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PostPosted: Sat Sep 30, 2017 1:05 pm 
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Hi, I'm wanting to extend a bedroom into the roof space, what I'm not sure of is how to insulate between the rafters, I've read you need to leave a 2 inch ventilation gap, but the current rafters are only 2 inches deep already.

The space separated from the bedroom by a plasterboard wall, so it's just a case of knocking that out, putting down floorboards and insulating the roof before covering with plasterboard...I think ;)

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Any suggestions how to approach this ?

tia


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PostPosted: Sat Sep 30, 2017 8:51 pm 
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heeelllooo and welcome ramissin :welcome: :welcome: :welcome:

if the space is for light storage you wont need to comply with building regs
but if its for habitable use then you must comply with building regs with structural strength and insulation level and many other

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PostPosted: Sat Sep 30, 2017 9:03 pm 
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Hey bigs, yes it's for habitation, I just need to know how to insulate it, I intended to fill the gaps between the rafters but as they are only 2" deep I can't do this.
Everywhere I look the advice is to insulate the gaps leaving a 2" breathing layer, nothing tells me what to do if the gap distance is as small as this.

If I cover the whole area with insulation board that's going to make thinks tricky putting plasterboard on top as generally that's fixed by screwing into the rafters afaik.


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PostPosted: Sat Sep 30, 2017 9:16 pm 
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ramissin wrote:
If I cover the whole area with insulation board that's going to make thinks tricky putting plasterboard on top as generally that's fixed by screwing into the rafters afaik.


Not really, you can get 100mm long plasterboard screws, so can fix through the insulation board into the rafters.

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PostPosted: Sun Oct 01, 2017 9:44 am 
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I'm not entirely sure the ventilation gap that is needed, 25mm or 50mm?

How do I find out if this roof is built to NHBC standards ?

According to the regulations it can be as little as 25mm:

http://blog.kingspaninsulation.co.uk/air-gaps-insulation-and-roofing-membranes-in-pitched-roofs/

http://www.nhbc.co.uk/NHBCPublications/LiteratureLibrary/Technical/filedownload,45119,en.pdf

• where the ceiling follows the slope of a
‘cold roof’ regardless of pitch or where a
cold roof has a pitch less than 15°, cross
ventilation should be provided to the
roof void equivalent to a 25mm slot
running the full length of the eaves. A
nominal 50mm clearance should be
maintained between the insulation and
the roof underlayh


What would you suggest I use for the insulation layer, and what thickness ?


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PostPosted: Sun Oct 01, 2017 10:35 am 
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ramissin wrote:
I'm not entirely sure the ventilation gap that is needed, 25mm or 50mm?

How do I find out if this roof is built to NHBC standards ?

According to the regulations it can be as little as 25mm:

http://blog.kingspaninsulation.co.uk/ai ... hed-roofs/

http://www.nhbc.co.uk/NHBCPublications/ ... 119,en.pdf

• where the ceiling follows the slope of a
‘cold roof’ regardless of pitch or where a
cold roof has a pitch less than 15°, cross
ventilation should be provided to the
roof void equivalent to a 25mm slot
running the full length of the eaves. A
nominal 50mm clearance should be
maintained between the insulation and
the roof underlayh


What would you suggest I use for the insulation layer, and what thickness ?


I've not read the links, so if they say different to your quoted section, then apologies.

Where the quote refers to 25mm, that's the size of slot/gap required for any air providing the ventilation, to access the 50 mm void between the insulation and the roof.

This can be achieved with a vented soffit or an over fascia vent.

BTW, Are you sure that those rafters are only 50mm deep? I feel they must be at least 75mm.

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PostPosted: Sun Oct 01, 2017 10:44 am 
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They are 65mm, either way it's too small a distance for between rafter insulation.

All I need to know is if I lay insulation board across the rafters how thick does it need to be ?

edit:

Ok, I'm going for 50mm Insulation board, then a layer of Double Foil Insulation and topping it off with plasterboard, it should be enough for the job in hand, and leave a sufficient ventilation gap.

...apologies for my impatience..I need this space. :wink:


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PostPosted: Sun Oct 01, 2017 2:50 pm 
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You'll need to comply with the architects drawings that are submitted to the council.
You can't just bash on and hope for the best,you'll need a vapour barrier also.
Doing it without planning means you've a storage space,when you sell.


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PostPosted: Sun Oct 01, 2017 7:14 pm 
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Architects drawings for small roof space ? I don't think so. :roll:

I'm aware I need for a vapour barrier, this is incorporated into the foil insulation.

I'm hardly bashing on, there's plenty of information on the internet and I've been looking onto doing this for weeks, unfortunately a lot of the info is sparse, contradictory and deliberately obtuse...as is the nature of "asking a builder" :wink:


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PostPosted: Sun Oct 01, 2017 7:21 pm 
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ramissin wrote:
Architects drawings for small roof space ? I don't think so. :roll:

I'm aware I need for a vapour barrier, this is incorporated into the foil insulation.

I'm hardly bashing on, there's plenty of information on the internet and I've been looking onto doing this for weeks, unfortunately a lot of the info is sparse, contradictory and deliberately obtuse...as is the nature of "asking a builder" :wink:

So you aren't submitting any drawings to the council,aren't having building control look at the work and don't plan on it getting signed off? Doubling up of ceiling joists and probably some steelwork as well. So architects/engineers spec...I do think so!


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PostPosted: Sun Oct 01, 2017 7:23 pm 
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I don't think you've looked at the photo's or read where this space is...feel free not to contribute anything else to this discussion.

edit: I see you edited your post and now realise I don't need a staircase lol.

...also I'm asking advice about the roof NOT the floor.


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PostPosted: Sun Oct 01, 2017 8:52 pm 
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ramissin wrote:
...also I'm asking advice about the roof NOT the floor.


Your *floor* is currently a ceiling: it's not been designed with the required strength to be used as a floor.

(I was going to mention it earlier, glad I didn't now). :lol:

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PostPosted: Sun Oct 01, 2017 8:59 pm 
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Ayjay,I don't know why some folk ask for advice from pros on here,clearly they seem to know everything anyway. What do I know tho?


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PostPosted: Tue Oct 03, 2017 7:17 am 
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"Pro's" lol, let me know when you see one.


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PostPosted: Tue Oct 03, 2017 8:05 am 
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https://www.homebuilding.co.uk/loft-con ... ers-guide/

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