Polystyrene Under Floor Insulation
Filed under Insulation, Under Floor Insulation.
April 29th, 2009
What cost benefits arise from the installation of underfloor insulation (polystyrene) and what savings can be achieved on your energy costs given that the rest of the house is well insulated
editor, May 11th, 2009 on 12:42 am
Hi there and thank you for your question. We need to make quite a few assumptions to come up with a figure, but I can give it a go…
The equation for working out heat loss depends on the floor area, the temperature difference from the inside (top of the floor) to the outside (underneath the floor) and the increase in the R value.
We can work on a per square metre basis and assume a temperature differential of around 10 degrees in winter (say 18 degrees inside and around 8 degrees under the floor), and heating for around 5 hours in winter evenings. An uninsulated timber floor may have an R value of around 0.3 (although this can vary considerably). This would give an energy loss of 0.033 KW per square metre each hour. For a 20 meter square lounge, this would equate to around 3.3 KWhr over the 5 hours (around 70 cents worth of electricity each winter day, or around $20 per winter month).
If the R value is increased to 1.3 (the current building code standard), using all the same assumptions as previously, the energy loss reduces to 0.008 KWhr per square metre per hour. For the 20 square metre lounge, this would equate to 0.8 KWhr over the 5 hours (around 17 cents worth of electricity each winter day, or around $5 per winter month).
This gives a saving of around $15 per winter month. However, the savings would increase substantially if bedrooms and hallways as well as the lounge are heated, if rooms are heated to a higher temperature, or heated for longer, or it is colder outside than I have assumed or the existing floor is very draughty and has effectively a lower R value.
If you are interested in more information on the probable savings in your particular situation, I could do a more detailed calculation with more information on the construction of your floor, your climate zone and your heating patterns, so please do feel free to contact me on 0800 388 588 if you are in the greater Wellington area or 04 389 3415 if you are elsewhere in New Zealand.
Kind regards,
Sarah Free (energy advisor, Wellington HEAC)