Just got back from a very busy Scottish Homebuilding & Renovating Show at the SECC. One of the recurring questions was about glass extensions and how to meet the building standards (as the building regulations are called in Scotland.)

Several frustrated readers who want to build extensions using large areas of glazing are being told by their architects that it cannot be done because the total glazed area of an extension cannot exceed 25% of the new floor area.

This is true if the ‘elemental’ method of demonstrating compliance with the standards for energy efficiency are used, but there are two alternative methods that allow more flexibility.

The “Carbon Index’ method allows the relative inefficiency of the glazed area to be offset against improvements in energy efficiency elsewhere in the building, such as additional insulation, high efficiency double glazing, a more fuel efficient boilers and renewables. As long as you can demonstrate that the CO2 output (carbon footprint) of the house will be equal to or lower than it would have been had the 25% max glazed area been followed, it will comply. This is how frameless glass extensions are able to comply with the standards for energy efficiency. You can read more about his in my feature in the current June issue of HB&R on Contemporary Design Features – extract below.

How do you include a large glazed area and meet the building regulations?
The problem: Many contemporary style homes and extensions incorporate large areas of glazing, but even the best glazing has a relatively poor insulative value making it difficult to meet the building regulations requirement, which on the elemental method allows a maximum glazed area of 25%, including doors, windows and rooflights.

Solution: Use The Carbon Index Method: The Carbon Index Method is an alternative way of demonstrating compliance with the building regulations Part L – Heating. Instead of meeting minimum standards for each element, e.g. roof, walls, floors etc. the building’s performance is related to the CO2 release associated with heating and using the building. The requirements are met where the Carbon Index (CI) for the dwelling is not less than 8.0. The CI method gives homebuilders/designers greater flexibility to include larger areas of glass or other alternative materials, providing the poorer energy performance is traded off elsewhere in the building.

Upgrade Energy Efficiency Elsewhere: Reducing CO2 output across the remainder of the building will work as a trade off (although it will be harder again in 2016 if the zero carbon target is maintained). Options include improving insulation levels, renewables such as solar panels and photovoltaics, heat pumps and bio mass boilers/generators, and mechanical whole house ventilation with heat recovery.

Use More Energy Efficient Glazing: Low emissivity glazing is now standard with a U-value for a 20mm argon filled unit (Optifloat, argon, Pilkinton K) achieving 1.5 W/m2K. Using triple glazing will allow a larger glazed area: Pilkington energiKare™ Triple, has a U-value of 0.7 W/m2K.

Useful Contacts
Pilkington Glass: www.pilkington.com
Saint Gobain: www.saint-gobain-glass.com

  • Dave Cornett

    Interestingly we’ve recently had such an issue with a new Orangery Extension on a 1900’s detached house. Whilst we did only just exceed the 25% rule we had to provide SAP calcs and some upgrading of the existing envelope to make it comply with the regulations.

    Hope this helps.

    David Cornett
    SNOW ltd
    RIBA Chartered Architects providing fresh, crisp and clear design solutions for your project.
    Blackburne House, Hope Street, Liverpool, L8 7PE
    tel 0151 703 0500 | http://www.snowltd.com

  • Michael Mongan

    Extensions with more than 25% glazing need a sensible Approved Inspector who understands the Building regulations.
    A sap calculation can often prove that the co2 emissions from a dwelling with a highly glazed extension are better than a notional "compliant" dwelling. BUT there are often simpler ways. We often use an area weighted U value calculation to do the same job in less extreme situations.

    Michael Mongan (@capcake)
    Assure Building Control.
    Approval Matters.


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