Insulation for the shed.

[David WE Roberts]

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

[Roger Mills]

Is your shed going to be heated? If not, what's the insulation *for*?

[stuart noble]

To keep it cool in summer maybe.

[John Rumm]

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.

[Rick Hughes]

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.

[Harry Bloomfield]

Rick Hughes used his keyboard to write :

Is plaster-board a good idea in what might be a cold unheated shed in the winter?

[Cicero]

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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.

[Tim Lamb]

er... Is fire risk not an issue?

regards

[Harry Bloomfield]

Tim Lamb formulated on Thursday :

Not with flame retardant polystyrene.

[Cicero]

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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.

[Tim Lamb]

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

[Harry Bloomfield]

Tim Lamb explained :

[Cicero]

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And fireproof your overalls as well, I hope! Soak in a solution of washing soda and hang to dry.

Cic.

[Duncan Di Saudelli]

My workshop (12'x12') was built from 25mm tongue-and-groove loglap. The floor of the workshop was the paving slab foundation itself because I was sometimes working oin a car in there.

I used cellotex on the roof and the main wall that caught the wind, and I skinned all of the insides with 6mm plywood.

1) A 2kW hot air heater warmed it up in about ten minutes 2) Next time I do it I will use thicker plywood 3) All of the plywood developed mould so next time I will 4) Make sure the floor is watertight or maybe I will use a polythene water-vapour barrier as well as plywood on all walls and roof.

I'll be doing it again in the next month or so and I hope to avoid all of the mould (anbd condensation), most of which was due to the semi-permanently damp floor (paving slabs down onto concrete and hardcore with no DPC underneath - next time the workshop won't need to take a car so it will have a wooden internal floor)

DDS

[David WE Roberts]

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.

[David WE Roberts]

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.

[David WE Roberts]

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 :-)

[robgraham]

There was one thing that came out of the discussion for me and one thing that was missed.

For a start my shed roof is made this way, though I did it from the bottom up using just light polythene sheeting to hold the f/g in place. I used a stable gun to hold everything up - the f/g was stapled in a couple of place to the joists to hold it, and then the polythene stapled on top. Face mask and gloves were necessary !

Now IMO the polythene is essential as a humidity barrier - there will be a temperature gradient across the insulation and in winter condensation will occur in the f/g as the warmer air carrying a larger amount of water vapour cools if it's omitted.

The rain ingress under the sheeting I hadn't thought of. I used Ondoline over OSB for my roof with a weather seal at the bottom end. It might be worth considering doing something similar here. The one thing I did have to allow for was a bit of a ventilated air gap then between the underside of the OSB and the fibreglass.

Rob

[Roger Mills]

It's not actually that difficult to fit from below. Provided it's the right width to be a tightish fit between the joists, it will stay in place on its own long enough to staple membrane under it and/or fit plasterboard.

[John Rumm]

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! ;-)