The new residential TFA rules

Posted by on Jan 7, 2013 in Guides | Comments Off

PHPP Version 7, introduced earlier this year, contains some significant changes with regard to what’s counted as Treated Floor Area (TFA).

The UK builds the smallest homes in Europe, and smaller buildings can struggle within the Passivhaus methodology. Unlike the UK convention, where the floor area is taken as everything within the external walls (including partitions and stairs), Passivhaus considers the actual rooms within the home. As the heating energy demand within PHPP is measured per m2 of TFA, a building with a small TFA will generally have proportionally greater heat loss areas (walls floor and roof) than a building with a bigger TFA.

Consequently, maximising TFA is important, as is a clear understanding of what’s included, and what isn’t. TFA, as a measure of internal space, comes from the German Verordnung zur Berechnung der Wohnfläche (Wohnflächenverordnung – WoFlV), a standard designed for assessing the space in houses and flats for sale, and not one to put on your Christmas list. As a result of its role, to help home buyers, it penalises areas which don’t fit into a strict definition of ‘usable space’. On mainland Europe, with larger homes, this may not have been too problematic, but we found that in the UK TFA measurements were having a major impact on the viability of designs.

The new TFA rules seem to have recognised some of the limitations of the Wohnflächenverordnung and that should make designers lives a bit easier. The residential changes are listed below (non-residential TFA will be covered separately)

For floor area to be counted in TFA it has to be within the thermal envelope. Most floor area is counted at 100%, with some areas being counted at 50% or 60%:

 

Counted at 100%

Living areas, bathrooms, hallways, plant rooms and store rooms/cupboards. This includes full height reveals more than 0.13m deep, the areas covered by built in furniture, and flights of stairs with 3 or less steps.

The notable changes here are that

  • plant rooms (eg cupboard with a hot water cylinder/ MVHR unit) are now included at 100%
  • stair heads and landings are now clearly included.

 

Counted at 50%

Areas with an internal height between 1m and 2m are included at 50% (in other words a 1m2 space would be entered in the PHPP model as 0.5m2). Understairs cupboards and the eaves spaces in rooms within pitched roofs fall into this category.

 

Counted at 60%

Rooms outside dwellings or in basements are counted at 60%.

The rules with regard to basements within houses are quite confusing, but essentially the way spaces are treated depends on whether there are windows. If over half the basement can be classed as a ‘living area’ (which is defined as having a window area greater than 10% of the surface area) then the TFA is counted at 100%. If less than half the basement is classed as a ‘living area’ then access storage and plant areas are counted at 60%. The diagram on page 49 of the PHPP Version 7 manual shows this quite clearly.

For multiple occupancy dwellings the big change is that access areas outside dwellings are now included at 60%. This has a significant impact on blocks of flats, where the communal areas were previously ignored with regard to TFA.

 

We would always recommend double-checking your TFA measurements, particularly for smaller schemes. An over-estimate in this area could cause you a real headache later on when it comes to certification.