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The term "thermal mass" comes up time and time again on building related forums and discussions, yet as a parameter it has one notable feature - it does not really exist. There is no such thing as "thermal mass" and never has been. Mass is a simple physical property, in simple terms it's approximately how much a given volume of something weighs at the surface of the earth. This, in turn, depends on the density of the material. For example, here are some densities for some common building related materials, in terms of the weight at the Earth's surface for a 1m square cube of the stuff (1m2? Brick ~ 2000kg/m2 Concrete~ 2400kg/m2 Plaster and plasterboard ~ 2700kg/m2 Water ~ 1000kg/m2 Structural softwood ~ 550kg/m2 Typical hardwood ~ 700kg/m2 Granite ~ 2700kg/m2 On its own the mass of a given volume of material isn't that useful for working out how much heat it would take to either raise the temperature of the stuff, or for it to give off heat as it cools down. What we need to know is the specific heat of the material, expressed as the amount of heat energy (called sensible heat, which can be measured in Joules, J) needed to change the temperature of a certain mass (lets say 1m3 to match the data above) by 1 deg C (or more correctly a deg K, but it's the same thing for this purpose). So let's list the same materials as above, with the amount of heat energy we need to put into increase the temperature of 1kg of it by 1 deg ? Brick ~ 840 J/deg C Concrete ~ 880 J/deg C Plaster and plasterboard ~ 1080 J/deg C Water ~ 4200 J/deg C Wood ~ 1700 J/deg C (This is an average value, as the true range is dependent on variety, with a wide range, from 1200J/degC/kg to around 2300J/deg C/kg) Granite ~ 790 J/deg C So, if you want to create a house with the highest "thermal mass" (i.e. Heat capacity per unit mass, if that's a reasonable way of trying to define this unknown term), then here is a list of materials, with the highest heat capacity for 1 kg at the top, and lowest at the bottom: Water Wood Plaster or plasterboard Concrete Brick Granite You may well spot a few odd things here. The first is that you cannot build a house with water (but you can include water as a heat distribution or storage system). The second is that concrete, brick and stone aren't great materials in terms of storing heat for a given mass. Surprising, isn't it? Even more so when building professionals keep harping on about the virtues of so-called "thermal mass".

I don't know what the extent of knowledge on here is for this sort of thing (probably deep and wide) but all opinions appreciated as always... We're properly off-grid as far as electricity goes. The quote for connection was well over £20k eight years ago, the nearest juice-poles are half a mile away and coming off the back of a few years living off all of the grids on a sailing boat it made sense for us to remain un-connected - we were familiar with the demands and had got used to a low-consumption boat-based existence. Our current P.V. array is only 1kW but together with a small wind turbine it easily keeps up with our demands living in a caravan. Once we're in the house the array will increase to 4kW and I don't anticipate problems with juice abundance. The fly in the ointment is the battery bank. We currently have a 345ah, 48V bank of flooded, deep-cycle lead-acid batteries made by Crown. Despite regular equalisation and diligent maintenance we've had two fail in two years necessitating a eight hour round trip for a replacement in the first instance and in the second recent case the purchase of a new battery - £170. I'm contemplating replacing the bank with something more robust, reliable and if possible lower maintenance. I've thought about AGM, about lithium and about NiFe... I'm wondering if anyone has any opinions or even better - real world experience - of living with a bank?

...... getting myself motivated is another : sometimes. Especially today for some reason. 3 full years in and the frayed edges are starting to show. Living with my mistakes and the mistakes of others. I'm tired. Dog tired. Bone tired. And all the things that I hate about myself bubble up to the surface too readily. I stamp hard on those annoying niggles when I can, and try to take pleasure in the very real achievements. But sometimes I just bloody well can't. Today, I can't. I used to teach students how to motivate themselves (!Ha!): so, in line with what I taught, I set SMART targets (Small Achievable Realistic Time-bound) for the next day, the next few hours, the next few minutes. But I recognise from previous experience ( marathon running, fell running ) that all too quickly, I have my eyes fixed on the floor, looking at the next few paces. I'm knackered. Had enough. Tearful sometimes. Big girl. I'm in that ungrateful, self-indulgent, whining mode. And I bet I'm not alone. If you recognise my description, how did you dig yourself out of a similar hole?