I'm in the process of measuring my house to do heat loss calculations throughout. I haven't done this before and a couple of things have struck me:

All the sources I've seen on this give air change figures that appear to be taken 'as standard' from I don't know where. This may be fine dealing with new build where there is design intention for specific rooms to have specific ventillation rates, but is there some way I can make a more informed estimate for the rooms in my 1880s house?

Further to this, a number of rooms on the ground floor have been knocked through at some point. This includes open plan access into a glass-roofed kitchen extension, that I'm guessing wouldn't pass regs today. Airflow is therefore hugely important in working out where all the heat is going. Rather than treat this interconnected area all as one room (which it isn't) It seems sensible to treat the regions as separate rooms. This would also allow me to model what happens if I re-partition them in various ways, as well as telling me if the existing rads are sized correctly for the current layout. But how do I work out air changes based on the size of the openings between them?

to do any serious calculations you need to get a copy of SAP 2005, free from DEFRA on 0845 120 7799 or (www.bre.uk/sap2005).
Air changes are quoted at Appendix P (assessmant of internal temperature in summer). For houses the figures quoted for 2 storey houses with cross-ventilation possible / not possible are: trickle vents only 0.2/0.1, windows open 50mm 1/0.6, windows open fully 8/5 air changes per hour.

Charlie,

Do you think SAP can be anywhere near accurate for calculating losses due to air infiltration?

Mike,

I wasn't planning to use any particular software. I was hoping to get away with doing it on a spreadsheet, looking up u-values from various sources. I'll have a look at SAP and see if that helps. I've looked at a couple of flash-based freebies like the one on www.heatweb.com, but they don't seem to do anything a spreadsheet won't and are less flexible. Any more info on free software or air movements gratefully accepted.

i can not understand why you might want to do this because in a house of that age you will not be able to meaningfully estimate what you infiltration losses (through the fabric and its gaps) are unless you have constructed a complete plastic bag around the house and had a pressure test done. what are trying to show?

Ken

The current central heating is a) burning tenners like you wouldn't believe and b) leaving parts of the house too damn cold. I'm trying to work out what's wrong; whether the radiator and boiler sizing is competely up the spout or just needs tweaking (intend to replace boiler in next couple of years anyhow as it was only 65% efficient new in '96) and whether the glass-roofed kitchen extension is a complete liability or can be heated adequately at a reasonable cost. That's all. My interest in air changes relates to air movements between 3 interlinked 'open plan' rooms, one of which is this glass extension that drops to 15 degrees when the others are at about 20. I want to work out if I'm better off increasing the heater size in this extension or giving it up as living space, calling it a conservatory, shifting the kitchen into the main house and putting a door in.

The kitchen extension floor is solid concrete (unfinished, just sealed). It's quite low, perhaps the previous owners intended putting UFH in. Looks about 5" lower than you might expect relative to doors/ windows. Not sure. We are toying with either UFH, or just insulating and adding a suspended floor on top. Another option is to remove this extension completely sometime down the line and put up a new one positioned to act as a passive solar collector. (The current one is in shade most of the winter and in full sun midday in summer).

In any case I need some way to 'make do' for the time being (perhaps 2-5 years). Hence the calculations.

Other floors are suspended wooden floors which we are going to seal. Probably with Gapseal v-strip. I'm just about to try it out in a bedroom. Ground floor void is about 3 ft deep. Have considered blowing in loose insulation but don't know whether damp may be a problem. I'm not willing to pull up all the boards to lay batts between the joists.

Windows are old and draughty but I intend to use a combination of brush strip which I'm hoping to fit this summer (and I rescued all the original sashes from a house down the street when they fitted UPVC so some of them may be in better condition - a lot of ripping out going on round here, I've just salvaged some moulded skirtings too. Not to mention the fireplaces.) and possibly secondary glazing - I'm hoping to get some quotes in the next few weeks.

Any further ideas welcome.