By continuing to use the site you agree to our Privacy & Cookies policy

Getting a grip on alpha-values

technical & practice

Just when you thought you had got the hang of Part L, along comes a requirement for more calculations

Tucked away at the bottom of page 15 of Approved Document L2 is a reference to BRE Information Paper IP 17/01, 'Assessing the effects of thermal bridging at junctions and around openings'. The BRE's Robust Details are indicative models, which, if complied with, are deemed to reduce condensation and thermal bridging to an acceptable degree.

However, for all those standard construction details which are not included in the meagre selection in the Robust Details compendium (AJ 28.2.02), you will have to prepare alpha-value (a) calculations.

Even though you may never have heard of them, a recent straw poll suggests that hardly anyone else has either. But at some point, architects will doubtless be challenged to demonstrate compliance, and we understand that many Building Control officers, with little time on their hands to check through calculation sheets, are choosing to place great emphasis on the a-value for proof of Part L compliance.

If a design complies with the elemental method of calculation outlined in Approved Document L, for example, you may find that it no longer complies when tested against the criteria set by the a-value calculation; meaning that the design or specification will have to be altered.

What's it all about, Alpha?

Thermal bridging used to be seen primarily as the occasion where a material penetrated a building element and came into physical contact with a surface of different temperature; usually steel lintels spanning across cavities, or nonthermally broken concrete floors extending through to the exterior, for example.

Now, thermal bridging is equated with a variety of non-physical contacts or differential coefficients by which the internal temperature of a building element might vary, giving rise to potential 'cold spots'. Repetitive mortar joints, for instance, reduce the overall thermal performance of the wall and could lead to localised condensation. Similarly, mineral wool laid between ceiling joists results in different insulation values at various points along the surface. These types of thermal bridges are now factored into the Uvalue calculation of the particular building element. However, where junctions in building elements occur - between a floor and an external wall, say (and where such junctions are not shown in Robust Details) - a new method of calculating the condensation risk is needed.

The a-value is the overall thermal performance of the entire building envelope, taking into account the positive effect of the U-value of all plane elements (roof, rooflights, walls, etc) and the negative 'heat draining' effect of junctions, details and interfaces, which act as direct heat conductors from the inside to the outside of the building.

Essentially, the BRE's Robust Details has been calculated against the correct a-value calculations. So, for example, the recommendation that cavity insulation be extended 75mm past the level of the floor insulation has been calculated to take account of the linear thermal transmittance (Psi) and the critical surface condensation factor (f CRsi ).Both factors assess the effects of thermal bridging. The a-value is a factor of the allowable transmission heat loss coefficient.

Calculating alpha-values?

Two a-values must be calculated, one for a notional building and one for an actual building. The objective is to establish that the a-value of the actual building is lower than the notional building.

The notional building comprises all of the plane elements which match the actual building, with two exceptions: lit has 20 per cent of the actual rooflight area; and lit has 15 per cent of the actual personnel doors/windows/curtain walling (for industrial buildings - or 40 per cent for 'places of assembly').

Once all plane element U-values have been established, the total area U-value (sigma (S) AU) must be calculated by multiplying the relevant areas for each plane element by their actual U-values.

The notional building a-value is then established by adding 10 per cent to SAU (for non-domestic buildings; 16 per cent for dwellings).

The actual building a-value is established by calculating all linear 'thermal bridges' such as ridge details, party walls, gutter details, gables, jambs, lintels, etc, and then multiplying the length by the actual thermal transmittance (Psi [. ]) values for each condition, to achieve a new value, S.L.

The actual building a-value can be calculated by means of the sum of SAU and S.L. This figure must be equal to or less than the notional building a-value to prove compliance.

Information required

Usually, the specifier will have to obtain the . -values for all details and junctions from the supplier, unless the contractor putting the design together is confident in its ability to calculate all of the appropriate details. Given the complexity of the calculation method, this is extremely unlikely. Specifiers would be best served by working closely with an experienced manufacturer who is prepared to assume responsibility for all aspects of the envelope calculation, rather than individual component areas.

