Why not reduce your slab size, foundation run and masonry wall area, pull all the masonry inboard by 125, build it of 140 dense conc blockwork (your engineer can prob justify that) instead of (presumably) conc in EPS formwork, put 200 EPS outboard, finished as you wish. Sounds a lot cheaper, is classic heavyweight (ish) interior, and hard plaster interior finish that you can screw shelves to, instead of tacky plasterboard. And your windows can be fitted entirely within the insulation zone, instead of uncomfortably straddling the EPS formwork/concrete zone.

So was I right - poured concrete in EPS formwork - or what is the proposed construction exactly?

Posted By: RobinBYour'e right is is EPS formwork

Sounds like standard ICF construction. Since this is usually very airtight, thermal mass of the walls is not so important as the heat tends not to escape as it does with standard sieve-like British construction. As I've said many times, the better your insulation and lower your air leakage, the less useful thermal mass is as you have to compromise the insulation to get heat inside in the first place. I'd rather put my thermal mass in the form of wooden floors and nice wood furniture - where I can see it and enjoy it, rather than using lots of nasty concrete. As for putting up shelves and such like on "lightweight" construction, if you design it right, it's not an issue (i.e. you can put strapping or blocking where you want to attach things to before you put up the drywall. Though I'm talking about timber frame here, not blob and dob. Though even with ICF, you can use strapping to attach the drywall rather than gluing it onto the internal EPS - this also gives space to route cables on the airtight and warm side of the insulation.

Paul in Montreal.

The above construction won't really have any thermal mass effects at all.

It has too much insulation in the way.

That is not to say that would couldn't build internal walls from dense blockwork to get mass that way.

But as Paul says if you have really good insulation, really good air tightness and overshading to mitigate summer overheating risk, you will be laughing.

Do you stay in your house 24/7? if not then thermal mass is even less of an issue.

If you, like most people, go out to work in the morning, you want a fast heating response in the winter, so it warms up fast in the morning, and agian (if it has cooled during the day - clever solar gains will again reduce/eliminate that) in the evening.

In the summer, overshading is the key, and when you go out in the morning shut all the curtains to keep the cool in!

Timber

Mike, I do have it thanks - and jolly useful it is too.
Looked at it a couple of days ago but couldn't really see if it showed how the ordering of the insulation and conductive elements would make a difference - i.e. where they were relative to the outside, rather than what they where. This is likely to be my failing though. I'll have another look.
thanks!

The mass is directly dependant on the weight (mass) of the floor so how is that? Or are you thinking of a thinny wood floor?

Posted By: bot de pailleA wooden floor isnt going to have any mass to it at all.

Hmm, not so sure about that. We installed a new hardwood floor in the old house recently and there was almost three tonnes of wood. That's quite a lot of thermal mass I think. And yes, it was only 2cm thick as well.

Paul in Montreal.

Or 10cm of floor screed @ 200 m3 equals 24 metric tons

heat capacity of concrete 3300 j/kg K

heat capacity of wood 1700 j/kg K

10 cm 200m2 concrete floor has Specific heat capacity of 153120 Kj

10cm 200m2 timber floor has specific heat capacity of 24514 Kj

2cm 200 m2 timber floor has specific heat capacity of 4828 Kj


My concrete floor has 32 times as much thermal heat capacity as your 2cm timber floor.

Paul, you are using specific heat capacity which is wrong, you need to use volumetric heat capcity

from wiki:
Volumetric heat capacity (VHC) describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change, but without undergoing a phase change. It is different from specific heat capacity in that the VHC depends on the volume of the material, while the specific heat is based on the mass of the material. If given a specific heat value of a substance, one can convert it to the VHC by multiplying the specific heat by the density of the substance

if you go here it explains how to calculate the heat capacity of concrete and other materials
http://www.sunspace.org/en/housecons02b_en.html

Concrete, for instance, has a specific heat capacity of 880 J/kg·K, a volumetric heat capacity of 2060 kJ/m3·K and a mass density of 2250 kg/m3

what wood are you using and what figure do you have for its mass density and volumetric heat capacity?

Every where you look you get different figures

Its very possible my calcs are wrong and Ive mixed up J and KJ, however in my final calculation I did a conversion from J to KJ to reduce the number of digits in the final sums.

Im going to run through the figures to check.

My calcs were right based on a medium density pine floor,
Ive redone the sums based on your extra heavy Cumaru floor Paul

1 m2 10cm concrete floor - weighs 232 kg SHC 3330 J/Kg K = 777200 J
1 m2 2cm Cumaru floor - weights 21 kg SHP 2390 J/Kg K = 50190 J

My 10cm concrete floor is 5 times as thick, 10 times as heavy and has 15 times the thermal capacity as your wooden floor. Again, Cumaru is very much the heavy end of the timber spectrum.

A medium density pine floor (500 kg/m3) will have 30 times less thermal capacity as my 10 cm concrete floor.

Which supports my feeling that a timber floor is not that great for therml mass, all 3 tons of it.

<blockquote><cite>Posted By: Paul in Montreal</cite><blockquote><cite>Posted By: bot de paille</cite>1 m2 10cm concrete floor - weighs 232 kg SHC 3330 J/Kg K = 777200 J</blockquote>
No!!! Concrete is 880 J/kg. Your 232kg concrete has a SHC of 880x232 = 204160J

Paul in Montreal.</blockquote>

Paul, yess!
I think you are confusing calories and joules
the number you are quoting is actualy 880 Calories/Kg K

880 calories equates to 3330 joules.

So my figures are correct.

No, it's not OK. The figures I quoted were in J/kg - http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html has 0.75 kJ/kg for regular concrete or 750J kg, not calories. Your figures are off by a factor of 4.18! Concrete doesn't have a particularly high SHC per kg.
They have 0.18 BTU/lb/deg F and 0.75 kJ/kg for concrete. I'm willing to accept 0.88kJ/kg, but not 3300 J/kg when water is only 4186 - no solid construction material comes close to water

Paul in Montreal.

<blockquote><cite>Posted By: Paul in Montreal</cite>No, it's not OK. The figures I quoted were in J/kg - http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html has 0.75 kJ/kg for regular concrete or 750J kg, not calories. Your figures are off by a factor of 4.18! Concrete doesn't have a particularly high SHC per kg.
They have 0.18 BTU/lb/deg F and 0.75 kJ/kg for concrete. I'm willing to accept 0.88kJ/kg, but not 3300 J/kg when water is only 4186 - no solid construction material comes close to water

Paul in Montreal.</blockquote>

Well Paul, this is interesting, I have found a number of references to the 3330 J/kg K figure for concrete
including Randall McMullans "Enviromental Science in Building" fifth edition,
on page 15 there is a table which lays out the SHC for various materials including water at 4190 J/kg K
followed by concrete at 3330, ice 2100, Wood 1700, Aluminium 910 etc

Bot

...these are per kg... so Balsa could easily be as thermally massive for a given mass as you would need that a larger volume of Balsa to get up to the same mass....

J