Auditors

[POL_SC 10]

Afternoon,

I'd like to insulate roof in the garage. I know already that I have to look at cold roof solution. I know that there should be ventilation between roof board (wooden board with felt on top) and insulation. But I cannot have any ventilation in between, as beams are done across the garage (if it makes sense). So my question is

- shall I just buy some polystyrene and stick it on the wooden board from the bottom

- or should I pack cavity with wool and stick plasterboard. This would make sandwich - roof board/insulation/plasterboard

Last thing which I want is to do something wrong, have condensation problem, which would lead to rotten roof.

 

I will upload some photos later.

[BlueBob + 700]

Its a garage so condensation should be expected.  Its all about layers - felt, wood sheet, breather gap, insulation.  You may find that plasterboard is a waste of time as the room will be expposed to the damp when opening the door ad the gypsum will just suck the moisture straight in to it and give you a saggy ceiling.  Insulating half the room (as thats what it will become) just concentrates where the hot/cold collects

[POL_SC 10]

Thanks @BlueBob,

But what I can do if there is no possibility of having breather gap? 

[Sceptre + 2,704]

Depending on how much height you've got to spare a false ceiling topped with insulation board with a few inches' gap for ventilation will make a noticeable difference - you shouldn't have much of a condensation problem unless there's a big temperature differential, in which case a wooden board covered with felt is probably the wrong sort of roof, so treating it with Probor or similar should protect it for a good few years. I'd use proper fire retardant insulation boards rather than polystyrene if I were you as it's much safer and your home insurer may require it. 

If the ceiling's that flimsy then the walls and floor probably aren't the model of thermal efficiency either, so for it to be worthwhile you might have to do the lot. Some pictures and an idea of your budget would make it easier to say for sure what I'd do in your place.

[POL_SC 10]

(...) Some pictures and an idea of your budget would make it easier to say for sure what I'd do in your place. (...)

And this is where I should start in first place  

I have found some old photos. 

Garage is with brick walls and I have door to the kitchen. Budget is always an issue, but I rather wait and save some money and do it properly.

[Sceptre + 2,704]

That makes it clearer. I'm still thinking false ceilings, as the joists are easily deep enough to have 50mm of insulation board between them with an air gap against the roof itself and plywood or whatever ceiling you opt for fitted against the joists themselves. If you stopped the ceiling just shy of the walls at either side then you'd have plenty of airflow over the whole roof to minimise condensation.

The problems I see are firstly the walls - are they a single layer of brick, if so then there's no point installing gold-standard roof insulation as the garage will still be cold, likewise if the floor is solid concrete or there's a big garage door which will leak heat. The peeling whitewash suggests issues with damp which makes me suspect the wood in the ceiling might be suffering too. Before you cover it all up it would be sensible to have it all inspected and any rot fixed, all the timber treated and the felt checked while you're at it.

There's never anything to lose by getting a few tradesmen in to suggest ideas and come up with prices, there's no obligation and they'll give you some ideas. 

[POL_SC 10]

OK, so you are suggesting breather gap on 'warm' side @Sceptre ? False ceiling makes sense, and I think that would be one of the best options here.

Wall are warm (-ish). One is side wall of my house with cavity insulation, opposite wall is wall of my neighbors garage. Garage door and concrete floor is still on my TO DO list. Just have to do it one by one.

I'm having felt replaced this next month so this is why I have focused on roof. At the end of the day warmth will go mainly through the roof.

I do not mind garage to be cold, but it would nicer to work on my motorbike when it is slightly warmer

Anyway, thanks for pointing in good direction.

[Sceptre + 2,704]

To be honest I'm not 100% sure about the air gap as it's been a while since I did a house up myself rather than get people in, but I'd think you'd be best having an air gap between the wooden roof board and the insulation, so on the cold side. You may need a membrane over the insulation as well - this is why getting someone to have a look is not a bad idea. I seem to recall 50mm being the minimum for pitched roofs.

Perhaps hang a spare duvet inside the door too, a big metal door is going to conduct a lot of heat. Happy to help anyway