Insulation in garden office.

[Andy7]

I just read with some interest the thread:

//www.diynot.com/forums/building/new-project-garden-office-research-stage-help-needed.313707/

In which mointainwalker advocates using 100mm fibrewool insulation over 50mm celotex. I was VERY interested to hear that it gives a better insulation and costs MASSIVELY less - handy, as I'm approaching my budget with my build. I've got about £750 of 100mm celotex spec'd at the moment.

Does anyone else have anything to say to back up the argument to use fibrewool over celotex (or equiv) in a timber constructed building?

Would need to find out how to fix it so it doesn't slump - I guess that's why celotex is popular with builders. I saw my builder just pop it in between the cavity of our extension... simple, no slump.

Ta all !

 

[r896neo]

You could use cavity batts which are much stiffer but still rockwool. bout middle of the two price-wise.

 

[Andy7]

You could use cavity batts which are much stiffer but still rockwool. bout middle of the two price-wise.

Yes, I was just looking at those in Wickes. 100mm is £9.45 / sqm vs £10.07 for 50mm Celotex. (roughly the same insulation value?)

Now, Celotex comes in 1200 wide sheets and the bats are 600.
I was thinking that the stud spacing would be 600mm between centres, but would I need to cut the celotex in half or could I have 1200mm spacing on my studs. Seems wrong, but saves loads on timber (I'm sheathng in 18mm OSB3 for strength), also less cold spots due to less studs.

Celotex might be a LITTLE more expensive, but my studs would be half the depth and so even at 600mm spacing I'd save a little on the timber!

Sings and roundaouts....

 

[r896neo]

celotex is more commonly used because people don't want to lose space. If you can live with the extra thickness no reason not to use wool of some sort.

 

[Deluks]

Wickes isn't the cheapest place for insulation tbh.

 

[Deluks]

ps. My choice would be 100mm rockwool in the studs and overboarded with 30mm insulated plasterboard.
Just sayin'

 

[Andy7]

ps. My choice would be 100mm rockwool in the studs and overboarded with 30mm insulated plasterboard.
Just sayin'

What, foil backed PB? Can't be, I hear you need an air spac for foil. Do elaborate, sounds interesting.

(Oh, I'll probably get the stuff from Buildbase but Wickes has a better website for looking at materials and specs - then the final price is a pleasent surprise)

It looks like whether I choose 100mm rockwool or 50mm Celotex I'm not going to make a TRAGIC mistake so long as I get all my vapour barriers / breathable membranes right, and I 'think' I understand how that all works.

 

[Deluks]

You don't need an airspace for foil backed plasterboard. You are referring to the superquilt multifoil stuff for loft rafters.
Insulated plasterboard is just that, PB with a celotex-ey or polystyrene backing. Polystyrene being much cheaper. Both are a vapour barrier in their own right.
Your rockwool will give a decent amount of insulation, and a thin layer of the insulated plasterboard will reduce cold bridging through the studs.

It does increase your budget slightly, but worth the little bit extra IMO

From inside Plasterboard>vapour barrier>insulation over studs>insulation between studs>breather membrane>batten> exterior cladding.

The only airgap you need to worry about is the space between the breather membrane and cladding.

You should make the insulation depth in the roofspace at least double what you have in the walls. (double the performance anyway)

 

[Andy7]

nice one delkus.

Next thing: The rear of the unit is made from an existing single skin of breezeblocks (I'm extending an existing workshop).
I won't be timber cladding the rear wall, just front and sides, so any advice on what layers to use there.

I'm going to put rockwool>VB>PB on the inside, but what's good on the outside? I guess latex paint will just trap moisture inside.