Ferdinand

Gravenhill Solar Gain Factor 0.5-0.7 in Design Code

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Just for interest, a brief extract i came across from the Design Code at Gravenhill:

 

Quote

Energy performance criteria

Most self builders want to attain a good level of energy efficiency to ensure long-term low running costs.

At Graven Hill, there are some specific – but eminently achievable – performance requirements over and above what’s laid out in the Building Regulations. These include:

 

  • Structural envelope Walls, ground floors and roofs must have a U-value of 0.15 W/m2K or less.
  • Windows Must achieve a U-value of 1.4 W/m2K or less, and a solar gain factor (g-value) of 0.50-0.70.
  • Airtightness Maximum air leakage of 3m3/hr/m2 at 50 Pascals (to be pressure-tested on site).
  • Ventilation Good-quality mechanical ventilation and heat recovery (MVHR) is required, with an efficiency of at least 70% as well as a power consumption of below 1.5 W/L/s.
  •  

The Design Code also recommends the inclusion of solar photovoltaic panels if the planned house’s roof orientation is suitable, but this is not a requirement.

https://www.self-build.co.uk/graven-hill-plot-passport-explained/

 

That line highlighted in red seems to me to be encouraging solar gain, rather than limiting it - as is more the need in well insulated houses.

 

AIUI the solar gain factor (ie amount of energy from sunlight that gets through to the inside) should be below 0.5 in a highly insulated house.

 

eg from Wikipedia

Quote

A g-value of 1.0 represents full transmittance of all solar radiation while 0.0 represents a window with no solar energy transmittance. In practice though, most g-values will range between 0.2 and 0.7, with solar control glazing having a g-valueof less than 0.5.

https://en.wikipedia.org/wiki/Solar_gain

 

However, this is not my specialism and the difference may be marginal.


Ferdinand

 

Edited by Ferdinand

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That is indeed a contradictory rule, and one that will probably lead to some houses overheating, unless additional measures are taken to reduce the solar gain in some way, I suspect. 

 

I dearly wish that I'd opted to fit glazing with a lower g value, as that would have probably removed the need to retrofit (expensive) reflective film.

 

 

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Email sent to sales@gravenhill.co.uk, which is their only obvious email.

 

Quote

Please pass on to whoever is your Design Authority for your buildings.

 

Your Design Code seems to demand a specification of windows, which in highly energy efficient houses will cause overheating through excess solar gain.

 

That is:
"Windows Must achieve a U-value of 1.4 W/m2K or less, and a solar gain factor (g-value) of 0.50-0.70."

 

Low energy houses should be in a range more like 0.2-0.5 as I understand it.

 

Example of a reference:

https://www.self-build.co.uk/graven-hill-plot-passport-explained/

 

and on page 5 of the Design Code itself.

https://www.gravenhill.co.uk/wp-content/uploads/graven-hill-design-code.pdf

 

This is being discussed on a thread at the Buildhub self-build forum, if anyone wishes to drop over.
https://forum.buildhub.org.uk/topic/9261-gravenhill-solar-gain-factor-05-07-in-design-code/

 

Given that overheating is becoming a key issue, this needs to be changed urgently.

 

Rgds etc.

 

 

F

 

(In case anyone is despairing, I am not THAT much of a nerd to know this off the top of my head.

 

I think it came up in consecutive search results in the Google snippets, while I was tracing up the spec of the chappie's Thermal Roof product from The Street programme, and then checking that against the required Specifications for Gravenhill.)

Edited by Ferdinand

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Has Graven Hill people/person answered your email about Windows solar gain.

Regards

Richard

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Depends whetehr "better" means quality of build or ease of understanding.

 

The Gravenhill one is about 5% of the size of the Building Regs, for example, and for the layperson probably does not seem to be written in Chinese.

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19 hours ago, quintain said:

Has Graven Hill people/person answered your email about Windows solar gain.

Regards

Richard

 

No.

 

I have not checked whether the update has been made.

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