Today I had a chance to see how fast the house cools down. Yesterday evening at 8pm the furnace turned off after heating the house to 19.5*C, after that it switched to night mode until 8am. In the morning it turned out that there won't be anyone in the house for most of the day, I'll just be coming and going a few times, so no use keeping the house warm. Set the thermostat to night temperature for the whole day. Wrote down the temperature in the dining room whenever I was back for a minute. At 4pm, after 20 hours, the indoor temperature had fallen to 17.5*C. The outdoor temperature was around 7-8*C the whole time, no wind, and very humid (light rain). I should add that the weather was exactly the same for the last 2-3 days. (This is important, as I've noticed that the thermal inertia of the house can take 2-4 days to adjust to weather changes.)
Here are the details:
| Hour | Time | Indoor temp | Outdoor temp |
| 0 | 8:00pm | 19.5 | 7 |
| 11 | 7:00am | 18 | 7 |
| 12.5 | 8:30am | 18 | 7 |
| 15 | 11:00am | 17.75 | 7.5 |
| 20 | 4:00pm | 17.5 | 8.5 |
So, with this data, can I get a value on how efficient my house's insulation is? Something like how coastdown testing gives you the drag coefficient of a vehicle, rolldown testing gives you its rolling resistance, etc.? Or would I need more data, like recording the temperature every 1-2h, repeating it 2-3 times on days with similar weather? Or maybe the 'cooldown' test should be repeated in different outdoor temps: 5*C, 0*C, -10*C?
This data is a good baseline for any large insulation projects. After any insulating work the cooldown test should be repeated in similar weather conditions and this should quantify the improvement. Just looking at your heating bills doesn't work that well, since each winter is different. Only if the project improved insulation by 10%-20% would it be noticeable compared to a multiyear average.