Thermal Performance
The Government is committed to the reduction of greenhouse gases as a result of the Kyoto agreement on Climate Change. The Building Regulations for England, Wales and Scotland, which cover Conservation of fuel and power, have been updated accordingly. The revised Regulations require buildings to:
Have more insulation in the building envelope.
To limit heat loss from pipes and ducts.
To provide more energy efficient lighting, heating, cooling and ventilation systems.
In the drive for energy efficiency the revised Regulations set minimum acceptable levels for natural daylighting and refer to CIBSE LG10 for additional guidance. That publication explains the value of natural daylight on human performance and thus on energy efficiency in its widest sense. Widespread research links natural daylighting to tangible work place benefits: improved retail sales, lower staff absenteeism, faster hospital recovery rates, and improved school exam results.
Natural lighting should be provided in all buildings. Windows can provide daylight to areas within 6 metres of a window, but rooflights are the only practical means of introducing daylight to any wider buildings.
The Building Regulations – Part L Click here for copies
The Building Regulations 2000 (England and Wales) Approved Document L (2002 edition) came into force on 1st April 2002 in England and Wales, in two parts:
L1: Conservation of fuel and power in dwellings.
L2: Conservation of fuel and power in buildings other than dwellings.
The above documents are referred to simply as Part L in this document.
The Building Standards (Scotland) Regulations 2001 Part J: Conservation of fuel and power, which has similar requirements, came into force in Scotland on 4th March 2002. In this document all general comments and any specific reference to Part L will equally apply to Part J unless indicated otherwise. Paragraph references are specific to Part L unless otherwise stated.
Daylighting
Workplace (Health Safety and Welfare) Regulations 1992 state, “Every workplace shall have suitable and sufficient lighting which shall, so far as is reasonably practicable, be by natural light”. These comments are restated in HSG 38 – Lighting at Work.
The most effective method of providing even, consistent daylight particularly in large buildings, is via rooflights. They are up to three times more efficient than windows of similar area. Diffusing materials should be used wherever possible to provide even light distribution and avoid glare. Wall glazing is less effective and can create internal shadows and dark corners. However it does offer good psychological benefits and ‘mood’ lighting must not be ignored.
The existing regulatory requirements are now reflected in the revisions to Part L. Para 1.14 states that “special care needs to be given to confirm that levels of daylight are adequate” and Para 1.55 states “where it is practical, the aim of lighting control should be to encourage the maximum use of daylight and to avoid unnecessary artificial lighting during the time when spaces are unoccupied”.
Daylight Levels
It is clear that there is a regulatory requirement for natural daylight, but Part L does not include definitive guidance on how to determine adequate daylight levels. It does define minimum glazing levels; para 1.45 says:
“… a daylit space is defined as any space within 6m of a window wall provided that the glazing area is at least 20%… Alternatively it can be roof-lit, with a glazing area at least 10% of the floor area… "
This means that for any building space which is more than 6m from a window, roof lights should be provided to a minimum of 10% of the floor area. Greater areas may be required in many applications.
General Design
There are three alternative methods in the Regulations to demonstrate compliance with the requirement for the conservation of fuel and power.
1. Elemental Method
This method considers the performance of each element of the building envelope individually. To comply, a minimum level of thermal performance should be achieved in each of the elements. This is stated as U-value; a lower U-value indicates less heat transfer per square metre, i.e. better insulation.
The simplest method for the designer to demonstrate compliance is to ensure that the U-value of all exposed elements meets the minimum level of thermal performance required and does not exceed the maximum allowable area for any element.
Some flexibility is also provided for trading off between elements of the construction. For example, a designer may choose to use less than the permitted maximum rooflight area (provided it can be demonstrated that daylight levels will remain adequate) which will result in a heat credit which can be used to justify use of less well insulated rooflights, or traded off against performance of other elements such as doors, windows or thermal bridges.
Compliance would be achieved providing that the overall heat loss from the proposed building does not exceed that from a notional building of the same size and shape.
2. The Whole Building Method
This considers the performance of the whole building, and applies to offices, schools and hospitals but not to industrial or storage buildings. Environmental design guidance issued from other authorities is also referenced. Performance of all environmental systems is considered – including heating, lighting, air conditioning and ventilation.
For example, for schools DfEE Building Bulletin 87 is the referenced source. The bulletin provides a holistic approach to school design encompassing acoustic, lighting, ventilation, heating, and thermal performance standards. An energy rating method is defined.
