Garage ceiling insulation - Struggling with requirements

[JB-BJ]

Good afternoon,

I will shortly be having the garage ceiling being taken back to bare joists, so will need to reinsulate and plasterboard. Half the ceiling is under a warmed bathroom. Half the ceiling is under a small, uninhabited roof space.

It's a 60's build so I appreciate that per building regs I just have to improve on the original construction, which I will. As it's a garage I will however observe fire regs (30 mins?)

At the moment I'm thinking 50mm Celotex or 100mm of rockwool between the joists, with 12.5mm fireboard underneath, leaving at least 50mm gap to the bathroom floor above. I don't particularly understand how a vapour barrier would work (on the warm side so closer to the bathroom?) and whether I'd struggle with cold bridging through the joists with nothing but plasterboard under the joists. I could perhaps stretch to 20mm Celotex under the joists as well, especially if only in the part under the bathroom.

I'd be grateful for any thoughts.

Please do assume I'm an idiot and spell it out like I'm 5 years old - there's a huge amount of often contradictory info out there in regards to condensation etc, and I want to do a decent job.

Many thanks.

Edit: I should add that this is somewhat on a budget (the vast majority being chewed up by AIB removal), so that is a consideration. I might be tempted by the rockwool between the joists at least as it's so much cheaper. Perhaps 100mm rockwool between the joists then 20mm celotex underneath would be a good compromise?

 

[Notch7]

I would be tempted to put 50mm celetex flush with the bottom of the joists, then use 75mm foil tape and apply tape across each joist to join up the foil on the celetex.

you have no opportunity to fit the VPL on the warm side, as it should go across the top of the joists in the bathroom.

The disadvantage is that the bottom of the joists are on the cold side and allow a cold bridge -so if you can go over it with insulation that would be helpful.

 

[JB-BJ]

I would be tempted to put 50mm celetex flush with the bottom of the joists, then use 75mm foil tape and apply tape across each joist to join up the foil on the celetex.

you have no opportunity to fit the VPL on the warm side, as it should go across the top of the joists in the bathroom.

The disadvantage is that the bottom of the joists are on the cold side and allow a cold bridge -so if you can go over it with insulation that would be helpful.

Thanks for your feedback.

I'm torn between the 50mm Celotex (£200 worth) vs. 100mm Knauf Earthwool (£40 worth). I'll definitely do the 20mm Celotex under the joists to form a continuous insulative barrier, and to prevent some cold-bridging.

In regards to moisture control and ventilation, am I trying to completely seal from the joists upwards? I keep seeing the importance of ventilation but can't grasp that if it's a sealed area.

And for moisture, oes having foil on both sides of the celotex potentially cause me problems?

 

[SFK]

My method would to be fill the joists. So from bottom up:

Plasterboard
50mm Celeotex/Kingspan
200mm of Knauf Earthwool (or whatever height is of Joists) so no air gaps or space under-floorboards

Note that 50mm Celetex is always slightly curved (in my experience) and a right pain to screw plasterboards ontop. Last time I gave up and used seemingly flatter 25mm boards.

SFK

 

[JB-BJ]

My methiod would to be fill the joists. So from bottom up:

Plasterboard
50mm Celeotex/Kingspan
200mm of Knauf Earthwool (or whatever height is of Joists) so no air gaps or space underfloorboards

Note that 50mm Celetex is always slightly curved (in my experience) and a right pain to screw plasterboards ontop. Last time I gave up and used seemingly flatter 25mm boards.

SFK

Forgive me but I thought that at least 50mm air gap under the floor above is an absolute must?

 

[SFK]

JB,
I am not 100% sure, and note that I am a DIYer.

But I do not want dead air that might change its humidity and/or temperature relative to other surfaces. I feel that leads to condensation. So I want to fill up to teh boards.

Also the issue for me was that I have drafts coming in from the sides. So I want no dead air that might condense onto cold surfaces and additionally if I didn't fill the gap 100% I got drafts moving around the insulation (making it pointless), making the floor cold and in places passing through gaps in floorboards (especially around skirting boards).

I guess I was aiming for an inverted warm roof effect (issue is with the VC layer), o all voids filled.
SFK.

 

[bobasd]

OP,
fwiw, given that work is going on then BCO's we deal with would require the ceiling under the bathroom (living quarters?) to be fire proofed.

come to think of it, they'd also want all walls/partitions and doors from garage to living quarters to also be fire proofed.
with good, reasonable common sense if not exactly Regs.

 

[Leofric]

Building regs and NHBC Standards approved treatment of floor over garage in my experience - if 200mm deep joists at 400mm centres, 18mm t and g flooring grade chipboard on joists, 200mm thick mineral wool insulation packed between joists, 2no layers of 12.5mm plasterboard ceiling with 5mm skim finish to give full half hour fire resistance. Cold bridging across timber joists was never considered to be an issue.
Any door between garage and house to be half hour fire resisting doorset reference FD30S,as required under building regs Part B.

 

[^woody^]

At the moment I'm thinking 50mm Celotex or 100mm of rockwool between the joists, with 12.5mm fireboard underneath, leaving at least 50mm gap to the bathroom floor above. I don't particularly understand how a vapour barrier would work (on the warm side so closer to the bathroom?) and whether I'd struggle with cold bridging through the joists with nothing but plasterboard under the joists. I could perhaps stretch to 20mm Celotex under the joists as well, especially if only in the part under the bathroom.

No gap, no barrier. Either fill it with insualtion of your choice, or shove the inualtion tight up against the floor joists. All info as per the other thread you posted in