Hi, long-term lurker, recent poster. I've just finished building a solar passive house and I've been collecting temperature data through this Canberra summer. I've got some questions about slab temperatures.
It's a raft slab on ground with edge insulation. Approx 150sqm with hydronic tubing throughout. Wide eaves means no direct sunlight on the slab except 3sqm near the laundry between 5-8pm. R2 walls, R5 roof, low-e double glazing. Without active cooling the slab temperature peaks at 27 degrees around 9pm, and cools down to 24 degrees by 9am. The inside air temperature varies from 22 degrees to 30 degrees over the same cycle.
This isn't what I expected from the books I'd read. I thought the thermal coupling with the ground meant the slab would average much lower, less than 20 degrees. The house gets uncomfortably warm in the evenings. I think the higher starting temperature of the slab is contributing to that.
I'm using cross-breezes and night-purges but the slab is stubbornly staying warm. Does anybody in Canberra have a similar slab and data they could share? I'd like to know if 24 - 27 is the expected temperature range for January. It could just be I have the wrong expectation.