# Full fill Kingspan K103 Insulation Condensation Risk

a suspended timber floor

most of the heat loss from a floor is by draughts through the gaps, between the boards but mostly the gaps at the edges of the room under the skirting.

Consider.

There is no loss by convection because convection does not go downwards

there is imperceptible loss by radiation because the temperature difference from the underside of the floorboards into the void is very slight

there is very little loss by conduction from the underside of the floorboards to the air layer in contact with it.

Mineral wool can easily be stuffed into the gaps to block the draughts. No precision cutting is required.

There is no loss by convection because convection does not go downwards
That's not quite true, a well ventilated floor has air blowing past it, which means you can get convection cooling. The typical convection image is cold air being heated, rising up and cooling to fall, but convection is where you have heat being transferred by moving particles so it still counts.

Just look up the Kingspan specification for suspended timber floors and follow that. It will tell you to insulate between joists. 100mm will be fine and will reach u-value. Heat loss through the joists is accounted for in the calculation. As long as you have good sub-floor ventilation, condensation is not a problem. I don't know why you mentioned breathability but it isn't a factor.

That's not quite true, a well ventilated floor has air blowing past it, which means you can get convection cooling. The typical convection image is cold air being heated, rising up and cooling to fall, but convection is where you have heat being transferred by moving particles so it still counts.

You are using a meaning for the word "convection" that is unfamiliar to me.

1. "the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat."

Regardless of theory, when a piece of underfloor pir fell into the void from between the joists under our kitchen floor, i could tell exactly where it was based on not being able to stand in that cold area with only socks on in winter. So whether it's conduction, radiation, or whatever, the insulation makes a big difference to comfort.
Building regs are more concerned about heat loss, so don't care so much about a pool of cold air sitting at floor level.

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