" kingspan, but I'm not sure the gap is enough.


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## fyall (18 Jan 2010)

PeterBassett":3g7kijki said:


> I'd favour a 1/2" air gap and 2" kingspan, but I'm not sure the gap is enough.



I think 1/2" would be fine. Kingspan is alot more robust in a moist area than Rockwool hence its preference old buildings when wall construction and vapour levels are a bit of an unknown quantity. Remember of course the building is wood not masonry built so it will breathe a bit naturally. Don't get too hung up on air gaps and expensive membranes as inevitably there will be alot of small gaps which will not be visible but will allow the building to breathe. The air gap you are thinking of should be fine in the walls but for the roof a bigger air gap would be better as this is where naturally the moisture can occur if there is not a good air flow. For my ceiling I coombed the ceiling instead of going to the rafters as it was easier to avoid having to cut round rafter ties etc, I laid the rockwool up there on a scrap strips of membrane to hold it up and then sheeted it in my vapour barrier. I have plenty photos if you are interested. I think the easiest would be a few nails 0.5" in to hold the Kinspan off the external wall and 2" of Kingspan in the void which will give you your gap without having to stud anything and you will not lose any space. Stud the roof out a bit to give you a bigger gap and use the same technique.


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## MikeG. (18 Jan 2010)

fyall":rom5b0y5 said:


> Kingspan is alot more robust in a moist area than Rockwool hence its preference old buildings when wall construction and vapour levels are a bit of an unknown quantity.



I think it is really important on open forums to be careful that advice can't be misinterpreted. This advice, above, is fine for sheds. However, for houses it needs a few caveats.

Listed buildings officers need a lot of convincing in most circumstances when it comes to modern vapour resistant materials of all descriptions, including insulation. It would not be an appropriate way of insulating a 400 year old oak framed cottage, for instance, where wall "breathability" is an important part of making the whole building work.

In general, before you do any insulation work in an older property seek the advice of a profesional. There are so many options, and what is applicable in one circumstance may be completely wrong in another.

Mike


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## fyall (18 Jan 2010)

Mike Garnham":2h7khnva said:


> fyall":2h7khnva said:
> 
> 
> > Kingspan is alot more robust in a moist area than Rockwool hence its preference old buildings when wall construction and vapour levels are a bit of an unknown quantity.
> ...



I gree with Mike and maybe should have been a bit more specific. I live in a 300 year old cottage which we are renovating and extending and while the extension is using modern membranes and insulation the old renovated part which had rough plasterboard studding and rockwool installed in the last ten years has been replaced with a good sized air gap, Kingspan and plasterboard and I can still feel some of the old draughts so I think we will be ok. Main thing to remember is old buildings need draughts to keep them dry and sound hence the reason its still there after 300 years.


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## MikeG. (18 Jan 2010)

I never specify draughts!

In principal I would use a breathable hygroscopic insulation like Warmcell (recycled newspaper) or natural sheepswool, and fully fill all the gaps......no draughts!............then lime plaster on, for instance, a wood-wool board such as Heraklith. Control the ventilation effectively within the building using a MVHR system.

If you have draughts coming through or around your insulation then it isn't going to be doing its job, I'm afraid.

Mike


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## fyall (18 Jan 2010)

not draughts coming through the insulation but behind in the air gap between the original stone wall and the insulation.


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## MikeG. (18 Jan 2010)

.......ah, and stone wall too......altogether different.


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## Shultzy (18 Jan 2010)

Steve with the Black Dog":1860k19g said:


> I have been meaning to ask whether you think the plastic membrane has made a difference. I already have waterproof paper between the noggins and the T&G boards.
> 
> Steve



Glad the photos were useful.

I haven't experienced any damp so the plastic membrane must be doing it's job.


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## Steve Blackdog (18 Jan 2010)

fyall":3paf8qlb said:


> PeterBassett":3paf8qlb said:
> 
> 
> > I'd favour a 1/2" air gap and 2" kingspan, but I'm not sure the gap is enough.
> ...



good tip with the nails - I'd planned to put battens in - you've just saved me a good few hours. Thanks


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## MikeG. (19 Jan 2010)

The very best way of fixing Kingspan removes the necessity for nails or battens. 

Cut your Kingspan such that it has a gap of about 10mm all round, and cut youself a whole lot of big wedges out of scrap. Wedge the Kingspan in place, about flush with the face of the studs, and then spray expanding foam into the gaps all round. When this has gone off, remove the wedges and spray some more into the holes they leave. Trim excess foam when dry.

This method ensures a perfect fit to the studwork, which is something that can never be achieved by "friction fitting". It also speeds up the cutting process, because you don't have to be particularly accurate.

-

Those following Shultzy's photoset.........be careful! It appears to me that there is a layer of plastic outside the studs, and this just should not be done. If this is plastic it is a fundamental mistake. The same with the floor. The only membranes to use in those positions are breathable timber-frame type membranes, or building paper.

