IECC Floor Insulation - Must be in contact with underside of deck

[redeyedfly]

Can anyone explain why this provision is in the code? Why would it matter if say there were a soffit framed below the slab and the insulation was on the bottom of the soffit with an air space between the insulation and the bottom of slab?

 

[redeyedfly]

 

[steveray]

So you don't end up with an "unconditioned cavity" below the floor.....

 

[redeyedfly]

What if it is conditioned? This doesn't provide an exception for that.

Also, even if it's unconditioned it's still within the thermal envelope and should stay close to the internal air temperature. I'm trying to figure out what negative result they are trying to prevent.

 

[Genduct]

So you don't end up with an "unconditioned cavity" below the floor.....

I agree, but may I expand your clear, simple observation and add that the Floor Insulation needs to be in contact with both surfaces so that insulation stops any convective air movement that would reduce the effectiveness of the insulation in Walls and Roof Raters with finish ceiling on bottom of Rafter. Joist
Of course this is not true of the air space in roof rafters where the outside air enters the eaves and leaves out the ridge vent
SO walls and floor FILL THE SPACE And roof rafters , like insulation in an attic with ventilation, LET IT BREATH

 

[redeyedfly]

I don't think there is any air barrier requirement on the interior side of insulation. A floor is opposite of your attic example (obviously) so the air between the floor and the open insulation should be relatively close to the interior temperature.

 

[steveray]

I agree, but may I expand your clear, simple observation

You just call me simple?.....I prefer terse or succinct....