Garage Roof Felt Replacement/Insulation Query

Section62

bloke91 said:

...
I heard Celotex and Kingspan are very good, resistant to mould and fireproof, but also heard fibreglass or mineral wool could be even better from a fire resistance perspective?

With there being a bedroom above I am keen to get the insulation right.

There should already be a fire barrier between the garage and the habitable room(s) above - in older properties this was typically asbestos.  It appears this protection may have been removed in your case, or never installed in the first place.

Inuslation placed between the joists is of limited benefit in fire protection terms because the joists will still be exposed.  There should be a continuous fire barrier across the ceiling of the garage - typically two thicknesses of plasterboard is used. So unless you want to do the job properly it doesn't matter a great deal which type of insulation you use.

What you do have is a fair number of electric cables which will make it difficult to install sheet insulation, which would lean towards fibreglass or mineral wool... but again this is only OK if you aren't affecting the cables in terms of airflow and cooling, or that they were sufficiently oversized when installed that the insulation won't take them outside their rated capacity.

On the outside part of the roof fitting the insulation externally would avoid some of these isssues, and be a lot easier to do.

FreeBear

bloke91 said: I heard Celotex and Kingspan are very good, resistant to mould and fireproof, but also heard fibreglass or mineral wool could be even better from a fire resistance perspective?

PIR/PUR insulation boards manufactured by the likes of Celotex & Kingspan are not fireproof - This was the main issue when it was used on the Grenfell tower block.

Fibreglass & mineral wool are inherently non combustible due to the materials they are made from. However, you need twice the thickness to achieve the same thermal performance as PIR/PUR. So when selecting insulation materials, there is often trade-offs to be made. Above a garage (even if it isn't currently used to park a vehicle), fire resistance should be the overriding consideration. So non combustible materials should be used.

Section62

FreeBear said:

bloke91 said: I heard Celotex and Kingspan are very good, resistant to mould and fireproof, but also heard fibreglass or mineral wool could be even better from a fire resistance perspective?

PIR/PUR insulation boards manufactured by the likes of Celotex & Kingspan are not fireproof - This was the main issue when it was used on the Grenfell tower block.

...

The problem was that there are different grades/specifications of "Celotex", some of which are more fire resistant than others.  People talk about "Celotex" (or "Kingspan") as if it were one product, whereas it is actually just a brand name.  Part of the Grenfell issue was that products only work if they are installed as part of a 'system' rather than used on their own or in combination with the wrong products - the problem in the building industry was (and still is) a lack of understanding in both design and installation.  Specifying the correct product and then using it correctly is the issue.

There's not much point worrying about the use of combustible insulation if it is to be installed between unprotected combustible timber joists - the 'system' required here is to create a barrier to fire at ceiling level.  Otherwise there is a risk of the timber joists burning through sufficiently to cause the floor above to collapse before any PIR/PUR insulation has become a significant problem.

TBH designing and installing fire protection isn't a DIY project.  It is difficult to get right and done wrong it can raise the level of risk above the 'do nothing' alternative.  If the OP is concerned about fire risk they should seek professional advice.

Jonwicksfan536

it sounds like you've got a solid handle on it

something like the ga4050 boards is a good call for the section of the ceiling that sits under the bedroom. product gives decent thermals without taking up too much space. If youre going to plasterboard over it, just make sure the insulation is tight with minimal gaps (use foil tape to seal if needed), and supported or fixed if the joists aren't deep enough to allow full board embedment.

Also, consider using a vapour control lay between the insulation and the plasterboard as this will prevent warm moist air from the bedroom condensing on the cold side. especially relevant if the garage gets damp in winter or lacks good ventilation. for the rest sounds like you're right to leave it be for now.