Extension: Bedroom over Garage

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Good Afternoon,

We are currently in the throes of a house extension, part of which is a side extension with a bedroom above the garage.

I am just calculating how many sheets of plasterboard to order for the ground floor ceilings and was wondering if anyone could point me in the right direction. I have read online and have been told by another friend that their bedroom above the garage is cooler that in other parts of the house (assume because of garage door).

Our plans specify that we will use 15mm fire board on our garage ceiling, i was planning on sealing the edges with intumescent mastic and ceiling will most likely be skimmed with plaster (maybe not yet decided).

Is this the best way of achieving the fire rating (garage will not house any vehicles but want to adhere to BC standards - or will be be simpler to do a double layer of plasterboard. the different is cost isnt really a factor as it will be a child bedroom so we would just like to to be warm and have the extra required fire protection.

I had thought that we would/could insulate the floor above with celotex fitted snugly in-between the joists - do we also need to use a vapour barrier here? What U have read so far online about this is a little confusing.

Thanks in advance
Lou
 
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We fill between the joists with glass wool insulation then fix another 50-70mm across the joists before fixing the plasterboard.
No need for a VCL if you have a 'warm' floor.
12.5mm fireboard will give you the rating you need.
 
For insulation, it is easier to pack mineral wool tightly between joists than to cut rigid foam to a perfect fit between them. It's also cheaper, with less wastage.
 
We have a small bedroom over the garage and hence outside the main thermal body of the house. It has three outside walls and a sloping roof and was freezing cold because there's no heat rising from below.

So we got a pal who's a builder to do a proper job insulating it; they fitted battens on the outside walls with Kingspan in between then a layer of plasterboard backed with polystyrene and they took down the ceiling and put in thick slabs of Kingspan between the joists. We took the opportunity to fit a Velux skylight in the slope. The room has gone from being cold to amazingly warm and quiet; when my son cranks up his computer and plays Grand Theft Auto the 750 watts output from his cooling fan makes the room pretty toasty. An amazing transformation that has made a miserable room into a superb teenage den. Not cheap though; I think it cost the thick end of £2000.
 
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For insulation, it is easier to pack mineral wool tightly between joists

And in doing so squeeze the air out and significantly reduce the insulation properties of the product!
 
wrong

It is still at full thickness, the width is slightly reduced.
 
So not tightly packed as you originally posted?

Compressing mineral wool removes air and reduces it's insulation properties, it doesn't matter if it compressed laterally or vertically it has the same effect.
 

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