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Thermal Specification Guide

George Barnsdale can produce BFRC approved thermal simulations in-house. This means thermal performance is designed into all windows.

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There has been a huge increase in the importance placed on the thermal performance of buildings in recent years, due to the environmental effects and cost increases of energy consumption. Approved Document Part L gives guidance on what is required to satisfy building regulations, but to achieve a more environmentally friendly and low cost building, many will want to go further. This guide details the important elements of windows and doors effecting thermal performance, the regulatory requirements and the methods for satisfying these requirements.

The Basics

The thermal performance of a window or door is based on the net energy gain/loss based on energy lost to the outside due to a temperature difference (conductivity, defined by a U-value) and energy gained from the sun (radiation, defined by solar gain). In the UK, the usual approach is to minimise the U-value to reduce heat loss from the building and maximise the solar gain to increase the heat gain into the building. This is demonstrated in the following diagram (Source: Saint Gobain).

Thermal Window Illustration

There are however a number of points that need to be considered with this:

  1. Many buildings use air conditioning and therefore heat gain is detrimental.
  2. Solar gain is extremely variable and therefore difficult to manage. There are many alternatives that are more predictable to make heat management simpler.
  3. Advanced coatings for high solar gains can reduce light transmittance.

Building Regulations – Document Part L

New Build

To determine the U-value required for windows and doors, the Whole Building CO2 Target Emission Rate (TER) must be determined using notional values. The Dwelling Emission Rate (DER) is then determined using actual values and must not exceed the TER. Only with these figures can the U-value for windows and doors be reached. The Building Regulations limit the maximum acceptable whole U-value to 2.2 W/m2K.

Replacement Windows & Doors

Replacement windows should be limited to a maximum whole window U-value of 2.0 W/m2K or use glass with a centre pane U-value of 1.2 W/m2K or better. Alternatively, it may have a Window Energy Rating (WER) performance of E or better. (See point 2 below).

Replacement doors will only need to meet the recommendation where they have more than 50% glass area (measured overall of frame) which must then be proved by hot-box testing, calculation or by using glass with a centre pane U-value of 1.2 W/m2K.

When replacement windows and doors are used consideration should also be given to compliance with other parts of the Building Regulations notably parts B, E, F, J and particularly part N.

Extensions

Windows in extensions must achieve a whole window U-value of 1.8 W/m2K or better or use glass with a centre pane U-value of 1.2 W/m2K or less. Alternatively for the first time, windows must have a WER performance of D or better. (See point 2 below).

Notes

  1. Some buildings, such as those in conservation areas, may have certain exemptions from some (or all) regulations in Part L. Contact your Building Control Officer for details.
  2. In Scotland (Section 6: Energy), all windows are expected to achieve a U-value of 1.8 W/m2K or better. For more information refer to www.sbsa.gov.uk.
  3. In Northern Ireland (DOE Technical Booklet F1 and F2 2006 – Conservation of Fuel and Power from 30 November 2006) U-values are equivalent to those for England & Wales. For more information refer to www.dfpni.gov.uk/br-technical-booklets.

Methods of Energy Rating

Glass Centre Pane Values

This is based on the glass manufacturers stated U-value and is based only on the type of glass and cavity gas. Whilst being the simplest of method of conforming, it is the least accurate and for this reason the strictest of the methods.

Overall Window U Value

Unlike Glass Centre Pane values, this takes into account the energy loss of the whole window, including the glass, spacer bar and window frame.
The value is calculated in accordance to EN ISO 10077-1, using a value for the window frame derived from either thermal simulation in accordance to EN ISO 10077-2 or hot box testing in UKAS approved laboratory.

The benefit of this is that it gives an accurate figure for the heat loss through the window allowing accurate calculation of the DER. It does not however take into account solar gain and air loss, which can have a significant effect on the thermal performance.

Window Energy Ratings

The British Fenestration Ratings Council (BFRC) have developed a window energy rating scheme for windows on domestic properties in the UK. Based on average temperatures and sunlight levels across the UK, the scheme calculates the energy losses and gains to give a window energy rating and a related grading between A & E (in accordance with the methods defined by the Energy Saving Trust). Whilst WER’s are the most accurate method of assessing thermal performance in theory they suffer from the following weaknesses:

  1. Based on average temperatures and sunlight levels – Due to significant variations across the country, and the direction the window is facing, the relative importance of the two factors varies and therefore, creates inaccuracies.
  2. Domestic properties only – The scheme has been developed around a formula for domestic properties and therefore, cannot be applied to commercial property.
  3. Treats all Solar Gain as good – There are many situations (see introduction) where solar gain is not positive and therefore, the WER’s are not applicable.
  4. Doors not yet rated – Doors are not as yet included in the scheme, due to the complexity of different designs.

Factors to consider when specifying

There are three fundamental elements of window & door design that affect the thermal performance:

  1. Glass – Affects the thermal loss out of the building and the solar gain into the building.
  2. Glazing Unit Spacer Bar – Affects the thermal loss through the edge of the glazing unit.
  3. Frame - Affects the thermal loss out of the building through conduction and air loss.

Glass

When specifying the glass it is important to consider the following:

  1. Coating Type – Softcoat has become common because of its balance between low U-value and high solar gain, as well as better clarity and light transmittance. Hardcoats tend to perform better for solar gain, but on its own this would very rarely be a consideration.
  2. Light Transmittance (LT) – Advanced thermal coatings can have a negative effect on the amount of light transmitted through the glass, although with modern softcaots this effect is negligable.
  3. Low Iron Glass – This has the iron content in the glass reduced resulting in more of the suns energy passing through the glass. Due to manufacturing processes the glass is more expensive, but the benefits in Solar Gain are significant.

Glazing Unit Spacer Bars

The spacer bar separates glass panes in double or triple glazed units forming a sealed gas cavity. The standard spacer bar is made from aluminium and is a very cost effective product. It suffers from being a poor insulator and thus is an extremely weak point in the glazing unit.

In the past 5 years, the introduction of Warm Edge spacer bars, which use materials with much better insulating properties have been introduced. These are more expensive than aluminium

Recently, a range of warm edge spacer bars have been developed to address the thermal bridging affect by using better insulating materials.

Window Frame

The material choice and design of the frame have a significant on the overall window U-value, the amount of air leakage and the solar gain (the width of the profile affects the proportion of glass and thus the amount of solar gain).

Whilst the amount of options with the design is endless there are a number of key principles: