Greetings,

There is an issue on applying F factor about which I'd appreciate some assistance.

The F factor in general refers to the floor slab edge insulation. Its value depends also on how the rest of the slab is further insulated or not at all (Table A6.3).

In case of unheated slab, there is a compliance value for F factor to be entered in Baseline listed in tables 5.5. For the proposed case however this F factor usually is different because almost always the designers are applying some more insulation below the floor slab.

The questions are:

1. Is the F factor the only heat transfer coefficient to be applied in the floor slab?

2. Should F factor be applied also to the interior spaces, not only the exterior spaces?

3. Should the Proposed F factor value be applied the same also for the Baseline?

4. If the floor slab has different insulation thicknesses for different distance from the exterior, should the area averaged method be applied for determining the Assembly F factor?

Thanks.

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## Jean Marais

b.i.g. Bechtold DesignBuilder Expert824 thumbs up

January 26, 2015 - 10:28 am

1) 90.1 regulates F-Factor only for "Slab-on-grade" which has a very narrow definition...check it out. Otherwise, identical to proposed.

2) F-Factor applies to the entire slab under the building and is aspect ratio, perimeter dependant...which means you need to calculate it (read it off the table) for each new building for the baseline case and the proposed case is deturmined from Table A6.3 (interpolation is allowed).

3) Both proposed and baseline use F-Factor for slab-on-grade, but are not the same. Your software should know what to do with input F-factors.

4) For most element types 2D calc methods are allowed as alternitives. F-factor is the exception. You have to deturmine the F-Factor from the table A6.3.

If my slab-on-grade does not appear in table A6.3 (including interpolation), then I consider it not regulated by standard 90.1. Then I have to model the slab identically in both models. And then I try to do it as accurately as possible.

However, when it does appear in the table, it is usually in the Fully insulated section.

## Vassil Vassilev

ManagerTermoservice

9 thumbs up

January 27, 2015 - 2:41 am

Thanks for the reply.

Something however still remains unclear.

The Slab on grade heat losses are function of F factor and perimeter of the spaces exposed to the ground. What I've been asking is how to proceed with interior spaces, which do not have such perimeter. Should they have heat losses to the ground or not and if yes (logically) how this F factor could be be applied and if it can't be applied, then should an U value for this surfaces be used. This issue is somehow not quite clear.

An answer to this question would be highly appreciated.

Thanks.

## Jean Marais

b.i.g. Bechtold DesignBuilder Expert824 thumbs up

January 27, 2015 - 3:22 am

Logically, you are absolutely correct. In fact F-Factor is an ancient concept. I spoke to Joe Huang (whitebox technologies) who was partily responsible for creating the F-factors back in the day. He is still a bit flabbergasted that we are still using F-factors. Modern modelling tools can deal with slabs properly and the whole F-factor rating needs to be updated, if it hasn't been already.

Sadly, we are bound to use the standard "as is", which in this point in time means applying it to the whole slab (including sections which are more interior than others).

However, all that being said, generally the error in heatloss via the slab is only a very small fraction considering a) the total building energy use and b) the inherrent errors in the model due to other factors which I won't get into here.

This is my interpretation and opinion...anyone can contribute to the conversation, I'm all ears.