To show compliance, drawings of all details must be available showing the relevant . -values and f-factors.

Where necessary, substantiation of these values should be sought. Once all these values are available, the avalue can calculated.

However, Brian Watson of CA Roofing, who is currently preparing advice for local authorities on this issue, is fairly relaxed about the new calculations. In essence, he says, Building Control will be satisfied 'if you can show them that you have made the effort'.

Purpose-designed products are beginning to appear in the market offering low . -value and high f-factors. Use of these minimises most of the risks and contributes significantly to final compliance. Watson says: 'As a rule of thumb, architects and specifiers should focus on those elements that they believe will cause the biggest problem, rather than spending inordinate amounts of time checking every single detail.

'At the moment, ' he says, 'it is a case of using common sense.' Here's hoping that Building Control and Approved Inspectors concur.

Thanks go to Brian Watson of CA Group, who brought this subject to our attention and assisted with the article.

Contact tel 01388 834242 or e-mail help@cagroup. ltd. uk

Tips for industrial buildings

Do not use gutters that have a continuous steel leg.

Whether membrane coated or not, they provide a direct thermal path from the inside to the outside of the building.A target Psi ( . ) value between 0.5 to 0.8 W/mK is realistic.Anything higher will jeopardise compliance.

A . -value of 0.00 (zero) W/mK can be achieved when the drip flashing is connected to the intermediate spacer bar.

Aim, wherever possible, to connect potentially heat conducting details to the external skin of the envelope.

Drip flashings are installed all round the building perimeter and a large metreage of a . -value in the region of 0.6 W/mK makes compliance extremely complex. A zero . -value allows the detail to be disregarded while performing the required calculation.

Parapets now present a serious problem.The higher the parapet, the bigger the . -value.

Maintaining the required thermal performance of the rooflights plays a significant role in achieving the a-value while at the same time minimising the potential risks of severe condensation within the rooflight cavity.

What Building Control looks for Here we publish a checklist of information used by Building Control officers to appraise the compliance of basic steel-framed industrial buildings:

Proof that the roof design meets the minimum U-value (0.25 W/m 2K) and that this takes into consideration any variation in the purlin span.Provide calculations or independent certification ie BBA Agrèment.

Proof that the wall design meets the minimum U-value (0.35 W/m 2K) and that this also takes into consideration any variation in the rail span.Provide calculations or independent certification ie BBA Agrèment.

Proof that the metal rooflight design meets the minimum U-value (2.2 W/m 2K) and that this takes into consideration any variation in the purlin span. Provide calculations or independent certification ie BBA Agrèment. (If the construction is only two skins of GRP expect a strong challenge to the a-value) lProof that the firewall design meets the minimum Uvalue (0.35 W/m 2K) and that this takes into consideration any variation in the rail span.Provide calculations or independent certification ie BBA Agrèment.

Certification of compliant pressure test.

Thermographic survey result or Competent Persons statement that the insulation is continuous.

Confirmation that f-factors are in line with the humidity class designation.

Credible supporting calculations for Psi ( . ) values for all details.

Proof that the a-value has been considered.

Brian Watson will be speaking at AJ's Part L conference at the RIBA on 17 October 2002.

Copies of a free CD-ROM and drawings of junction details to help design a-value-compliant industrial buildings are available from Chris Taylor at CA Roofing.

References

1.BS EN ISO 10211-1:1996, 'Building components and building elements - Thermal resistance and thermal transmittance - Part 1: General methods.

2.BS EN ISO 10211-2:2001, 'Building components and building elements - Thermal resistance and thermal transmittance - Part 2: Linear thermal methods.

Note: Part 2 is suitable for threedimensional details which can adequately be represented by the intersection of two or more twodimensional models.

Have your say

You must sign in to make a comment.

The searchable digital buildings archive with drawings from more than 1,500 projects

AJ newsletters