Shed insulation

[JWF]

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

 

[9fingers]

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

 

[Steve Blackdog]

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

 

[frugal]

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.

 

[Shultzy]

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.

 

[Steve Blackdog]

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

 

[MikeG.]

post deleted as it contained nonsense!

 

[MikeG.]

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

 

[fyall]

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.

 

[MikeG.]

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

 

[Steve Blackdog]

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

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

 

[PeterBassett]

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

 

[fyall]

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.

 

[MikeG.]

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

 

[fyall]

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

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.

 

[MikeG.]

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

 

[fyall]

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

 

[MikeG.]

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

 

[Shultzy]

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.

 

[Steve Blackdog]

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.

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