Scottish Building Standards, equivalent to Building Regulations for the rest of the UK, outline the required safety and sustainability standards that must be met during conversions and new builds in Scotland.
Wherever architectural glazing is used in buildings, there are various aspects to consider to ensure compliance with Scottish Building Regulations. This ranges from the surface area of the glass in relation to the floorspace of the room, to the energy efficiency of the glass and how it contributes to the overall thermal performance of the building envelope. It is the responsibility of the homeowner to make sure that all glazing within their building project is compliant.
The Building (Scotland) Regulations 2004 lays out the required standards, with a recently updated handbook that will apply to any building work commencing from, or building warrants submitted after, the 1st March 2021. This article summarises the key points relating to glazing requirements for building works in Scotland, which can be found in Standards 3 and 6 of the updated 2020-21 technical handbook.
Conservatory = a building attached to and thermally divided from a dwelling, with translucent glazing (including frames) forming not less than either:
- 75% of its roof area and 50% of its external wall area or
- 95% of its roof area and 35% of its external wall area
Dwelling = a self-contained unit designed to accommodate a single household
Building envelope = the materials making up the foundation of the outer shell of the building, including the walls, roof, glazing and doors
TER = Target CO2 Emissions Rate. This is measured as the mass of CO2 emitted in kg per square meter of floor area per year (kgCO2/(m2year). It is calculated using SAP 2012 methodology.
DER = Dwelling CO2 Emissions Rate (kgCO2(m2 year)
Notional Dwelling = an example building specification that achieves all building requirements of L1A
Minimum glazed areas under Standard 3.16, Natural Lighting
Standard 3.16 lays out the minimum amount of glazing required as part of a habitable space. It calculates the acceptable amount of daylight required for a room to ensure the health and safety of the occupants as they go about their day-to-day activities.
This building regulation applies to living rooms and bedrooms, however kitchens and toilets are not classified as habitable rooms, so do not require a certain amount of glazing to comply with building regulations.
All build types, Part 3.16.1: every habitable room must have a minimum of one glazed opening that is equal to at least 1/15th of the floor area of the room. The glass can be located in an external wall or roof, meaning that a homeowner could choose to opt for a rooflight or glazed roof instead of incorporating a window into the design.
Conservatories, 3.16.2: conservatories are often built over the top of a glazed opening, such as a set of rear patio doors. In these cases, the area of the glass doors that link the two spaces must equate to at least 1/15th of the floor area of the original room.
Extensions, 3.16.3: In contrast to the higher amount of architectural glazing found with a conservatory, traditional extensions can restrict natural light from entering the internal living spaces. In order to prevent this, home extensions must:
- Have windows or glass roofs that equal at least 1/15th of the combined floor area of the extension and original room.
- Add a new window or rooflight to the existing room. Where this isn’t practical, the joining wall between the extension and original room should be opened up to create one single space. For this option, the opening should be a minimum of 1/10th of the total combined area of both rooms.
Thermal efficiency for Glazing under Section 6, Energy
Scottish building regulations have seen various updates to improve energy standards within building construction. CO2 emissions from heating, light and ventilation in buildings were identified as a key area for improvement, achieved by reducing the U values of the building fabric.
Using SAP: Standard 6.1 sets out recommendations for the performance of architectural glazing used in a building project. Energy performance is calculated using the government’s Standard Assessment Procedure (SAP 2012) for Energy Rating of Dwellings. This method looks at a range of parameters, meaning that new dwellings must be designed with a collective package approach with regards to energy performance.
Design flexibility within the TER: This offers a large degree of design flexibility when it comes to glazing design. Rather than setting out maximum sizes and Uw-values for glazing, Scottish building regulations state that:
“The dwelling emissions rate (DER) should be less than or equal to the target emissions rate (TER).”
As long as all building materials work together to both achieve this standard and find a balance between heat loss and solar gain, designers are free to create a bespoke building.
