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PART L, JOIST-END LEAKAGE and THE CAPIT SOLUTION

Part L of the Building Regulations

Part L of the current Building Regulations and associated Approved Document L1A place tighter limits than before on the thermal insulation and air-tightness of new dwellings.

Limits are placed on:

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the thermal conductivity ("U-values") of elements from which the dwelling is built,

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the average U-values of the building’s walls, floor, roof, and door and window openings,

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the measured air-tightness of the building envelope, and

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the CO2 emission rate from the building.

Once built, dwellings need to be pressure tested to determine their air-tightness. If the measured value is outside the limit, remedial measures need to be taken.

The CO2 emission rate is calculated to provide an improvement on the CO2 emission rate of a notional dwelling having the same size and shape and constructed according to particular reference values that were current in 2005. Currently, the required improvement is 20%. In time, a greater improvement will be required; the Government’s aim is to achieve a reduction in CO2 emissions of 60% by the year 2050.

The calculation of the CO2 emission rate depends on the air-tightness of the building envelope. During design, a value for the air-tightness can be assumed. However, once built, the CO2 emission rate needs to be recalculated to take account of the actual air-tightness measured by the pressure test and also any changes in design made during construction. If the CO2 emission rate, as constructed, is outside the limit, then again remedial measures need to be taken

During a transitional period until October 2007, the remedial measures merely needed to improve the situation in prescribed ways. However, since November 2007, it has been necessary to remedy the situation completely.

In most cases, remedial work will be difficult, expensive and time consuming. It is therefore imperative, during the design and construction of a dwelling, to take all possible steps to reduce air leakage.

In summary, a leaky dwelling may fail to comply (a) simply because it is not sufficiently air-tight, or (b) because the CO2 emission rate as a result of the leakage is too high. Reducing air leakage can therefore assist with regulation compliance on two fronts.

 

Joist End Leakage

Joist ends can either be built into the supporting wall or be hung on joist hangers. Due to the risk of floor collapse during construction (see for example HSE warning, 29 January 2007 and prosecution case nos. 4037242, 4011144, 4022849, 2012395, 2016853), it is preferable to build joist ends into the supporting wall, rather than hang them on joist hangers. However, a joist end built into the internal leaf of an external wall can result in significant air leakage due to:

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an insufficient initial seal between the joist end and surrounding mortar,

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shrinkage or splitting of the joist,

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shrinkage of the mortar and

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flexing of the joist end in the wall with varying loads on the floor.

To reduce the leakage, Accredited Details MCI-IF-02 (version 1.0) (page 5) states that "mortar joints around built-in joists should be recessed or struck and carefully pointed with flexible sealant." Air leakage tests carried out by CERAM show that the air leakage past a joist end sealed in this manner is 0.99 m3/hour (at 50 Pa). It will be appreciated that the application of flexible sealant entails an additional time-consuming step in the construction process, after the wall has been built around the joist ends and before the floor is laid on the joists.

The Accredited Details go on to say that, alternatively, joists may be fitted with proprietary shoes as they are installed. A few proprietary joist shoes, including CAPIT, are now available on the market.

Which Joist Seal?

In order to achieve low air leakage, a joist shoe needs to:

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facilitate a good initial seal between the joist shoe and the surrounding mortar, and

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maintain that seal despite:

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shrinkage or splitting of the joist end,

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shrinkage of the mortar, and

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flexing of the joist end in the wall with varying loads on the floor.

There is only one joist shoe on the market that satisfies all of these requirements, the CAPIT joist seal A from Capit Building Products Limited.

The CAPIT joist seal is a plastics box with an open bottom B which is simply slipped over the end of the joist c at any time before the wall is built up around the joist end, and which has a 10 mm flange d around its sides and top which becomes embedded in the mortar part-way through the thickness of the wall as the wall is built up.

Unlike other joist seals, the CAPIT joist seal does not have any large exposed flanges efg that impede the proper tooling-in H of the mortar around the joist and joist seal. Therefore a good seal can initially be achieved without the use of any additional flimsy foam rubber sealing J or flexible sealant.

Unlike other joist seals, the CAPIT joist seal’s small flange d is embedded in the mortar and maintains the seal despite any shrinkage of the mortar.

 

How effective is CAPIT?

Tests carried out by CERAM showed that the air leakage past a joist end sealed with a CAPIT joist seal was 0.37 m3/hour (at 50 Pa). That amounts to a reduction of 63% compared the figure of 0.99 m3/hour for a joist sealed with flexible sealant and is 32% better than any of the published leakage figures known to us for the other proprietary joist seals.

For a typical dwelling having, say, fifty joist ends built into external walls, the total reduction in air leakage by using CAPIT rather than flexible sealant therefore amounts to over 30 m3/hour (at 50 Pa).

That reduction in air leakage enables a looser tolerance to be employed for other leaky areas of the envelope, and could make the difference between compliance with the regulations, or non-compliance and expensive remedial work.