Insulating a garage ceiling under a bedroom

[mateus]

Hi,

Any suggestions on insulating a garage ceiling under a bedroom, the loft above the bedroom has plenty of insulation but the room is still quite cold...
was thinking of fixing some of those foam panels used in walls to the garage ceiling somehow..

 

[PrenticeBoyofDerry]

They will do fine, rook/mineral wool will be a better sound insulator and the use of duplex plasterboards will give an added vapour check barrier to prevent condensation.
It is advisable if you have any electric circuits within this void, not to cover the cables up with insulation, as they can over heat, lighting circuit should be okay if on a 5amp or 6amp fuse/breaker but other power circuits may need to be upgraded.

 

[ajrobb]

mineral wool will be a better sound insulator and the use of duplex plasterboards will give an added vapour check barrier to prevent condensation.

I don't think it is a good idea to put a vapour barrier on the cold side of mineral wool like that; Duplex board will trap moisture and cause condensation.

The garage should already have a fire-proof ceiling. Maybe paint the original ceiling with a couple of coats of acrylic eggshell or any old gloss paint and THEN put up a false ceiling with insulation.

The OP's original suggestion sounds OK to me, if not the cheapest. A closed cell insulation like PIR in Kingspan K18 or Celotex acts as a vapour barrier in both directions so there should be no problem with condensation.

 

[^woody^]

There wont be no condensation issue with using insulation board on/under the ceiling, but it would really then want some 12.5mm plasterboard for fire protection as you don't really want this insulation exposed to potential fire

 

[mateus]

Ok now for any ideas on where to get this stuff cheap?
Seems there are some seconds places?? Anyone had good experience or bad?

Seen this place - http://www.secondsandco.co.uk/ - seems ok, have some on ebay - but £100-£150 so need to double check size I need..

Thanks

 

[PrenticeBoyofDerry]

[quote="ajrobb";p="1746289]
I don't think it is a good idea to put a vapour barrier on the cold side of mineral wool like that; Duplex board will trap moisture and cause condensation[/quote]
if it's a full fill or ventilation is offered to the top side of the insulation, surely not!

 

[ajrobb]

I don't think it is a good idea to put a vapour barrier on the cold side of mineral wool like that; Duplex board will trap moisture and cause condensation

if it's a full fill or ventilation is offered to the top side of the insulation, surely not!

I'd want to see the dew point gradient against temperature gradient to convince me that putting a vapour check on the cold side won't raise the dew point above the temperature. An unheated, well ventilated garage can get below 5°C (because boilers in garages need frost protection).

I wouldn't want to put ventilation on the warm side between the insulation and the bedroom floor - that would just be a source of cold draughts and defeat the whole purpose. Ventilation on the bottom/cold side would be good and, with mineral wool and ordinary plasterboard, could allow spills on the bedroom floor to drain through into the garage.

Maybe full fill with mineral wool with porous plasterboard and porous paint on the cold side.

 

[mateus]

Is it likely that the garage ceiling will have rock wool in it then? Under the bedroom floor????

Very confusing and opposing opinions on what to use/do...

 

[^woody^]

I'd want to see the dew point gradient

WTF

LOL this is a garage not a turkish bath

I think you are way over complication the theoretical side of things

 

[ajrobb]

I think you are way over complication the theoretical side of things

Just don't use Duplex board on the cold side of insulation - simple.

Is it likely that the garage ceiling will have rock wool in it then? Under the bedroom floor????

There was a suggestion that you batten out the ceiling with 2x4s and put 100mm of mineral wool between the battens and then another layer of plasterboard. The original suggestion was to put Duplex plasterboard on the cold (garage) side of the insulation. I am just saying it is not needed and might even be a bad idea.

I don't think there is anything wrong with your initial suggestion of using something like Kingspan K18 insulated board.

It is possible that there is already some insulation in the bedroom floor. Bedrooms over garages often have 3 outside walls and no heat coming from below. This makes them colder than other bedrooms even if they are as well insulated.

 

[mateus]

Sounds just as easy to do... suppose I can just leave the boards unfinished too? Or do they need to be taped and skimmed??

The weird thing is it is already skimmed in there... would that suggest this has already been done?

Maybe I should make a couple of inspection 'holes'

 

[ajrobb]

Sounds just as easy to do... suppose I can just leave the boards unfinished too? Or do they need to be taped and skimmed??

The weird thing is it is already skimmed in there... would that suggest this has already been done?

Maybe I should make a couple of inspection 'holes'

I think regulations require a garage ceiling be a fire barrier, which probably needs the joints taped. Even though the existing ceiling might still be intact, nobody will be able to inspect it. It might be as well to make the new ceiling a proper fire barrier (no inspection holes) to keep future surveyors happy. I suggest you have a word with your local building control.

Refer to British Gypsum Site Book for proper instructions. Basically, if you put 12.5mm plasterboard ceiling on battens, they should be spaced no more than 450mm between centres and there should be noggins between the battens by the walls (within 50mm). (Alternatively, 600mm batten spacing with noggins between the battens to support the joins.)

Finally, it is possible to put mineral wool under the floorboards instead. (I suspect that would be more disruptive.)

 

[tfish]

just done this, i stuck 170 mineral wool in the joists and put up some large sheets of celotex directly under the joists and pinned them up, this keep the wool from dropping out... then taped the joints with alui tape and boarded over with fireboard using extra long plasterboard screws.

oh and before i started a sealed ALL holes and gaps under the bedroom floor with Hilti thermal foam this made a MASSIVE difference