Uw-values for Glazing in a Notional Dwelling (New Builds)
SAP 2012 looks at the individual U-values to calculate the overall thermal performance of a building. In order to meet the TER for a dwelling, Table 6.1 from Standard 6.1 recommends the following values for architectural glazing:
|Windows, doors and rooflights||Uw = 1.4|
|Allowance for thermal bridging ||0.08 x total exposed surface area|
In comparison, Table 6.3 below stipulates the maximum, or worst performing, Uw-values for glazing on a notional new build or conversion project. Column (a) highlights the required average for the combined performance of all glass in the building, while column (b) shows the maximum Uw-value for one glass unit in isolation.
|Type of element||(a) Area-weighted average U-value (W/m2K) for all elements of the same type||(b) Individual element U-value (W/m2K)|
|Windows, doors & rooflights||1.6||3.3|
Recommendations vs requirements: the above values are recommended guidelines, meaning that designers may opt for slightly higher U-values in one building element, if this can be balanced by a lower value in another. Equally, one glazed element may have a slightly worse performance as long as the remaining glass units have a higher specification that counteracts this.
Achieving a fully glazed home design: New Builds
Buildings that want to use a much larger area of glass may find it difficult to show compliance with Standard 6.1 and 6.2. The guidelines acknowledge that for these types of houses, “innovative solutions will need to be considered” to ensure both high thermal efficiency and to avoid overheating during the summer.
Architects and designers are still free to create a luxury, fully glazed new build home, despite these recommendations. In order to show compliance with Standard 6.1 and 6.2, the building regs acknowledge that innovative systems should be considered to reduce solar gain and heat loss from using oversized structural glass elements.
This includes specifying high performance architectural glazing with fully thermally broken frames, as well as additional technical glass solutions such as solar control and low-E coatings. You can also compensate by choosing materials for the rest of the building that far exceed the recommended standard performance values, in order to still meet the TER for the home.
Glazing Requirements for a Notional Extension
Home extensions are not subject to the TER requirements from Standard 6.1. Instead, Standard 6.2.9 states that energy efficiency relies mainly on the performance of the new building fabric in isolation.
High spec architectural glazing should be used for any extension project, with better Uw-values than those of the original windows, in order to limit emissions from heating the extension.
Table 6.5 lays out two levels of Uw-values that should be met for windows, doors and rooflights in extensions, depending on the performance of the existing building and its external wall:
|Type of element||Area-weighted average U-Value (W/m2K) for all elements of the same type||(c) Individual element U-Value (W/m2K)|
|(a) Where U-Values for wall and roof of the existing dwelling are poorer than 0.7 and 0.25||(b) where parameters for column (a) do not apply|
|Windows, doors, rooflights||1.4||1.6||3.3|
Achieving a fully glazed design: extensions
Uw-value flexibility: In line with new builds, glass in extensions may have a poorer performance, as long as this is compensated by the rest of the glazing to create an acceptable area-weighted average. However, the maximum recommended U-values should not be exceeded even for individual elements, as this will raise the risk of condensation.
The 25% recommendation: Section 6.2.9 recommends that the area of windows, doors, and roof lights within an extension should be limited to 25% of the floor area of the extension, plus the area of any existing openings that are built over as part of the extension.
However, this shouldn’t deter you from designing a fully glazed extension. These guidelines are designed to make sure that the project complies with required energy efficiency standards. As long as you can demonstrate compliance with the overall U-values of all the building materials combined, you are free to create a modern, light-filled renovation project.
The compensatory approach: when designing a structural glass box extension or floor-to-ceiling rear glazed facade, you may need to compensate for additional heat loss and solar gain in other areas. This compensatory approach can be calculated in two ways:
- When the original dwelling and extension are built to the same standards, SAP data can be used to show compliance for the building as a whole.
- Alternately, U-values from the relevant column of Table 6.5 (see the handbook for all building material values) should be used to show that the overall average heat loss from the extension is not greater than that of ‘notional’ extension of the same shape and size.
Requirements for conservatories and other stand-alone buildings
As conservatories are required to have a large amount of glazing in order to promote natural light levels in the adjoining room (see key terms section) consideration must be given to thermal efficiency. Section 6.2.12 recognises that conservatories are now being used year-round, resulting in higher energy usage. As a result, they should be built using high specification materials to limit heat loss and solar gain.
Conservatories of less than 50m2 in area are considered stand-alone buildings, meaning that they are thermally separated from the dwelling. All doors, walls and windows should have U-values that equal or exceed those used in the original dwelling building, and ideally should be built to the same maximum U-values listed in Table 6.2.9 above.
