Creating a 'warm' storage space in old house loft - floor vs roof insulation thickness'
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Creating a 'warm' storage space in old house loft - floor vs roof insulation thickness'

by amg » Mon Nov 06, 2023 8:10 pm

I'm a newcomer to this site and I was hoping somebody may be able to advise how best to go about insulating my loft space to create a practical 'warm' loft which can be used for the storage of items such as clothes etc.

For context, my house is an old semi-detached built in the the early 1900's which appears to still have its original slate roof with the underside of the tiles being visible from the rafters alongside a layer of crumbling 'mortar', most of which has fallen away and rendered the loft a no-go area!

Although relatively small, there is adequate space that can be utilised for storage but unfortunately the headroom is very limited, measuring a max. 1.5m between the apex of the roof and the floor joists (both constructed from 70mm thick timber sections which appears to be in reasonable condition despite their age).

To avoid extremes of temperature I understand it will be necessary insulate the roof but my query relates to how best to do apportion insulation between the floor and roof space whilst maintaining sufficient headroom that the room remains viable as a storage area.

Given the age of the house and the intended function of the loft (for storage only) I am assuming the latest (new-build) building regs wrt insulation thickness don’t apply, which gives me some freedom to play ‘tunes’ with the floor and roof insulation so that I enhance the current space but not to the point where headroom becomes so restricted that it defeats the original objective.

As it stands only the floor of the loft is insulated with mineral wool (supposedly to 270mm according to the contractors who laid it) but the roof itself uninsulated. I don’t think it’s feasible to lay a floor and maintain an air gap with the floor insulation at 270mm, so my initial thoughts were to reduce the thickness of this insulation to say 100mm (raising the height of the floor joists by 50mm to preserve an air gap beneath the floor) and to insert 40mm deep insulating boards between the rafters (positioned slightly to maintain >25mm airspace) with some additional boarding overlaid on top (thickness c.50- 60mm).

To those far more knowledgeable than myself, does this sound like a viable strategy? Are there any general rules of thumb for creating a warm space in the loft e.g. is it even necessary to insulate the floor if the roof is insulated? Can anyone recommend a better split between the floor and roof insulation (noting existing timber is 70mm thick) to optimise headroom?

I appreciate there is probably no right or wrong answer but any advice would be much appreciated! :-)

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Re: Creating a 'warm' storage space in old house loft - floor vs roof insulation thickness'

by stoneyboy » Wed Nov 08, 2023 10:01 pm

Hi amg,
Given the physical constraints you have in the loft, suggest you remove excess existing insulation and board direct on the existing joists with no air gap. Insulate as you have suggested between and under the rafters but use one of the pir boards which have low thermal conductivity.
Regards S

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