Building Bulletin 87 refers back to Part L for minimum acceptable rooflight U-values and glazing areas, so rooflight limits as defined in the elemental method above still apply. As in Part L the recommendation for daylight provision also applies: “Priority should be given to daylight as the main source of light in working areas, except in special circumstances. Wherever possible a daylight space should have an average daylight factor of 4-5%”.
3. Carbon Emissions Calculated Method
This method also considers the whole building performance including building services. To comply the annual carbon emissions from the building should be no greater than the notional building that meets the compliance criteria of the Elemental Method. A variety of software modelling tools are available – since these are complicated it will be normal for most industrial buildings to be designed to the Elemental Method. In Scotland the calculations are done by the Heat Loss Method.
Elemental Method
The simplest method for a designer to demonstrate that rooflights comply with Part L is to ensure that:
1. The rooflights achieve a U-value of 2.2 W/m2K or lower.
2. The rooflight area on the building is no more than 20% of the roof area.
However these values may be varied by the designer using:
3. Heat credit trade off calculations.
Other requirements relate to:
4. Air leakage.
5. Solar overheating.
6. Thermal bridging.
Lonsdale ThermGard & SpanGard systems will normally meet the standard U-value requirement of 2.2 W/m2K when incorporating low-e double glazed units. See Quikspecs for typical examples.
1. Standard U-values
To show compliance the building envelope has to provide certain minimum levels of insulation. Part L defines standard U-values for each constructional element as given in Table D.
Table D: Standard U-values of Construction Elements
| Exposed Element |
|
U-Value (W/m2K) |
|
|
|
Flat Roof, Pitched Roof below 10°
All roofs with integral insulation
(composite or site assembled) |
|
0.25 |
| |
|
|
| Walls |
|
0.35 |
|
|
|
| Walls Part J Scotland |
|
0.30 |
|
|
|
| Floors |
|
0.25 |
|
|
|
| Rooflights* |
|
2.20 |
|
|
|
| Windows translucent wall areas |
|
2.00 |
|
|
|
* the standard applies only to the performance of the unit excluding any upstands. Reasonable provision would be to insulate any upstand or otherwise isolate it from the internal environment.
U-values should be determined by physical test or
by finite element analysis in accordance with BS EN ISO 10211 Part1: 1996 as specified in L2 Para 0.15.
2. Maximum Glazed Areas
To show compliance the total area of windows, doors and rooflights should not exceed the values given in Table E, unless compensated for in some other way.
Table E: Maximum Areas of Openings Unless Compensating Measures are Taken
| |
|
|
|
|
Building Type |
|
Windows*
Doors as % area of wall |
|
Rooflights as % area of roof |
| |
|
|
|
|
Residential Buildings |
|
30 |
|
20** |
| |
|
|
|
|
Places of assembly, offices and shops |
|
40 |
|
20** |
| |
|
|
|
|
Industrial and storage buildings |
|
15 |
|
20** |
| |
|
|
|
|
* For the purposes of this calculation dormer windows in a roof may be included in the rooflight area.
** L2 Para 1.14 states that if the rooflight area is to be reduced below 20% of the roof, the designer must ensure that the natural daylight levels are sufficient for the building purpose
Rooflights that achieve a U-value of 2.2 W/m2K, at areas up to 20% of the roof, thus fully meet the requirements of the new Regulations.
3. Trade Off
Area of rooflights, doors and windows, and their respective U-values, can be varied from standard values given in Tables D and E so long as total heat loss meets allowable limits. A notional building is used to demonstrate that a design meets the energy conservation objectives of Part L whilst allowing these elements to deviate from standard allowable U-values or areas.
A notional building is a theoretical building of the same size as the building under design, with maximum areas of rooflights, doors and windows and with each element meeting its standard U-value. To achieve compliance, the total heat loss from this notional building must not be exceeded by the building being designed.
Care should always be taken that rooflight area is sufficient to provide adequate daylight, but this may be less than the maximum permitted area of 20%. If rooflights with a U-value of 2.2W/m2K are fitted to a reduced area, there will be a heat credit, which can be traded off against the performance of other elements such as doors, windows and thermal bridges.
