U value calculations for renovating flooring thermal element

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I am looking to insulate my 1969 build ground floor flat.

External / Party walls & ceilings I have a handle on what I'm doing.

When it comes to the flooring, I am wondering how much leeway my local LBA might give me in new flooring thermal elements.

I am looking at installing Spacetherm C on the floor to minimise loss of headroom (I'm loosing 70mm with doing the ceiling). I'm assuming a solid 125mm concrete base, vapour layer & 50mm screed on top (which seems to fit with what I found when I dug out some blown screed a while back)

My question is to do with P/A ratio. If I use the total floor area, and exposed perimeter I can get a U value of 0.25 (to BR spec) with 10mm Spacetherm bonded to 19mm chipboard. However, if I do the calcs on a room by room basis two of the rooms which have a larger exposed perimeter would need 20mm spacetherm (which starts to get very expensive).

Which is the correct approach ? Room by room, or can I argue whole slab ? Obviously I'm hoping that I can go towards the latter. If the former, a couple of the rooms have a calculated U value of 0.3. Would the LBA give me that much lattitude ?

Also, when looking at exposed perimeters, Do I need to include walls boundarying unheated spaces (e.g corridor between flats) in the P value ?

So far as I can tell, the internal dividing walls are all built directly on the concrete slab. The only exception being the central dividing wall, which is load bearing.

Any guidance would be appreciated

Matt
 
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You can count the whole perimeter of the flat and, if memory serves, you can also count the perimeter of an adjoining building (eg if it is a semi).
 
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RonnyRaygun / Tony1851 thanks for your responses.

Just to be clear with regards to including the perimeter of an adjoining building does that mean I can include the perimeter / area of both flats either side of me ? Or, can I even go so far as to include the whole ground floor of the 4 flats that make up the two adjoined blocks (one to the left of me, two to the right)?

Obviously the lower I can make the P/A ratio the better for my insulation thickness calculation

M.
 
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RonnyRaygun / Tony1851 thanks for your responses.

Just to be clear with regards to including the perimeter of an adjoining building does that mean I can include the perimeter / area of both flats either side of me ? Or, can I even go so far as to include the whole ground floor of the 4 flats that make up the two adjoined blocks (one to the left of me, two to the right)?

Obviously the lower I can make the P/A ratio the better for my insulation thickness calculation

M.
I believe you can include the perimeter of the whole block, the idea being that much of the floor's heat is lost through the external edges. Any edges of your flat which aren't external therefore won't count against you.
 
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