Insulating a detached garage workshop

[breezybadger]

My garage is detached from the main house, is constructed from breeze blocks internally and stone on the outside and I’m assuming cavity in between. There is a damp proof course visible externally.

The roof is pitched so has a bit of open loft space with joists spaced at 400mm.
The garage has both power and water.

I would like to make this space warmer and dryer for spending more time in as a workshop / hobby room, but I’d also like it to be usable as a garage infrequently.
My broad plan is to plasterboard the ceiling and put 200mm (or more) of rock wool in between the ceiling joists. I would drill more air vents into the soffit boards to ventilate the loft space.

For the walls I would put a vapour control layer up and then put up insulated plasterboard up all around secured mechanically.

On the garage door itself, I’d use some garage door insulation boards and then add some draft exclusion strips.

I’m not sure about the floor - currently it’s covered by an old carpet but possibly I’d replace with some Lino or similar.

Does this sound like it would make the space more usable?

Am I likely to have mould or damp issues? Currently as it is uninsulated you can’t really store fabrics or soft furnishings in it without getting mildew growing on them over the course of a year.

 

[jnw010]

Your insulation plan sounds reasonable to me, however even with decent insulation you will get temperature variations in an unheated space which can still result in condensation and mildew. My solution has been to use a dehumidifier to manage moisture variations. This may be useful if you park a car in there after it's been raining (in my case motorbikes) as all that moisture has to dry off to somewhere.

 

[Jameshow]

I'd cut out the insulation at the edges and put in some 2x2 then you'll have a solid point for fixing of cabinets etc every 4ft.

Otherwise you'll need long frame fixers to go into the block work.

 

[MarkAW]

What are garage door insulation boards?

It made a huge difference to mine when I boarded and insulated the ceiling (after the door of course)

 

[breezybadger]

Your insulation plan sounds reasonable to me, however even with decent insulation you will get temperature variations in an unheated space which can still result in condensation and mildew. My solution has been to use a dehumidifier to manage moisture variations. This may be useful if you park a car in there after it's been raining (in my case motorbikes) as all that moisture has to dry off to somewhere.

The dehumidifier idea sounds good as I do also have a motorbike stored in there too!