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Part O – the Inkling appraisal – Part 2

This blog follows on from one published in February that you can read here:

The new Part O of the Building Regulations comes into force in under 3 months.
DLUHC are adamant that these tough new criteria will be beneficial for reducing overheating risk across all new homes built under them. I have been working with the Future Homes Hub and others to better understand the implications of this new legislation.
Here are some of my (constantly evolving) personal opinions on the subject.

Simplified method

I am still worried that the simplified method is far from simple to assess (or sometimes pass). The FHH hub are working on templates and advice to make this process easier to follow, and suggestions such as allowing use of frame factors could reduce the amount of detailed measuring involved.

Another worry is how easy it will be to complete a sufficiently accurate assessment pre-planning, given the level of detail needed. In particular, assumptions will need to be made for window opening angles and areas (the actual hole dimensions when a window is open are hard to measure accurately from drawings as the opening frame obscures the opening itself). Provided conservative estimates are made then this may be ok, but the calculations are sensitive, pass margins are tight, and there is a risk that detailed design might uncover non-compliance which could be hard to mitigate without going back to planning.

Advice from DLUHC suggests that they do not intend to revise the glazing and free area targets, but there may be room for compromise with the West facing glazing area limits for high risk locations (which are oddly higher than for medium risk locations) being used in moderate risk locations if suitable shading is also provided. Confirmation is awaited.

The simplified analysis is sensitive to the unit floor area. The figure used should be the GIA (Gross Internal Area) which Appendix A clarifies should be measured to the inside face of perimeter walls i.e. excluding internal walls and therefore a higher value than the sum of all the room areas. Any changes to the GIA affect the glazing and free area targets as they are based on % of the unit GIA, so this number needs to be assessed carefully early on.

The classcool calculator assumes a top hung example so the height and width dimensions need to be swapped to assess side-hung openings – this is confusing and a calculator tool provided specifically for ADO would be helpful.

The free area requirements in bedrooms in high risk locations is more than 3 times the provision needed in moderate risk locations at 13% of the floor area – this could lead to larger windows in bedrooms and increased solar gains, or reduced glazing in other occupied rooms to balance this.

Allowable shading measures include low g-value glazing (g<0.4). This is sensible in many ways, but might impact on the FEEs within SAP. I assume that the benefits of low g-value glazing are best realised when the windows are closed (unless top hung). If all windows are open in hot weather the shading effect from the low-g glazing won’t be felt. This is a criticism often levelled at dynamic modelling and I would be interested to know what others think about this.

Orientation of units – there are good reasons to think that West facing units are at higher overheating risk as they receive most solar gain in the afternoon when external temperatures are often peaking and internal gains increase as people get home from work/school. However, the 11% glazing limit for west facing units (moderate risk locations) is significantly lower than the 15-18% allowed in other orientations, and means that many unit types will pass in other orientations, but not when the rear of the property faces West (it’s usually the rear that is the most highly glazed). Given that sites are often laid out with units orientated in many variations, it’s possible that identical units orientated within a few degrees of each other, or on opposite sides of the road might have very different simplified method results.
This 11% limit is based on the whole unit, so if minded, developers could reduce glazing on the East (opposite) façade in order to maintain preferred glazing levels to the West (provided no single room exceeds the 22% limit) which would secure a pass but without any real change to the level of overheating risk.

The simplified method requires very large free areas, especially in high risk locations. Homes are unlikely to comply with the free area requirements with window restrictors installed. DLUHC maintain that this is why sill heights have been raised so that restrictors will not be needed, but this may cause problems for windows opening directly onto pavements where there is a safety risk of injuring pedestrians and local authorities and housing associations may have to reconsider their policies on requiring restrictors.

The simplified method results do not fully correlate with TM59 results especially in lower risk locations in the North of England and on the coast. This could penalise homes looking to optimise passive solar gains to reduce heating loads in the North of England.

TM59 Route

DLUHC have confirmed that mechanical cooling is only allowable if using the TM59 route, and probably only in response to noise issues limiting use of opening windows. Homes with mechanical cooling will also have to comply with Part L which will take cooling energy use into account – I am interested to understand how onerous this will be – please let me know if you have any experience to share on this.

