Membrane to go behind cladding

[SteveA]

On a side note what sort of boarding would you use for the roof? its just going to be felted

I was thinking sterling board or plywood which would be best suited?

 

If you're using silver-foil backed foam insulation the Celotex data sheets are really handy as it will show you all the materials & dimensions. Need to register an account to see them: Celotex - UK PIR Thermal Insulation Manufacturers

[jamesd]

I would insulate personally - it is expensive trying to heat the whole district otherwise.

 

There are two basic approaches - one is to stop water getting in, the other is to let it out if it does.

 

I am not a big fan of relying on a perfect seal. When it is breached the water will get in and then not get out again.

 

I would infill the walls with something breathable - eg reed board, sheep's wool or fibreglass (compare prices and lambda values). I would cover the outside in thin ply (wbp) if I hadn't used reed board, with a breathable membrane if I didn't trust my cladding to be fully waterproof. I would then overlay 20mm battens, lined up with the frame below, and clad over this, so there was an air gap behind the cladding. Inside I would board out the walls with half inch wbp ply to minimise absorption of water into the structure. For a Rolls-Royce job I would paint the back of the ply first. The ply doesn't need to be this thick but you can then screw shelves etc directly to it.

 

Roof, I would use Kingspan or Celotex expanded foam boards - thin seconds are pretty cheap. I would then put wbp ply over the top, probably 9mm so I could climb on it and nail it straight through with helical nails. Felt or corrugated bituminous sheet to finish. Infill between rafters or ply under, to cover the foil surface and prevent condensation.

 

Perhaps overkill for a shed, but it would be reasonably quick and cheap on a construction that size.

 

Alec

 

Ok many thanks for taking the time to reply so would you use marine plywood over sterling board for the roof then? the board i was thinking of using was this:

Norbord Sterling OSB3 Board 18 x 1200 x 2440mm - OSB3 | Jewson

 

I agree with how you do the side walls as well although i won't have a sheet of ply before the cladding my intention was to have (working outside to inside) feather board, breathable membrane, 25mm air gap, 50mm insulation, thin ply to cover up on the inside.

 

How does this sound to you?

[Guss109]

kingspan sounds expensive i would use 100mm glass wool insulation between framework then on the outside of the frame have a sheet of ply to hold the insulation, on the ply i would mount a building paper or roofing paper usually rubberoid then at 600mm spacings vertically fix a 10mm x 40mm strap (i call them drip strips because they allow water to run off your paper) on the strap fix a 40mm x 50mm batton horizontally this gives your air gap, then mount your cladding

 

The kingspan will be a far higher insulation rating than the equivalent glass wool. But I do agree with you reference the batten to allow airflow but I'd only put the one one as opposed to two.

[Graham w]

The kingspan will be a far higher insulation rating than the equivalent glass wool. But I do agree with you reference the batten to allow airflow but I'd only put the one one as opposed to two.

 

yes one batten for feather/waney edge, i was thinking about scotch lap.

 

the type of insulation is not so important for the size of the build, any heat saved with high performance insulation could be quickly lost through windows, frames or doors, or when you open the door.

 

will there be insulation under the floor?

 

why not use box profile or corrugated iron for the roof, it would almost guarantee a water tight roof. it may also work out more cost effective with a longer lasting material. also you wouldnt need to have OSB or ply with tin as it would be mounted on battens above your membrane that sits above the insulation and trusses

 

 

i think the membrane should be on the insulation side of the air gap to allow the cladding extra air to dry when damp. i can see the membrane holding moisture if its hard against the cladding

[agg221]

Ok many thanks for taking the time to reply so would you use marine plywood over sterling board for the roof then? the board i was thinking of using was this:

Norbord Sterling OSB3 Board 18 x 1200 x 2440mm - OSB3 | Jewson

 

I agree with how you do the side walls as well although i won't have a sheet of ply before the cladding my intention was to have (working outside to inside) feather board, breathable membrane, 25mm air gap, 50mm insulation, thin ply to cover up on the inside.

 

How does this sound to you?

 

Re. the walls. As Graham W has said, you want the air gap between the feather edge and the membrane, rather than between the membrane and the insulation.

 

If you use loose fill type insulation such as glass fibre or sheep's wool, you will need something rigid to stop it falling forward against the cladding. If you use an insulating board then you won't need this. Try looking at the Lime putty, mortars, plasters and limewash at unbeatable prices supplied by Mike Wye & Associates. - they have quite a range of breathable sheet insulation ranging from cork board to their diffutherm boards. I have found them very helpful and places like Buildbase and Jewsons can order their products in.

