Insulation for metal workshop roof

[aesmith]

Hi,

My shed has a metal roof, that modern squared corrugation stuff, not sure what its official name is. The sheeting's screwed onto timber purlins.

What I was wondering, is whether there's anyway to add insulation to this sort of roof as a retrofit. I'm concerned that if I do it wrong I'll trap moisture and condensation between the insulation and the roofing. Ideally I suppose it would be something applied directly to the sheets themselves.

Any suggestions?

Thanks, Tony S

 

[OPJ]

Polyurethane foam may be your best bet.

 

[liam8223]

The steel you are referring to is called 'box profile'.

We let out steel shipping containers, and to stop the condensation problem we use 60mm Celotex insulation, with battens to hold it secure.

We use 20mm x 20mm steel angle, but anything rigid enough not to sag will do.

My workshop is actually two 40ft artic trailers bolted together with the centre two sides cut out, so with single skin aluminium roof you can imagine what it does when the temp drops!

I have presently solved my condensation problem in the workshop with some tube heaters to raise the temp and a decent dehumidifier to remove the moisture, its currently working a treat.

Hope this helps your predicament.

Eh smith are by far the cheapest for celotex that i have found.
Another product, Kingspan is fundamentally the same stuff and will do the job, as will Jabbolite, which is expanded polystyrene, available in many thicknesses, we use 50mm.

 

[aesmith]

Thanks for the responses.

I was thinking maybe I could fit insulation boards as a snug fit between the purlins, held in place by battens on the sides of the purlins. Maybe duct tape between the boards and the purlins as well to seal the gaps before the battens are fitted.

That would leave the boards hard against the flat parts of the roofing sheets, but air spaces along each of the corrugations. Are those spaces a concern? I guess those spaces need to be completely sealed from the inside of the building.

 

[liam8223]

I have never had a problem with the 'spaces'.

You will see a massive improvement, trust me.
If you do intend to warm the shop slightly, it will become quite a dry environment.

 

[aesmith]

Thanks again. Could I ask you to put me straight on U values and what that means in relative terms?

I quick Google suggest that plain steel cladding would have a U of 1.2 - and that's watts, per square metre, per degree - does that sound correct?

If so, assuming my building's 4m from eve to ridge, 10m long, that's 80 square meters of roof. So that would be 480W to maintain a 5 degree temperature difference.

That sounds much too low, but I'm not sure where I've miscalculated.

 

[jimi43]

I had exactly the same problem and it was an absolute NIGHTMARE...it actually rained in the workshop! There is a mention of this problem on the humidifier thread here recently.

Also mentioned was the very logical pros and cons of dehumidifiers. The desiccant type being the only solution. I had a refigeration type and it was running constantly and freezing - oh and bloody expensive!

What I used eventually was used estate agent boards (which I manged to get for free) which are basically plastic corrugated sheets with air space in the cavities. I glued these to the roof directly between the joists...then I put foil backed insulation for warming walls over the whole lot and it has worked!

I am now going to put up a false cavity filled with fibreglass insulation...when I get a spare day or two!

So far it has cost me about £30 for the full roll of foil insulation from a bootfair...

Jim

 

[9fingers]

I can't really believe that figure of 1.2 for sheet steel but I have been unable to find an alternative value.
I mainly get hits with steel /foam sandwich figures around u= 0.2-0.25.

Bob

 

[MikeG.]

I don't know where you guys are getting your figures, but you can work on a U value of around 16 for a thin piece of sheet steel........in other words.......bugger all!

You all know why it is "raining" off a bare steel roof without me explaining, and you all know that the answer is insulation. Ideally, there would be ventilation above the insulation (unless there is absolutely no void or gaps whatsoever).

Mike

 

[OldWood]

I used Onduline (corrugated bitumen saturated fibre material) for my workshop roof. In terms of it's insulation it will be better than steel but in contrast to any insulation material, the difference is not significant -ie Mikes figure of 16 for steel doesn't sound unreasonable to me, and on that scale the Onduline is probably 10 and any good insulant will be down around something like 0.1.

