Overheating Frequency Target

[Anonymous]

PHPP calculates an overheating frequency for a property which appears to be defined as the proportion of hours during the year that the property exceeds 25C (default temperature).

It further states that houses with an overheating frequency of more than 10% then additional measures are necessary to prevent overheating.

If I assume that all of the overheating occurs in the summer three months, then this implies overheating for 30% of the summer. If I further assume that overheating mainly happens during the day then at 10% we're into overheating (above 25C) during the day for 60% of the 3 month summer period. This doesn't sound very comfortable to me.

What do you consider is an acceptable target for overheating frequency?

[Anonymous]

For your information, CIBSE recommends a maximum figure of 1% of the occupied hours over 28 deg C throughout the year – although that is based on an office environment (naturally ventilated building). This recommendation is also picked up in Part L – however it is only a recommendation.

The recommendation for schools is less than 120 hours over 28 deg C during occupied periods between May to September. Under BREEAM for schools you get credit if you can reduce this to less than 60 hours. Again under BREEAM you need to ensure that the maximum is less than 32 deg C at any time.

Trust this provides an idea on other similar overheating criteria.

Any naturally ventilated buiding will follow the line of the external air temperature – a lightweight building will follow quite closely.

[Anonymous]

For your information, CIBSE recommends a maximum figure of 1% of the occupied hours over 28 deg C throughout the year – although that is based on an office environment (naturally ventilated building). This recommendation is also picked up in Part L – however it is only a recommendation.

The recommendation for schools is less than 120 hours over 28 deg C during occupied periods between May to September. Under BREEAM for schools you get credit if you can reduce this to less than 60 hours. Again under BREEAM you need to ensure that the maximum is less than 32 deg C at any time.

Trust this provides an idea on other similar overheating criteria.

Any naturally ventilated buiding will follow the line of the external air temperature – a lightweight building will follow quite closely.

Thanks for the info Nathan.

So 120 hours works out around 1.3%, although these are talking about 28C rather than 25C as used by PHPP, so is probably quite a bit less comfortable.

Sounds like I should be aiming for significantly less than 1% and ideally 0.1% or less @25C in PHPP.

[Mark Siddall]

Mark
I appreciate your line of thinking but I think that you're distorting things a little. East and west windows are much more likely to result in over heating during the autumn and spring as well as the summer – due to the low angle sun.
Also, 5% at 25C is the good practice for PHPP (refer to AECB Feist interview).

Mark

[Anonymous]

Thanks Mark.

I see the 5% target quoted as good practice, and maybe the overheating is more distributed through the year than I thought, particularly with large amounts of east and west facing windows. Even so, over 25C for 5% of the year still seems a very significant proportion – on average it is over an hour a day for every day of the year (looking from an extreme and unrealistic viewpoint). Personally I struggle to think that this would be comfortable.

Has anyone experience of living in a house with overheating thresholds similar to 25C for 5% of the year? I very much doubt my existing house is anything close to this (mainly beacuse it is high thermal mass and as leaky as a sieve).

[Anonymous]

Mark, sorry to go over old ground…
If the Cibse Guide A recommends 28degC for offices that equates to 31hrs of the working day/year assuming occupancy of 12hr/day 5days/week.
Whereas Passivhaus recommends 25degC in a given year. I am no Excel wizard so I cant see what assumptions it is making on occupancy and I may be missing something the German translation too, but even at 5% that looks like 438hrs of temperatures >25degC
Chris

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