Shed insulation

[MikeG.]

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.

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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

 

[Steve Blackdog]

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

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.

 

[MikeG.]

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

Mike

 

[OldWood]

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

 

[MikeG.]

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

Mike

 

[Shultzy]

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.

 

[MikeG.]

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

 

[OldWood]

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

 

[Steve Blackdog]

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

 

[MikeG.]

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

 

[Steve Blackdog]

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

 

[9fingers]

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

 

[Steve Blackdog]

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

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!)

 

[Chris 83]

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

 

[Chris 83]

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

 

[MikeG.]

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.

 

[Steve Blackdog]

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.

 

[MikeG.]

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.

 

[Steve Blackdog]

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.