Building Regulations and Planning Permissions

Thermal Performance in Modern House Design - All You Need to Know

One of the first questions architects often ask about the glazing for their modern house design is; what is the thermal performance?.

It is understandable as historically glazing has been thought of as the lowest-performing element of an external building envelope. With advances in glazing for modern house architecture, advances in installation techniques and clear data on residential energy consumption glazing has become highly energy efficient, able to be used on high specification house architecture with great effect.

Here is everything you need to know about the thermal performance of glazing in modern house design:

What is a U Value?

Before you begin you first need to understand how the thermal performance of glazing is denoted. All building materials for modern contemporary home design use U values to show you what their thermal performance is.

Architectural glazing is a little more complicated that your standard wall structure. The overall U value of a glass installation (the Uw value) is created using a combination of the Uf value (the thermal performance of the frame), the Ug value (the thermal performance of the glass) and the PSI value (the thermal performance of the glass spacer bar).

These are measured in W/m2K and the lower the number the better the glazing will insulate. You can read into more depth about Uw values and how they are calculated here:

What is a U Value?

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What Thermal Performance Do I Need in Modern Home Design?

The local building regulations or rules will dictate what the thermal performance of your glazing needs to be. In England this is given by building regulations.

Building Regulations Approved Document L gives the rules on thermal performance for modern home design in England. Wales and Scotland have slightly different rules.

The U value your glazing needs will depend on whether the glazing is part of a new dwelling (Approved Document L1A) or an extension or renovation to an existing dwelling (Approved Document L1B).

The rules for renovations to existing houses are simpler, requiring all glazing to achieve a minimum of 1.6 W/m2K.

New build modern houses have a more complicated approach to thermal calculations as it takes into account the entire envelope and energy usage of the building. The glazing cannot have a Uw value worse than 2.0 W/m2K however, glazing typically must achieve much better values than this to satisfy all the other criteria.

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You can read about the glazing thermal requirements for various builds in more depth below:

Building Regulations Part L1A for New Dwellings

Building Regulations Part L1B for Existing Dwellings

How Can I Improve the Thermal Performance of My Glazing?

As the thermal performance of glazing is a combination of different elements of the glass installation you can improve the thermal performance by improving the value of any of these.

The glass is normally the largest aspect of a glass installation on a modern home design so the greatest change in thermal performance can be seen by improving the Ug value. This could be done through a general change in the glass specification, the addition of more coatings or the uplift to triple glazing.

Although the framing is a small element of a modern architectural glass install (and normally hidden on modern house architecture) ensuring the frame is thermally broken is a big element of getting an insulated building.

If non-thermally broken framing is used not only will the Uw value of the glazing reduce, but you will also see condensation building up internally on the cold frame.

You can read more about thermal breaks in glazing and their importance here:

Thermal Break Technology in Windows and Doors

There is also a very informative video/webinar below that discusses thermal breaks in detail, please like comment and share with anyone you feel would benefit from this or any of our other content.


The Thermal Performance of Roof Glazing is More Complicated

When conducting your SAP or building regulations calculations the thermal performance of roof glazing is always given in its vertical position.

That is because, in reality, the Ug value of a glass unit changes depending on the angle it is installed at. The convection currents within the insulating gas filling in the glass unit increase when tilted which increases heat transfer to outside.

All building authorities understand this and understand that it is too complicated to cover all pitches and eventualities in regulation documents for roof glazing. That is why the Uw value requirements for roof glazing are given in the glass’s vertical position.

If you are keen to ensure the highest possible thermal performance for your roof glazing read our below article which explains it further:

The Thermal Performance of Roof Glazing

More Information?

Further Reading:

What is a U Value or Rating?

Building Regulations Part L1A for Glazing (new build homes)

 Building Regulations Part L1B for Glazing (extensions/renovations)

True vs Applied Glazing Bars in Windows and Doors

Thermal Performance of Horizontal Glass

Thermal Breaks in Metal Windows and Doors

Do you have a specific Uw value you need to achieve from your glazing? Contact the team at IQ and we will be able to offer advice and guidance for your glazing specification. Our team of glazing experts are ready to answer your questions.

Visit our Contact Us page for all the ways you can get in touch.