Conservatories and other stand-alone buildings larger than 50m2 are subject to Standard 6.1, meaning they must be built to meet or exceed the overall TER of the building when combined with the rest of the building elements. The average area-weighted U-value for these materials should be 1.8W/m2K and a maximum individual element U-value of 3.3W/m2K. If the glazing exceeds these values, a compensatory approach must be taken.
Requirements for other types of glazing renovations
Small infills that equate to 4m2 or less should have a U-value which matches, at minimum, that of the surrounding building elements. For walls or floors this should not be worse than 0.70W/m2K, and for a roof, not worse than 0.35W/m2K.
For larger infills that are greater than 4m2, the U-value should achieve those laid out in column (b) of the Table 6.5. Another way would be to follow guidance for small infills, but compensate for the energy efficiency deficit by improving the overall U-value of other parts of the building envelope.
Glazing Replacement (notional)
Where windows, doors and rooflights are being created or replaced, they should achieve the U-value recommended in column (b) of Table 6.5. A compensating approach may also be used.
For small scale glazing replacements involving 1 or 2 new windows, the frame may be disregarded when assessing the Uw-value, provided that the centre pane U-value for each glazed unit is 1.2W/m2K or less.
Where additional windows, doors and rooflights are being created, the total glazed area (including existing) should not exceed 25% of the total dwelling floor area.
Glazing Replacement in Listed Buildings
Listed building renovations are more complex and subject to the individual condition and requirements of each project. While the aim should still be focused on achieving the recommended Uw-values previously outlined, the regulations recommend a flexible approach that takes into consideration the historical construction and character of the original building, as well as the relevant planning laws and restrictions.
Similarly, each building will need to be assessed to see if improvements can be made without damaging any existing fabric or key listed characteristics. In some cases, innovative solutions may need to be implemented to improve the energy efficiency of the building. Specialist advice specific to the building in question is recommended for these instances.
Thermal Bridging & Limiting Air Infiltration: all build types
Clauses 6.2.3 and 6.2.4 state that heat loss from thermal bridging and air infiltration should be limited where possible through the use of thermal breaks and insulation of the building fabric.
Particular attention should be paid to repeating thermal bridging within building elements and non-repeating bridging at junctions with structural openings. For building alterations involving new or modified glazing, thermal bridging around the new windows, doors and rooflights should be minimised through the use of thermal breaks.
To limit air infiltration, new builds should have a continuous barrier within the insulation envelope that limits air movement from entering:
- The inside of the dwelling
- The warm side of insulation layers
- The spaces between components of exposed building elements
Air infiltration rate is used as part of the TER calculation and it is recommended that a new build’s rate equates to or exceeds 10m3/h.m2 @ 50 Pa. On the other hand, low infiltration rates of less than 5m3/h.m2 @ 50 Pa may result in issues with internal air quality, smoke control and moisture removal, unless planned ventilation is incorporated into the design.
Air testing should be carried out for new builds to demonstrate compliance with ventilation and heat loss. However, testing is not necessary for work to existing buildings.
Escape Windows under Section 2.9.4
Certain rooms should have an escape window to provide a last resort emergency escape route. The criteria for this falls under the following points:
- Habitable basement spaces and upper storey living spaces of not more than 4.5m above ground level should have an escape window
- Inner rooms located under 4.5m above local ground level should be provided with an escape window
- If any of the above has an alternate room of escape to other rooms or circulation spaces, the addition of an escape window is not required
- Rooms above 4.5m are considered too high to be able to escape from, so escape windows for these spaces are not required
Escape windows may be located above a conservatory, but in these cases it is the homeowner’s responsibility to see that the conservatory roof is constructed to withstand the load of occupants climbing onto it, in the event of a fire.
Escape windows should have an openable area of at least 0.33m2 in total, spanning 450mm high and 450mm wide. The route through the window may be at an angle rather than straight through and the bottom of the openable area should be not more than 1100mm above the floor.
Ventilation Requirements under Section 6, Energy
Ventilation is the last aspect to consider with regards to glazing design, as the combination of poor ventilation and larger amounts of glass will result in overheating and use of excess energy to cool the space. If an air conditioning system is being considered, the building must first demonstrate that it has been designed, as far as possible, to counteract this. The guidelines laid out in Section 6.6 detail further information on ventilation design.