Is floor insulation really necessary?

DrStriver · 13-03-2012 12:02AM #1

Hi all,
Can someone critique this calculation for me?

I have a house built in 1990, being renovated.

Floor is uninsulated, concrete.

I have an opportunity to lay 25mm of Kingspan inbetween battens beneath new oak floors.

So, the calculation.

perimeter = 35m, area = 75m. This ends up giving the floor a u-value of 0.72.

Putting 25mm Kingspan on gives it a new u value of 0.42.

A difference of 0.3.

Ground temp = 11C. Internal Temp = 23C.

75 (area) by 0.3 (u value saving) by 12 (temp diff) = 270 watts

Assuming maintaining 23C for 12 hours a day (maybe excessive).

270 by 12 = 3.24kW per day.

Assuming need to heat to maintain this level for 200 days a year.

200 by 3.24kW = 648kW

Gas heating, 6c approx a kW.

638 * 0.06 = €38.88 saving per annum.

€1000 worth of insulation and an extra week of hassle for a saving of €40 a year?

I'm thinking no thanks.

I'm I missing anything?

DrS

muffler · 13-03-2012 12:16AM

DrStriver wrote: »

I'm I missing anything?

You are, yes. See page 7 of TGD L - 2011 Building Regulations

DrStriver · 13-03-2012 09:30AM

muffler wrote: »

You are, yes. See page 7 of TGD L - 2011 Building Regulations

That link wont open for me, can you be specific as to what I am missing?

My main point is, as I put down floors, and I am limited to 25mm thick of insulation, is it really cost effective and worth the hassle? Bear in mind the amount of skirting boards that have to come off, doors have to be trimmed, problems with thresholds at front and back doors, first stair step will be odd etc etc.

Ripping up the floors entirely is not going to happen. Astronomical cost.

muffler · 13-03-2012 10:40AM

DrStriver wrote: »

That link wont open for me

Its a downloadable pdf file and could take a couple of minutes depending on your connection speed.

Alternatively click here and scroll about 2/3 down the page and under the sub heading "Part L: Conservation of Fuel and Energy: Dwellings" click on "Part L - Conservation of Fuel and Energy - Dwellings (2011) (pdf, 985kb)"

DrStriver · 13-03-2012 11:18AM

Hi,
Ok, thats a big document about new building regs, I dont really understand your point, you have stated I have missed something, can you be more specific about what I have missed, is my calculation wrong? I could do with a more explicit answer rather than a vague referral to a large document.

My analysis is purely fiscal, I am not considering legal or environment concerns at this point.

DrS

sydthebeat · 13-03-2012 11:43AM

im not sure what muffler is referring to as:

1 you are not replacing the floor and
2 you are meeting minimum u value regardless.

why did you take a ground temp of 11 c ?
in cold winters days ground temps may only be 1 or 2 degrees higher than ambient....

also, you should include a conservative energy unit cost increase of 7% on gas prices, current delivered gas prices are 6.7 cent per kwhr

so even using your temp diffs....
a 20 year cost of not doing this is €1779
a 50 year cost is €17,649

using a more realistic temp difference of say 17 deg = 918 kwhr / yr
a 20 year cost of not doing this is €2521
a 50 year cost is €25,001

muffler · 13-03-2012 11:47AM

I may have slightly misread your "am I missing anything" comment. I was linking to details of requirements in relation to works being carried out to an existing building.

sinnerboy · 13-03-2012 12:09PM

to add to syd's post -
also factor in 10 % on top as no heating appliance can convert fuel to heat at 100% efficiency and
factor in then a 10% loss for storage and distribution of space heating water

Is floor insulation really necessary?

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