Insulation for the shed.

All things being equal (which they usually aren't) I should be in a position to start on the shed roof next week. The roof will be gently sloping (almost flat).
Now is the time to think about insulation.
The joist depth will probably be about 200mm - give or take 25mm. [Between 7" and 9" in old money].
Now Kingspan is a wonderful thing, and easy to fit from underneath a roof, but is not especially cheap.
The sheds seem to have a lot of very cheap rolls of insulation (allegedly subsidised by the power companies) which are much more difficult to lay from underneath a roof.
However, as a new build I have extra options.
So, how about:
(1) Lining the underneath of the joists with some spare DP membrane left over from when I laid the base. (2) Adding rolls of (cheap) insulation from above. (3) Putting on the roof. (4) Adding some plasterboard later when I have the time and inclination.
Depending on joist height I could go to about 180mm of insulation (which is better than most sheds have) and still have ventilation space above the insulation..
The plan is to lay the joists flat then add a profile piece to the top of each joist so there will be more depth for insulation towards the higher part of the roof.
Any major problems with this approach?
Cheers
Dave R
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In an earlier contribution to this discussion, David WE Roberts

Is your shed going to be heated? If not, what's the insulation *for*?
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Cheers,
Roger
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Roger Mills wrote:

To keep it cool in summer maybe.
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wrote:

Firstly, yes it is going to be heated at times - wood burning stove in stock just begging to be used. The building is both a store and a workshop so it will need heating when used as a workshop in the winter.
Secondly, insulation under a metal roof will reduce temperature fluctuation summer and winter even without heating.
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David WE Roberts wrote:

Even easier to fit from above - right over the top of the joists and firings - creates a warm deck roof that needs no ventilation.
Might be worth checking the price of "seconds" locally. Round these parts, its about 12 for a 8x4' sheet of 50mm foil faced PIR.

That would work. The only minor problem would be if any water could blow under your roofing sheets! Then it would puddle in the membrane.

Sounds ok to me.
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Cheers,

John.

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<snip>
Height is an issue - the maximum joist height is governed by the 2.5m maximum height to avoid planning and building regs. So a warm deck doesn't seem to be an option.
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David WE Roberts wrote:

I may be out of date, but I was under the impression a temporary garden building could be taller than that - even if of a pent design rather than apex?

You would only need say 50mm - I am sure is would not be difficult to finish the final external floor level heigh enough to satisfy that measurement! ;-)
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John.

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I would hardly describe a block build workshop on a concrete base as 'temporary'. It also falls between the '15 sq m and 30 sq m floor area within 2m of a boundary' which makes the rules quite specific about what you can build without applying for planning and/or building regulations approval. As far as I know the regulations for outbuildings apply.
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you can do what you say ... although any condensation or blown rain could be trapped under DPM. Another option is Fit from below, and apply a layer of netlon (raspberry net) underneath stapled to joist.
If you decide to plasterboard later then you just board on top.
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Rick Hughes used his keyboard to write :

Is plaster-board a good idea in what might be a cold unheated shed in the winter?
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Regards,
Harry (M1BYT) (L)
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My last two cold, unheated garages (both, however, attached to the house) had plasterboard ceilings. Cold isn't a problem. Excessive damp could well be, but I hope to avoid this because the shed is also a store. Plasterboard seems to be a reasonable ceiling material - fire retardant, cheap, easy to fix and paint.
Obviously any suggestions of a cheaper and more effective ceiling are welcome :-)
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wrote:

If it's cold and damp, it survives better than hardboard does.
If it's too bad for plasterboard, you probably ought to improve it just for the sake of the tools you'll probably have out there too.
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I would not do this .. but OP mentioned he might want to use plasterboard .... I'd use OSB.
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On Fri, 23 Apr 2010 16:09:01 +0100, Rick Hughes wrote:

Even OSB has moisture issues... ply's probably the best bet, if you don't mind the expense (or at least, it's 3x more expensive than OSB this side of the Pond, but there's probably a good reason for that :-)
Having said that, if it's ventilated and regularly heated then you can probably get away with OSB....
cheers
Jules
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On Thu, 22 Apr 2010 19:04:33 +0100, David WE Roberts wrote:

================================================ Polystyrene slabs are much cheaper than Celotex and just as easy to fit. They might be adequate for a shed which doesn't require insulation to house standards.
Cic.
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er... Is fire risk not an issue?
regards
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Tim Lamb

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Tim Lamb formulated on Thursday :

Not with flame retardant polystyrene.
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Harry (M1BYT) (L)
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On Thu, 22 Apr 2010 21:59:02 +0100, Tim Lamb wrote:

=============================================== Mixed views on this, I think. I used it as retrofit underfloor insulation (suspended floor) with the full blessing of both BCO and supplier. It would depend on the kind of work going on in the shed. Polystyrene is still commonly used as tiles, damp proofing and coving in houses. The main caution offered is not to paint with gloss paint, so it's apparently not regarded as a high risk product.
Cic.
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OK.
Having set my overalls ablaze on a number of occasions; using an angle grinder, I'll stick to putting the polystyrene under the concrete:-)
regards
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Tim Lamb

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Tim Lamb explained :

I do that :-)
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Harry (M1BYT) (L)
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