So, my single skin (painted inside & out) garage build is almost complete, it has a felt on OSB roof with 200mm joists, currently no insulation. I’ve moved in now with bikes, tools, stuff stored on shelfs. No leaks.

During fitting i cut 50mm off the top section of the noggins to allow for air movement once insulated and boarded.

After a dip in temperatures, this morning anything with a metal surface has a layer of condensation on it and paper stuff is damp.

So i presume i need to insulate the roof to stop this condensation forming? It feels chilly in the garage whilst warm outside thus producing what i call the beer out of the fridge condensation problem.

It is only a garage and i’d like to keep this as simple as possible and keep costs down, whilst doing the job properly.

So, am i correct in thinking that 100/150mm Rockwool type insulation between the rafters, held in place with stapled garden netting, then finished off with some kind of boarding (plaster/ply?), and it seems the board requires taping?

OR… could i get away with just stapling some Kontrol R1.9 to the underside of the rafters and be done with it?

Also i believe that the void above the insulation needs venting to the exterior, yes or no?

Many thanks.

Wont rock wool turn into a soggy mess ?

See if nobeer posts the used celotex site. I used kingspan on mine – 100% stopped the condensation – cardboard boxes dont even go soggy any more 🙂

You might find it hard to hold insulation in place prior to the netting without shoving it up tight into the joists.

Wont rock wool turn into a soggy mess ?

I’d hope not but don’t honestly know, would it not hold the moisture until it’s vented and evaporates?

In a perfect world of every rafter being square & paralell i think i’d go with the Kingspan stuff, but on mine every piece would be a custom cut due to shonkyness, is this normal? If i used the Kingspan could i just wedge it between the rafters and NOT board it out? A silver ceiling would be nice and light.

TR – did you also board it out? If so what with?

You got plenty of ceiling height? If so nothing stopping you nailing full sheets up. Far quicker and less gaps!

To hold rockwool type insulation in place before the netting use bits of bamboo slightly longer than the gap and just push them in. They will hold surprisingly well which I can testify too after using this method to insulate under our floor.

No boarding yet.

My garage is like nothing on toast also- as i was reminded last weekend when i stuck up a shelf above the door.

When i do get round to boarding it itll be 6 mm ply.

You can see it here , kingspan and some expanding foam in any gaps ….not cut for added – bodge effect

celotex +1. I would only use the R1.9 as part of a complete package ie. Vapour barriers/ breathable membranes Plasterboard/ Timber cladding.

You got plenty of ceiling height? If so nothing stopping you nailing full sheets up. Far quicker and less gaps!

A eureka moment!!!!

Polystyrene sheets are very cheap and easy to work with.

TR / all – what thickness sheet did you opt for?

Its 50mm

For no other reason than i was using it in other bits of the house already and had a garage full of it …..snuck some off to the garage roof and ordered more 🙂

Was there a hint of sarcasm there qwerty? 🙂
If you use fifty mm board then 75 mm nails with a decent sized head it will hold no problem prior to boarding it with your chosen material.

Wrightyson – no not at all, I get so bOgged down with these thing I miss the obvious solution! I’ve picked your brains often enough and appreciate your advice. I’m liking the idea of a foil ceiling, very NASA. Can’t see the point of board if I don’t have too.

Still not decided what to use for my mancave roof insulation so watching this one!