All,
We are getting a house built by a local builder.
The house will be timber frame on a concrete slab (insulation under and around the slab) about 80-100m2, single storey.
The architect and builder have spent a lot of time refining the design to remove cold bridging and air leakage and increase insulation.
They have offered to achieve a certain score on the HERS rating (we are in New Zealand), but the HERS is a prediction not a measurement.
We have discussed including two tests at the end of the build before they get the last ~10%. Those tests would be air leakage and insulation success (for want of a better expression) by thermal imaging.
I see the benefit of this as two fold:
-> if the builder knows there will be a test at the end he will ensure all the work is done correctly (as I'm sure he will anyway).
-> if anything relevant to thermal performance gets missed during building (or design for that matter), it will show up in the tests.

Can anyone suggest a form of words that could go into a contract?
Something like:
-> For cold bridging, "no area greater than 100 mm2 will be more than 4 deg C different from any other 100mm2 within 1 m"
-> For air leakage, "no leaks of more than 10 m3/hour per 100 m2 of floor area per 0.5 bar pressure"

-> What values are reasonable for each of those figures (above)?

-> The house should be finished around March, which is late summer here, but still hot during the day (and night sometimes). Do I need cold weather for a good test?, ie should I wait for Winter (generally cold clear nights, sunny days).

-> Are there any other moderately priced tests that I should include to ensure good thermal performance.

Cheers,
Geoff.

Yes you need cold weather for a good test (at least with the camera used on our house). It was -5C here which was ideal. I guess anything under about 5C will work, but generally the colder the better as it removes environmental effects so you can 'see' what the house is doing.

My SAP and DER worksheet has an effective airchange rate of 0.24 and my TER worksheet has it at 0.61 - is this sort of discrepancy normal or is there an error in their maths? Does TER have an option for MVHR as it is not shown on the worksheet?

50pa is that the same as q50 ?

Our house has been designed to be as airtight as possible (and still breathe!) so I'm expecting very good figures.
The assumptions used on the worksheets for the MVHR are 3 cubic metres per hour per square metre - does that sound about right?

thanks

RobinB

%0 Pa and q50 are the same I think --- 3 cubic meters per hour sounds high to me -- is this an air leakage rate or an air exchange rate? These are very different.

But when there is a thermal bridge it shows up as generally the surrounding area is of the same material.

Sorry -- it may as well be written in a foreign language -- meaningless to me ----- are you there Paul?

M3/hr/M2 @50pa is the air leakage rate measured against the surface area of the building ie: floor, ceiling and
external walls. ACH, air changes per hour is calculated using the volume of the building. It's only the first that's
used for Building Regs purposes in UK

IR
For thermographic surveys a temp difference of 10 deg C is generally required. Surveys are generally carried out at night to minimise the effect of solar radiation on surface temperatures. In addition, dry conditions are required as rain is likely to distort the surface temperature of walls and roofs. Surveys are often undertaken from both the interior and exterior depending on access and viewing angles.
I believe the relevant standard is, "BS EN 13187:1999 Qualitative detection of thermal irregularities in building envelopes"

Airtightness
Anglia is correct , m3/hr/m2 @ 50Pa is the standard measurement used for Part L compliance. The m2 refers to the building envelope, which is simply the surface area of the walls, floor, ceiling etc
However, just because a builder knows he has to hit a low target value doesn't mean he'll get there. On commercial projects it's common to perform a drawing review to ensure the design includes a continuous air barrier. During construction a number of site inspections are often made by a specialist to ensure that the building is constructed correctly, trying to rectify problems once the building is complete is more diffficult and may be expensive.