This heat credit can also be used to compensate for the use of less well insulated rooflights, and Table F shows the maximum rooflight U-value at different areas, if the entire heat credit is traded against rooflight U-value. However, at Lonsdale we encourage the use of rooflights with a u-value not worse than 2.2w/m2.k
Table F: Maximum Rooflight U-values per Roof Area
Rooflight area (% of roof) |
|
Rooflight U-Value (W/m2K) |
|
|
|
30% |
|
1.5 |
|
|
|
20%* |
|
2.2 |
|
|
|
15% |
|
2.8 |
|
|
|
12% |
|
3.5 |
|
|
|
10% |
|
4.1 |
|
|
|
* recommended
Constraints to Trade Off
L2 para 1.14 states that care must be taken to confirm that levels of daylight are adequate. If the rooflight area is to be reduced below 20% the designer must ensure that natural daylight levels are sufficient for the building purpose.
L2 Para 1.16(c) states that no more than half of the allowable rooflight area can be converted into increased areas of windows (vertical) and doors. There is no provision for converting vertical openings into increased rooflight areas in the roof.
L2 Para 1.16(b) states that if the area of rooflights is less than the values shown in Table E the respective U-values of roof, wall and floor cannot exceed the appropriate values given in Table D by more than 0.02 W/m2K. Thus however much notional heat loss saving is made by reducing the rooflight area the U-value of the insulated roof cannot exceed 0.27W/m2K.
4. Air Leakage
Air leakage requirements do not apply to Building Standards (Scotland): Part J and for Part L1 – Dwellings – only reasonable provision is required.
Under Part L2 buildings should be reasonably airtight to avoid unnecessary space heating and cooling demand. There is a requirement that the permeability of the envelope (which includes the total area of the perimeter walls, roofs and ground floor area) should be no worse than 10 m3/h/m2 at an applied pressure of 50 Pascals. All buildings that exceed 1000m2 of gross floor area must be air tested on building completion to show compliance.
Buildings of less than 1000m2 gross floor area will be deemed compliant providing evidence that appropriate design detail and construction techniques have been used.
Provided that our systems are correctly fixed and sealed, Lonsdale products will usually comply easily with this requirement. We recommend that installers take care in the fixing and sealing of the rooflight where it abuts the building in accordance with our recommendations since leakage through the flashings is a vulnerable area and subsequent air test failure, will generally require extensive remedial work which could prove to be expensive.
5. Solar Overheating
Solar Overheating legislation is not included in the Building Standards (Scotland): Part J.
There is an important distinction between solar overheating and solar gain. Most windows and rooflights are likely to generate solar gain under normal daylight conditions. Solar gain is beneficial in that it reduces heating requirement during daytime hours when buildings are usually occupied. In some cases, such as use of unheated atria, solar gain may play a major role in energy efficiency strategies by reducing the effective exposed wall area of the building and offering a buffer zone between external and internal climates.
However improperly designed glazing may result in solar overheating and the workplace environment becomes unpleasant.
Buildings should be constructed so that occupied spaces are not likely to overheat when subject to a moderate level of internal heat gain, and so that excessive cooling plant is not required to maintain the required conditions.
There are various ways of achieving this:
Appropriate specification of glazing performance.
Incorporation of passive measures such as sun-shading.
Mechanical ventilation without excessive use of cooling plant.
Use of exposed thermal capacity combined with night ventilation.
By calculation ref Part L para 1.23.
If none of the above are suitable alternatives then:
By limiting the area of glazing facing only one orientation to the opening areas shown in Table G.
Table G: Maximum Allowable Area of Glazing
Orientation of opening |
|
Maximum allowable area of opening (%) |
| |
|
|
N |
|
50 |
| |
|
|
NE/NW/S |
|
40 |
| |
|
|
E/SE/W/SW |
|
32 |
| |
|
|
Horizontal |
|
12 |
| |
|
|
It is generally accepted that Horizontal will be applied to all roofs below 75º pitch.
Thus in England & Wales compliance is only demonstrated if at least one of the above six requirements to minimise solar overheating is adopted. If rooflight area at 12% is the chosen method to achieve compliance, then the designer needs to ensure that the daylight levels are adequate for the purposes of the building. If they are not, then the designer could chose to obtain compliance by adopting the Calculation Method (Part L para 1.23).
6. Avoidance of Solar Overheating by Calculation Method
Part L Regulations state that where the rooflight area is less than 12%, solar overheating will not be a problem and no further action is required. Wherever higher rooflight areas are specified, it must be shown that solar overheating will not occur by calculation or adoption of alternative measures.
Independent research carried out by the Institute of Energy and Sustainable Development, De Montfort University, has been submitted to the Building Regulation Advisory Council and being considered for inclusion in future revisions to the Building Regulations, has predicted the levels of solar overheating which will occur inside typical large span buildings using the latest computer modelling techniques.