Furthermore, best practice is to batten out on the outside of the studs to create a ventilated void behind your cladding. Any moisture that gets behind the boards will otherwise sit on the back of them unable to dry out, and could well develop mould, and rot in untreated timber.

Mike


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## Steve Blackdog (19 Jan 2010)

Mike Garnham":1up0c839 said:


> The very best way of fixing Kingspan removes the necessity for nails or battens.
> 
> Cut your Kingspan such that it has a gap of about 10mm all round, and cut youself a whole lot of big wedges out of scrap. Wedge the Kingspan in place, about flush with the face of the studs, and then spray expanding foam into the gaps all round. When this has gone off, remove the wedges and spray some more into the holes they leave. Trim excess foam when dry.
> 
> ...



That last tip is interesting - I have put dpc membrane on the floor sandwiched between the wood and insulation which is capped off with ply. If I put a vapour barrier in instead, surely the polystyrene insulation in the floor would have exactly the same issues as the solid DPC?

It would be easy the change if the consensus is that I should.


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## MikeG. (19 Jan 2010)

Steve, I don't understand. A DPM* is* a vapour barrier. What sort of floor construction are we talking about?

Mike


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## OldWood (19 Jan 2010)

I'll put my tuppence worth. 

Two points on the shed insulation - one I think is the missing explanation why the 'warm-side' dpm is necessary. It's the heating of the shed which is the driver here - keep the shed cold and the membrane isn't necessary !! Warm air holds considerably more water vapour than cold. Across the insulation there will be a temperature gradient - external wall temperature to internal wall temperature. This gradient will on cold days include the dew point temperature at which the water vapour will condense out into the insulation, onto the studding and onto the cladding. Therefore we need to stop the moisture getting there, hence the polythene membrane on the wall studding**inside**. And by the way OSB is so full of epoxy that it is an effective membrane in it's own right.

Secondly - although fibreglass insulation is cheap it isn't ideal, and Kingspan is more suitable. You do need to leave an airgap for the cladding to breath (and remember that holes are necessary through the horizontal bracing (dwangs in Scotland) to allow full air movement), and unless you put in some sort of barrier to the cladding, the f/g will just fill this gap. The other factor is that f/g loses it's insulation if it is compressed (smaller air pockets and more material in contact with itself), so filling a 2 or 3" cavity requires thinning the f/g sheet. I did use f/g but it was very time consuming - I used an industrial stapler to hold it in place.


Rob


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## MikeG. (19 Jan 2010)

Thanks Rob....that's saved me a whole lot of typing!

Mike


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## Shultzy (19 Jan 2010)

Mike, the outside plastic was going to be permanent until I read a few posts on the subject. I left it on to waterproof the structure until I put the cladding up. I then stripped it off and put building paper up. Unfortunately I didn't mention it in the postings.


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## MikeG. (19 Jan 2010)

At Bob (9Fingers) suggestion I will write a definitive guide to shed walls, floors and rooves in the next week or two, as a sticky. Seems that these things go in cycles, and in a few months time there will be another spate of shed questions......asking the same things all over again!

I hope this will help all aspiring shed builders, but in the meantime, I will as always answer all the queries I see.

Mike


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## OldWood (19 Jan 2010)

One thing I did like about this past snow episode is that it did show that I had the insulation in the new workshop at an adequate level, as it was passable warm inside, but the snow didn't melt off the roof until the thaw. 

Having said that I've a lean-to that was recycled from my father-in-law's house and I'm sure if I hadn't whip most of the wet snow off that (at 7pm one evening) it might well have collapsed as the timberwork is a way beyond minimal ! I did some calculations for a conservatory last year and included an allowance for wet snow - experience now shows I'd better re-visit those figures before it all goes to the build stage!! But then I won't be around when the next 30 year snow fall comes !!!

Rob


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## Steve Blackdog (19 Jan 2010)

Mike Garnham":fqfh7nlc said:


> Steve, I don't understand. A DPM* is* a vapour barrier. What sort of floor construction are we talking about?
> 
> Mike



Maybe I didn't understand your earlier post. My DPM is not breathable - just plastic sheet you might put under a concrete floor. 

My t+g floor is pressure treated tanalised, then there's the membrane then the insulation board and then the half inch ply.

BTW the T&G floor is on battens which are on runners on a concrete slabbed base.

I look forward to your guide Mike!

Steve


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## MikeG. (20 Jan 2010)

Steve,

you're not helping!! Give me a list, from the dirt all the way through to the surface that you walk on, of exactly what this floor is made from. At the moment I can't even tell if there is concrete involved.

Mike


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## Steve Blackdog (20 Jan 2010)

Mike Garnham":2b9r84ow said:


> Steve,
> 
> you're not helping!! Give me a list, from the dirt all the way through to the surface that you walk on, of exactly what this floor is made from. At the moment I can't even tell if there is concrete involved.
> 
> Mike



You can give up on this if you want to Mike!! But here goes:

Earth/clay
MOT1 sub-base
Concrete
concrete Slabs
Wooden runners (lots of air circulating)
Underside of floor comprising tanalised battens
Tanalised t&G boards
Plastic sheet (DPM) non breathable
Polystyrene
1/2" ply 

Hope that is clearer


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## 9fingers (20 Jan 2010)

I'll leave the insulation expert to comment on the DPM position but the only comment I'll make is that the 1/2" ply will not be very good for point loads on a workshop floor such as machine feet/wheels.
I'd suggest a minimum of 18mm.

just my 2p

Bob


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## Steve Blackdog (20 Jan 2010)

9fingers":33gacy5u said:


> I'll leave the insulation expert to comment on the DPM position but the only comment I'll make is that the 1/2" ply will not be very good for point loads on a workshop floor such as machine feet/wheels.
> I'd suggest a minimum of 18mm.
> 
> just my 2p
> ...



Actually, I bet it is 18mm - it is as heavy as anything. I've not measured it - I reclaimed it from my old outbuildings! (cheap skate!)


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## Chris 83 (17 Jun 2020)

Hello everyone, I'm new to this site, we've just bought a summerhouse delivery is due on 14th july 2020, if any people have any pictures of how to insulate the summerhouse with the correct Materials would be greatly appreciated, I've bought osb 3 boards for the internal walls, however I'm at a bit of a loose end of what to insulation it with, I was thinking polystyrene but then read it's a no no with electric cables, also moisture barrier is this installed before the insulation or after? This summerhouse will be used all year round by the way, thanks in advance guys


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## Chris 83 (17 Jun 2020)

Chris 83":3ay6pcxu said:


> Hello everyone, I'm new to this site, we've just bought a summerhouse delivery is due on 14th july 2020, if any people have any pictures of how to insulate the summerhouse with the correct Materials would be greatly appreciated, I've bought osb 3 boards for the internal walls, however I'm at a bit of a loose end of what to insulation it with, I was thinking polystyrene but then read it's a no no with electric cables, also moisture barrier is this installed before the insulation or after? This summerhouse will be used all year round by the way, thanks in advance guys


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## MikeG. (17 Jun 2020)

Chris 83":14t4dn35 said:


> Hello everyone, I'm new to this site, we've just bought a summerhouse delivery is due on 14th july 2020, if any people have any pictures of how to insulate the summerhouse with the correct Materials would be greatly appreciated, I've bought osb 3 boards for the internal walls, however I'm at a bit of a loose end of what to insulation it with, I was thinking polystyrene but then read it's a no no with electric cables, also moisture barrier is this installed before the insulation or after? This summerhouse will be used all year round by the way, thanks in advance guys



There is no correct way. It's impossible to insulate a pre-made building of this nature properly, which is why I say so often that this is not the place to start if trying to create a workshop or a home office.


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## Steve Blackdog (17 Jun 2020)

I wouldn’t say impossible. I had great success retro-insulating my ready made shed. 

I used polystyrene in the floor and multi-layer foil insulation (go for the best you can afford - check u value) for the walls and ceiling.

On the floor, I laid a plastic DPc membrane, with loose batonS laid over to support 19mm ply floor with polystyrene between batons. 

On the walls, the moisture barrier was on the outside skin, I attached multifoil by stapling to the Inside edges of the wall studs, making sure that there was an inch of air gap either side. I did the same for the ceiling so that the room was completely encased in the multifoil (apart from door and window - obvs). Tape seams. Then the walls and ceiling can be boarded over. 

This approach is also quicker and easier than cutting insulation board to fit the spaces between the studs.


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## MikeG. (17 Jun 2020)

Steve Blackdog":3ruwhffx said:


> I wouldn’t say impossible......



It is impossible to do it properly. A properly designed timber wall has a gap behind the cladding , a continuous breather membrane outside the frame(and inside the ventilated void), as well as a vapour barrier on the inside. The latter is easy. The rest...impossible. What you have done is superficially OK for the time being, but will inevitably lead to interstitial condensation and to a short life-span of the building. Laying plastic on the floor was a mistake, though. It is on the wrong side of the insulation. Your walls are OK (if you think multi-foils work) in the panels between the frames, but the frames themselves are left unprotected directly behind the cladding. Like I said, this is a job which is impossible to do well.


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## Steve Blackdog (17 Jun 2020)

Mike, I defer to your superior knowledge. I was just trying to help someone who had already bought a pre-fabricated summer house and was looking to retro-fit. It wasn't meant to be a counsel of perfection.

On the plus side, this is a ten year old thread and I sold the house with the shed about 5 years ago! My old shed was my workshop and it served me very well. Although, it wasn't perfect by any means, it was relatively warm - the dust extractor put out enough heat to keep it warm through winter.

My experience with multi-foil is that it really good if you buy the expensive stuff and poor if you don't. It is certainly much better than cheap polystyrene (as opposed to Kingspan). The plus side is that foil is relatively easy to install and by leaving air gaps you maximise its insulating performance.


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