Your TM59 Assessment Failed. What Now?

A TM59 failure is not the end of a development. It is information, and it is more specific information than most people realise. The mistake is reaching straight for the blunt fix.

First: read which test failed

This tells you what kind of problem you have, and the two are not solved the same way.

• The daytime test failed: the room is taking on too much heat. A solar gain problem.
• The night-time test failed: the bedroom cannot shed heat overnight. A ventilation problem.
• Both failed: usually a single-aspect dwelling with significant south or west glazing.

Also find the worst-performing room. Most developments fail on one or two rooms, not everywhere. You are solving a local problem.

Then work in this order

Ordered by cost, cheapest first.

1. Shading

The cheapest change you can make, and often enough on its own. A due south overhang should fully shade the pane when the sun is at 50° altitude or higher. Balconies above the window already do this. External blinds and shutters do more than internal ones, by a wide margin.

2. Glazing specification

Changing the g-value changes the solar gain without changing the size of the opening. This is the fix people skip, and it is usually cheaper than re-drawing an elevation. Remember that the g-values in Part O are centre-pane and exclude the frame.

3. Ventilation strategy

For a night-time failure, this is where the answer usually lives. Can the room get more openable free area? Is there a route to cross-ventilation that the model does not currently credit? Where a bedroom window must be modelled closed for security, mechanical purge or night cooling may be the only way through, but note that mechanical cooling has Part L consequences and must be reflected in SAP.

4. Orientation and layout

If the design is early enough, rotating a house type or swapping which rooms face west can resolve a failure for nothing. This is only available to you if the assessment happened early, which is the argument for running it early.

5. Glazing area

Last. Not first. Smaller windows are the fix that gets reached for because it is the one the Simplified Method tables imply, and it is the one that makes homes worse to live in. It also does very little for a night-time failure, because the problem there is not how much heat came in, it is that none of it left.

Check the model before you change the building

Before you redesign anything, make sure the failure is real. The common modelling errors all push the same way:

• Cross-ventilation misclassified. A corner flat with windows on two elevations is not automatically cross-ventilated. Getting this wrong is one of the most common causes of both false passes and false failures.
• Openable area overstated. Restrictors, guarding and handle positions reduce equivalent area.
• Wrong weather file. Check the location and the DSY used.
• Rooms modelled closed that need not be, or open that must be closed.

Remember what a pass means

Compliance means the dwelling meets the standard. It does not guarantee comfort in every summer, and it does not account for how the home is actually used.

If your assessment passed by a hair on a west-facing single-aspect flat, you have met the regulation. Whether you have built something people will be comfortable in is a different question, and worth asking while you can still answer it.

AutoTHERMAL lets you duplicate a dwelling, change one variable and compare results side by side, so you can find the cheapest fix that passes rather than the first. [Join the waitlist](/waitlist).