U-value OR R-value of slab on grade in WUFI-Passive 3.0

[russ@goodshacks]

I'm wondering if we are supposed to enter U-value or R-value for the "U-value of slab on grade" box under Foundation Interface. When I enter larger numbers for "U-value" the performance of the building in terms of Heating demand decreases. I would expect it to be the opposite since a higher U-value would be less insulation and should result in higher heating demand. This makes me suspect we should be entering "R-Value" in this box, especially since when you hover over the box it says "Would like R-value here". Was this something that was on a wish list and got fixed but not updated on the parameter specification labelling?

[mpazold]

Hi Russ,

the input is the U-Value. But of cause, the R-value is still on the wish-list.

The decrease of the heating demand with higher U-values for this foundation interface is also correct.

The inputs on this foundation interface form are to caluclate the gound temperatures. The U-value from the floor slab component can be detected automatically for this input, but not vice versa. If you increase the U-Value only on this form, you get higher ground temperatures because the ground get more heat from the building. But the U-Value of the floor slab component, to calculate the actual heat loss to the ground is still the same!

So you just decrease the temperature difference for the floor slab, but not the actual U-value, resulting in an reduced heat loss. You also have to change the component assembly to set the same u-value... We should add a warning for this.

[russ@goodshacks]

Thanks for your explanation. That makes sense to me now.