insulating a nissen hut whilst minimizing damp.

[colditzthepig]

Hello DIYnot folk

I'm building a rustic kitchen in one end of a nissen hut and would like some advice/suggestions for insulation and vapour barriers/breather membranes etc.

The buidling has corrugated metal roofing, which goes all the way down to the concrete floor. I'll be building an internal shell of plywood, with a 15cm gap for insulation between this and the metal outer.

We'll be heating it with a wood burner and also cooking in it frequently.

How do I insulate this shape and type of building without trapping lots of moisture between the insulation and the metal?

I would also prefer it if rodents couldn't nest in the insulation.

Any help much appreciated.

James

 

[theprinceofdarkness]

Corrugated iron roof are notorious for condensation as the metal is always at the outside temperature in the cold and above it in the sun. So as I see it there are two ways to go.

One way is to totally fill up to the iron with pumped foam. This then excludes air, so no condensation. However with the pumping pressure it could cause the iron to bulge, so specialist advice would be required.

The other way is to build a waterproof inner building, knowing that its going to be dripped on. Some way must be allowed for the drips to run away safely with a groove around the inside of the concrete base and ventilation for the underside of the metal roof. So this would call for a conventional roof construction, i.e. from the outside, breather membrane, insulation, vapour control membrane, plasterboard.
The big problem is that you are building it upside down, unless the roof void is big enough for you to get into.

One way could be to build a polyhedral roof, a bit like the way underground tunnels are constructed. So you make each section less then the widest breather membrane by about 100 mm and a manageable width 1m?. The side would have to have a taper, so that when say 8 of them are put on on top of another, they curve over to follow the roof curve. The over lap should be arranges so the top membrane overlaps the underneath one. So starting at the bottom, glue the first over lap to the concrete, put the next section on it and loosely bolt/screw, glueing the overlap and propping, continue until all sections are done. Figure out how to get the last section in. Remove props, frame should settle tighten bolts/screws. Fill section with insulation, staple on VCM and plasterboard.

Or just build a lightweight flat or ridged roof with the same layup, but you will be loosing space and the wow factor.
Frank

 

[colditzthepig]

Hi Frank.

Thanks for the ideas.
I would go for spray insulation, except I keep reading about possible health risks!

I think I'll go for the second option, but using plywood instead of plasterboard.

I'll try and leave a 5cm gap between the breather membrane and the metal roof, and keep the gap well ventilated.

Cheers

James