So my basic roof of Onduline over 9mm OSB board will be considerably better than steel but still in need of significant insulation.

If we now look for the similarities - my corrugations are allowed to ventilate so the top surface of the OSB is at outside temperature. Tony's roof needs I think the same type of barrier against the steel and then the insulation below that, but again that insulation should have some air gap to avoid any build up of moisture at the insulation/roof barrier surface - that is the insulation must not be hard against the barrier layer.

And further more to prevent serious moisture condensation within the insulation, due to the dew point being within the insulation depth, there must be an impermeable layer immediately above the ceiling cover - typically polythene sheeting if PB is being used. Kingspan and it's likes are moisture impermeable but any joins would then need to be equally so otherwise damp air will reach the barrier layer and condense there.

I think in Tony's case I would attach OSB to the purlins, put in some framing to attach the PB to, fill that cavity with fibreglass, staple up the polythene sheet (which will hold up the fibreglass), and then screw up the PB.

OW

 

[aesmith]

I don't know where you guys are getting your figures, but you can work on a U value of around 16 for a thin piece of sheet steel........in other words.......bugger all!

That makes more sense of the calculations. I found it quite hard to find U for plain steel, I guess not many people are interested in that, but on the other hand it can't really be infinite either.

At the moment I don't get condensation on the roof, but then I'm not able (or trying) to raise the temperature significantly above outside temp.

 

[aesmith]

OW,

I understand your explanation, but I think its probably a non starter for my building, unless I take the steel sheets right off and rebuild the woodwork. As it stands the roofing sheets come right down to the masonry at the eves, and I really can't think of any way that would allow them to be ventilated without trapping some of the timber on the cold side of insulation.

Turning it over in my head, I'm now thinking along the lines of something stuck directly onto the steel. Maybe something pretty crude.

 

[Digit]

something stuck directly onto the steel.

Foam!

Roy.

 

[9fingers]

50mm jablite polystyrene sheet will be cheap and will form a good vapour barrier. it comes as 8 x4 sheets and can be cut with a handsaw or breadknife. Costs about £10 a sheet but you should be able to strike a deal for 80 sq metres at your BM
Cutting slightly oversize allows it the be sprung into place between walls, battens etc and pushed up tight to the steel.
Tape over the joints to maintain the vapour barrier.
You can then clad it with whatever durable finish you want on to the battens.
I used 6mm mdf on my w/s ceiling - cheap, not too heavy and roller painted white for maximum light reflection.

Bob

 

[aesmith]

something stuck directly onto the steel.

Foam!

Roy.

Do you mean sprayed in place? Or sheets of foam?

 

[Digit]

Sprayed. No voids, no gaps.

Roy.

 

[9fingers]

I'm guessing that spraying would need pro kit and operators to do 80sq m without it ending in tears - literally!

Bob

 

[Shadowfax]

I know I have said this befire but if you value your workshop, your tools and your life please do not use expanded polystyrene in any form in your workshop.
If you must use it make sure it is very well covered with fire resisting material.

It is one of the biggest fire hazards you could use and will burn in the most evil fashion you can imagine.

The alternatives cost more but will be better for you in the long run. They will also be much better at insulating which is what is being asked for here.

SF

 

[Digit]

It is one of the biggest fire hazards you could use and will burn in the most evil fashion you can imagine.

And it drips flame as well. Sprayed foam is polyurethane...

http://www.diysprayfoam.co.uk/

...ask these people, and theres lots of others.

Roy.

 

[Shadowfax]

And it drips flame as well. Sprayed foam is polyurethane...
Roy.

If you have ever seen this you will probably agree that it is not suitable as a building material. A disaster waiting to happen!
The heat given off is phenominal and the smoke is thick and black, laden with toxic stuff such as cyanide and other lovely poisons!

Cheers,

SF