garage insulation

[Cuprager]

Hi again,

I am looking to insulate my shiplap wooden garage so that it wil be a more comfortable place for me to work in. What is the best method of achieving this at reasonable cost?

My initial thoughts would be polystyrene sheet insulation inserted in between the studs and that whole lot plaster boarded over. What I am unsure of is whether I need something like polythene sheeting to block any moisture from coming in. If thats the case, where in the sandwitch would I put the sheeting? (shiplap | Polythene | Polystyrene Insulation | Plasterboard) or (shiplap | Polystyrene Insulation | Polythene | Plasterboard) or something differenty again.

For the floor I thought about laying polystyrene and overboarding with flooring grade chipboard.

I know that plywood would be better than plasterboards but I dont know if the budget will stretch to it!

TIA

[The Natural Philosopher]

Use celotex if possible between studs

Leave 25mm airgap outside it.

Seal it with sealing tape supplied for that purpose so its totally airtight.

Use foil backed plasterboard.

Good.

Its actually not that much better. Use MDF for walls that need things attached. Chip is great for floors.

[Cuprager]

I presume that you mean between the celotex and the shiplap?

Ok, is there a need for the polythene as well as this or is that not required?

Thanks for your advice.

[Stuart Noble]

Polystyrene is half the price of celotex and probably good enough for a garage. The gap is normally between the insulation and the roof. I put a few nails in the joists to stop it pushing too far

[Andy Hall]

The back of the Celotex and the outside wall.

Celotex has foil on both sides so an additional barrier isn't required.

I found when doing this that the most cost effective way was to use ply on the inside face, but in order to get best prices, I put together a complete project materials list and faxed it to several merchants asking for quotes. I made it clear that it was competitive request.

This brought in prices that were substantially below initial asking prices.

I found that it was worth insulating the walls and the roof. The floor was much less interesting to do. This was partly because I didn't want a loss of height, partly because I have heavy workshop machinery (some machines > 1 tonne) and the heat loss through the floor was proportionally much less than the other surfaces.

[Ian Stirling]

It's also half the insulation value, so for a given price...

[Andy Hall]

It really depends on what is wanted.

In my own case I was looking for a solution to be able to achieve and maintain a comfortable working temperature in the workshop with low running cost. For that reason, I opted to use Celotex because the insulating properties are much better than those of polystyrene.

50mm polystyrene has an R value of 1.3 m^2.K/W 50mm Celotex has an R value of 2.15 m^2.K/W

One would have to install 85mm or so of polystyrene to achieve 2.15 (even if such a sheet size existed).

Therefore that justifies immediately a 50% greater cost for the same thing. One also has to add vapour barrier etc. for polystyrene.

Since I wasn't able to implement floor insulation for reasons of weight but more importantly loss of height, this alos provided a sensible solution. One could go for a greater thickness of insulating material, but that results in a loss of internal space.

[Cuprager]

Some times you just cant believe your luck. A mate called me last night and told me that he had about 5 'spare' rolls of Knauf glass wool insulation left over from doing his house and would I like them for the price of a couple of pints :O) Well, the answer is fairly obvious, the question is, is that type of insulation suitable for doing my garage?? Would I insulate, vapour barrier and then plasterboard / plywood the inside face?

Thanks plenty for the answers given so far.

[Andy Hall]

It's free, however 5 rolls probably won't do the whole job.

In order to match 50mm Celotex, you would need at least 100mm of glass wool.

However, the situation may be the opposite in that you will have 150 -

250mm material. Do you really want to lose that much area all round?

If you compress the material, you would lose insulating value.

A good solution might be to use the fibre for the roof space where the thickness doesn't matter so much (probably) and sheet material for the walls.

[Ian Stirling]

100mm is common.

Yeah - I looked at this, and I diddn't really see a good place for polystyrene.

For the same price - 50mm kingspan from jewsons costs me some 7 quid a square metre. 100mm polystyrene was not much cheaper, giving around the same insulation. And 100mm rockwool - around 2 quid a square metre.

I went with the rockwool.

[Ian Stirling]

Indeed. I note that homebase is at the moment doing 12m^2*100mm rolls for 23 quid. (jewsons pretty much the same)

There is an alternative - you can without too much trouble delaminate the fibreglass.

My current walls -

- the sections shown now have more battening against them to provide airflow against them, then fibreglass pushed onto the pegs, the pegs trimmed, and battening (which happened to be the original stuff) put back. Then foiled plasterboard to be put on. Works well.

[Cuprager]

I like your garden but cannot spot the battened walls :O)

[Ian Stirling]

Doh.

[Ian Stirling]