We have run a sensitivity study on one single aspect flat (south facing) to test the impact of applying the change to the night time bedroom window opening strategy. For this one example having the windows wide open all night if the inside temp is greater than 23°C at 11pm improves the results quite significantly. We think this is because the windows are forced to be open wide all night, while if left on the 22-26°C opening pattern they’d be less wide open, and might close during the night if it cools off significantly. This suggests that the testing we’ve done comparing TM59 results to those using the simplified method were conservative, and the TM59 results might indicate less overheating risk once the modelling software enables this facility without having to use a slow workaround.

Note that this is just one example and I have heard others reporting more mixed outcomes when implementing this change. Please let me know if you have any experience to share on this.

Changes in latest download (since previous blog was posted)

The latest download of the AD extends the especially low glazing limit for west facing homes to homes with no cross-ventilation (table 1.2), reducing the glazing limit in moderate risk locations from 18 to 11 (% of floor area) in-line with homes with cross-ventilation.

The latest ADO download includes an additional sentence:

The equivalent area of the opening should meet or exceed the free area of the opening.“

Intended to confirm that it is the equivalent area values from the classcool calculation or the Appendix D tables that should be compared with the targets in tables 1.3/1.4.

There are also two changes to the targets set in table 1.2 (limiting solar gains without cross-ventilation):

Expect to see:

  • Far fewer fixed glazing panes – it is likely that almost all windows will need to be openable, especially in bedrooms in order to meet the simplified method. This could impact on glazed front doors (or fixed lights above), sash windows and sliding patio doors.
  • Large windows replaced with increased numbers of smaller opening panes. The requirements on maximum reach not exceeding 650mm from the inside wall surface as part of the protection from falling will limit the dimensions of opening panes, especially where walls are thick.
  • Higher sill heights (min 1100mm) or more creative guarding measures. Part B requires windows that are used for escape to have a maximum height of 1100mm, so this gives zero build tolerance in these cases. It could also restrict views out from a seated position including for wheelchair users.
  • Larger windows in bedrooms (all openable panes) in order to meet the bedroom free-area targets – especially in high risk locations
  • Fewer single aspect flats – these will become very difficult to design to meet this AD unless they face East or North (less good for daylight). This might be appropriate as single aspect units are known to be at higher risk of overheating due to all glazing receiving solar gains at the same time of day, and the lack of cross-ventilation reducing the benefit of opening windows.
  • Increased use of TM59 route for London developments as it will be extremely difficult to comply with the simplified method in high risk locations. This will not be a magic solution however as the AD does not allow blinds to be included in the model making hard to meet unless we also see reductions in glazing areas and majority opening panes.
  • Design changes to homes built in the North of England (which might not be necessary, and will reduce beneficial passive solar gains) as these units will need to meet the same simplified criteria as homes built in Reading.
  • No more conservatories on new builds

What would we have done differently?

It’s easy to criticise, and much harder to come up with a simple method for predicting overheating risk reliably in all contexts (believe me, I’ve tried). So how else might this have been approached?

I don’t have access to the same research resources as DLUHC, but I think the principles in the simplified method could be maintained but made simpler for better usability. I have not tested these thoughts out, and there may be good reasons why they would not be sufficiently robust, so I put them out for discussion only.

  • High risk locations – straight to TM59 (the higher risk and land values associated with London and city centre units could justify this)
  • Cross ventilation: max glazing:floor area ratio of 15% with no room more than 30% (exact numbers to be researched and evidenced), plus free area limit requires glazing in every room to be 50% openable (to at least 70°)
  • No cross-vent:  max glazing:floor area ratio of 11% with no room more than 25% (exact numbers to be researched and evidenced), plus free area limit requires glazing in every room to be 90% openable (to at least 70°)
  • Glazing limits could be increased to 18% floor area in cross-ventilated homes where shading is applied

These are my current thoughts, and my views may change as we use the guidance more regularly. I would be very interested to other peoples experience and opinions. You can get in touch via email: or on Twitter: @InklingLLP

I’m adding links below to some useful responses:

This is the second in our blog series on Part O:

Part 1

Part 3

Part 4

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