How best to insulate my new garage?

[JesusNinja]

I've recently had a rather splendid prefab garage put up, but it's a bit chilly in there. It will be used as a mechanic's workshop, so whilst it doesn't need to be the height of luxury, I'd like to be able to warm it up a bit, and not just end up heating the garden outside! There's also a potential issue with condensation from the roof.

What are the suggestions for insulation? I had considered insulated plasterboard - but what insulation backing, and what thickness? I am wary of anything two thick that will eat into my usable space!

The panels of the walls are cast in concrete with ribs for reinforcement. The thinnest areas being about an inch thick, and the ribs about 3". It strikes me that something like rock wool could be packed in the recesses and then paneled over, but the ribs themselves would not be insulated.

If insulated boards are used - do they not end up feeling bouncy?

Another option as a halfway house might be to put wooden battens up the ribs and then fill the voids with insulating material. Still an issue where there is no insulation where the battens are, but timber being a reasonably intermediate layer as opposed to direct onto the concrete ribs. The timber could be significantly narrower than the ribs too.

Anyone got any better ideas (I'm sure they have!)

 

[ajrobb]

What's wrong with bouncy? How cosmetic or structural does it need to be?

How about just 25mm Celotex (PIR) on the joists and between the pillars held in place with battens with screws through the PIR into the walls? Silver surface means work lighting will be a doddle.

 

[JesusNinja]

Dunno, I suppose I just like the feel of something more solid!

Cosmetic isn't an issue. I won't be plastering over it - just painting it white. Structurally - it would be nice to be able to attach things soundly. For this reason I've also considered using ply for the walls instead - very easy to put screws in for hanging tools etc.

 

[Richard C]

Don’t use any form of plasterboard in an outbuilding if you cant guarantee a stable & low ambient moisture content. It wont tolerate moisture (even MR) to any degree, will warp all to hell & disintegrate in a very short time.

I would use external grade ply or OSB over the insulation.

 

[JesusNinja]

Thanks Richard - a good tip regarding the plasterboard. The aim is to maintain a pleasant environment, not disimilar to in the main building, but afterall, it is still a prefab garage, and damp cannot be completely ruled out.

So, how does OSB rate against marine ply in terms of cost and quality?

 

[Richard C]

So, how does OSB rate against marine ply in terms of cost and quality?

Firstly forget Marine ply, you definitely don’t need that; it’s expensive & WBP uses similar waterproof adhesive in construction & is effectively the same apart from the decorative appearance of the expensive, external veneers of Marine ply. Although I’m familiar with ply & most manufactured boards, I’m not that familiar with OSB; a quick search would seem to indicate OSB3 grade is what you need to go for as it’s broadly equivalent to WBP ply. WBP doesn’t look particularly attractive but it’s generally covered with something so doesn’t need to be; OSB is even less attractive (but that may be subjective ) & it’s not as structurally strong but you don’t need structural strength &, as I assume you will be painting it, raw appearance won't make a lot of difference either.

OSB should be somewhat cheaper than WBP but you need to search around for the best prices; I would probably rule out the national Builders Merchants chains, even with the hefty discount they will offer at the drop of a hat but check it. With the quantity your likely to be buying, an internet order may make sense, especially if you can hit that “magic” min order price as you’ll get free delivery. You may even be surprised at BNQ trade prices if you’ve access to a truck.

 

[JesusNinja]

I will certainly need a reasonable amount. The walls are only 2m high, but the garage is 4.5m X 11.5m, and there's the roof to do as well.

If I use rockwool or similar, do I need to concern myself with moisture - ie does it need to "breathe"

Thanks for the help!

 

[Richard C]

As long as you use an insulation product that’s suitable for vertical installation & is water repellent. You need fit something that has either an integral vapour barrier (facing the inside) or fit a separate barrier between the insulation & the inner wall liner.

 

[ajrobb]

I get the impression that concrete is less permeable than brick, so I don't know if you need to protect mineral wool from penetrating damp. For BRICK single skin walls, you could use treated mineral wool slabs like Crown DriTherm that don't 'wick' water from the outer skin. A polythene vapour barrier under the OSB should prevent condensation behind the insulation. You might get enough vapour barrier by caulking the joints in the OSB and a couple of coats of oil-based or acrylic paint.