 

Sterling board or WBP ply (which is exterior rather than marine) would be interchangeable. Both of them are there to form a rigid base, rather than provide the insulation. I would put one of these over the top of a layer of insulation such as Kingspan or Celotex.

 

From personal experience, it is a lot quicker to install sheet materials as a continuous layer than to infill between panels. You also then don't need the ply layer on the outside to keep it in place (although you will on the roof unless you go for the suggested box profile roofing). Sheet insulation over the outside of the structure, with battens over, nailed straight through both into the frame creates the airgap. I'll see if I have pictures of this from when I did my extension roof.

 

The point about Kingspan type sheets being much higher performance and that you will lose the heat faster through the doors and windows is well made, however the relevant number is its lambda value, which is thermal conductivity. The less conductive the material, the thinner you can go for the same performance. Roughly, fibreglass is .045, sheep's wool is .042, Kingspan or similar is .02. This means that 50mm of fibreglass would be equivalent to 30mm of Kingspan for the same performance (for comparison, I used aerogel insulation which gives the same value with only 15mm). These figures are simplified, but you can see that for small quantities at lower thicknesses it can work out cost effective to use higher performance material to simplify the installation. If you do go for Kingspan or Celotex (different manufacturers of the same thing) you can buy seconds at about half the price of new, which works out reasonably cost effective. I reckon, depending on the roof pitch, you could get away with three sheets for the roof which would be about £25 all in for 30mm boards. Breathable insulating boards will be more as you won't find them so easily as seconds.

 

Hope this helps!

 

Alec

[jamesd]

Re. the walls. As Graham W has said, you want the air gap between the feather edge and the membrane, rather than between the membrane and the insulation.

 

If you use loose fill type insulation such as glass fibre or sheep's wool, you will need something rigid to stop it falling forward against the cladding. If you use an insulating board then you won't need this. Try looking at the Lime putty, mortars, plasters and limewash at unbeatable prices supplied by Mike Wye & Associates. - they have quite a range of breathable sheet insulation ranging from cork board to their diffutherm boards. I have found them very helpful and places like Buildbase and Jewsons can order their products in.

 

Sterling board or WBP ply (which is exterior rather than marine) would be interchangeable. Both of them are there to form a rigid base, rather than provide the insulation. I would put one of these over the top of a layer of insulation such as Kingspan or Celotex.

 

From personal experience, it is a lot quicker to install sheet materials as a continuous layer than to infill between panels. You also then don't need the ply layer on the outside to keep it in place (although you will on the roof unless you go for the suggested box profile roofing). Sheet insulation over the outside of the structure, with battens over, nailed straight through both into the frame creates the airgap. I'll see if I have pictures of this from when I did my extension roof.

 

The point about Kingspan type sheets being much higher performance and that you will lose the heat faster through the doors and windows is well made, however the relevant number is its lambda value, which is thermal conductivity. The less conductive the material, the thinner you can go for the same performance. Roughly, fibreglass is .045, sheep's wool is .042, Kingspan or similar is .02. This means that 50mm of fibreglass would be equivalent to 30mm of Kingspan for the same performance (for comparison, I used aerogel insulation which gives the same value with only 15mm). These figures are simplified, but you can see that for small quantities at lower thicknesses it can work out cost effective to use higher performance material to simplify the installation. If you do go for Kingspan or Celotex (different manufacturers of the same thing) you can buy seconds at about half the price of new, which works out reasonably cost effective. I reckon, depending on the roof pitch, you could get away with three sheets for the roof which would be about £25 all in for 30mm boards. Breathable insulating boards will be more as you won't find them so easily as seconds.

 

Hope this helps!

 

Alec

 

Unfortunately most of the frame work is up so putting an air gap between the feather edge and membrane might be difficult, the only way to do this would be to set the membrane back an inch between the studwork. This would take more time, but i presume would do the same job?

If i were to put the membrane straight onto the studwork then put a batten on to create the airgap (which is obviously the easiest way) my feather board would be too far away from my dwarf brick wall.

[Graham w]

here is a quick cross section.

[jamesd]

Thanks for that, i intend to felt the roof would i still need to use a membrane underneath the felt or would you just tack the felt straight onto the plywood?

[Graham w]

Thanks for that, i intend to felt the roof would i still need to use a membrane underneath the felt or would you just tack the felt straight onto the plywood?

 

if i was to use felt i would put the battens running from top to bottom instead of side to side, screwed or nailed to the top side of your trusses

[jamesd]

if i was to use felt i would put the battens running from top to bottom instead of side to side, screwed or nailed to the top side of your trusses

 

Ok many thanks, so the battens would run in the same direction as my trusses then?