The Part L Regulations are met if the overall internal gain does not exceed 40W/m2; they assume an internal gain of 15W/m2; thus allowing a maximum solar load of 25W/m2; but the research demonstrates this assumption does not apply to many large span buildings, depending on the building use.
For typical activities in large span buildings, the heat emitted per person (male) ranges from 140W (seated light work) to 256W (medium bench work). Standing, light work or walking produces about 160W of heat.
Table H (extracted from the independent research) shows the maximum rooflight area which will avoid solar overheating, from various levels of internal gain.
Table H: Maximum Rooflight Area to Avoid Overheating
Internal gain (W/m2) |
|
Max rooflight area
(% of floor area) |
|
|
|
0 |
|
23 |
|
|
|
5 |
|
20 |
|
|
|
10 |
|
17 |
|
|
|
15 |
|
14 |
|
|
|
20 |
|
11 |
|
|
|
This table shows that where internal gains are 15W/m2, rooflight area can be up to 14% of floor area without risk of causing solar overheating; where internal gains are lower then rooflight area can be higher.
For example, in storage buildings, occupant densities are generally very low and can often be ignored; the main gains are from artificial lighting, typically only 5W/m2. It can be seen from Table H that rooflight areas up to 20% will not cause solar overheating.
Any large plant or process facility may produce considerable local heat gains. Where these are envisaged, it is recommended that localised heat extraction/removal and/or cooling is used to prevent overheating. Where these are known to be effective in eliminating the localised heat gain, the sources can be excluded from the internal heat gains for the assessment of overheating.
In retail outlets occupant density can be significant (typically around 4W/m2), and retail outlets are usually well lit, with internal gains due to lighting around 15-20W/m2. However, the period of highest solar gain is simultaneous with highest daylight illuminance, and provided rooflight area is sufficient, the internal gains due to electric lighting can be greatly reduced or eliminated by switching off the lights either manually or more reliably, by daylight-linked controls. Total internal gains may therefore be around 4W/m2 and Table H again shows that rooflight areas up to 20% will not cause solar overheating.
7. Thermal Bridging
The building fabric should be constructed so that there are no significant thermal bridges or gaps in the insulation layer within any elements, at the joints between elements and at the edges of elements such as those around rooflights or windows (para 1.9).
Part L2 refers to BRE IP17/01 and MCRMA Technical Report No 14, which details how thermal bridges should be assessed, and the limiting factors. Thermal bridges have two effects: there is a heat loss per linear metre (measured by the Y-value), and an increased risk of surface condensation due to localised cold spots (measured by the f-factor).
In practice heat loss through thermal bridges can be treated in the same way as that through any other element of the building. The maximum allowed heat loss through thermal bridges is a further 10% of the total heat loss allowed from the notional building described in 3 above.
In addition, the trade off principle can be used so any heat credits (e.g. a rooflight with a higher insulation specification) can be traded off, and thus compensate for correspondingly greater heat loss through thermal bridges in other areas of the building.
To avoid condensation risk in different building types, IP17/01 specifies that the f-factor for every detail must always be greater than a minimum permissible value, as shown in Table J.
Table I: Minimum Permissible f-factors
Types of areas |
|
Minimum f-factors |
|
|
|
Storage Buildings |
|
0.3 |
|
|
|
Office, Retail premises |
|
0.5 |
|
|
|
Sports Halls, Kitchens, Canteens, etc |
|
0.8 |
|
|
|
Swimming Pools, Laundries etc. |
|
0.9 |
|
|
|
Lonsdale patent glazing provides varying f-factors depending on the specification of the double glazed units and aluminium system. Please contact our Technical Office for advice.
Exempt Buildings
Buildings or parts of buildings with low levels of heating or unheated buildings do not require measures to limit heat transfer through the fabric of the building and are exempt from these Regulations. For such buildings single glazing is adequate.
A low-level heated building with a heating requirement no more than 25W/m2 could typically be a warehouse used for storing goods to protect them from condensation or frost.
A cold store building is one where insulation is required to a level that will be determined by operational needs.
Roof Refurbishment
The Regulations apply to both new buildings and refurbishing old buildings, however if a single component is defective and needs replacing it is exempt from the Regulations.
Thus if a roof is being stripped and replaced the new roof would need to comply with the new Standard U-values described above. However where only the rooflights are deemed to require replacement a direct replacement will be allowable.
Where the old roof is insulated and rooflights are in place but only single glazed and deemed to be defective, it would be advisable to replace the rooflights with double glazed rooflights that comply with the requirements if it is feasible to do so. |
