Wondering how to choose the right windows and doors to support and improve your home’s thermal performance?

Discover the two numbers, U value and SHGC, that tell you how well a window system will really perform.

Learn what to specify and what to ask your team, so your windows work for your climate and orientation, and don’t undermine your home’s energy efficiency.

Listen to the episode now.

Hello! This is Episode 421, and Way #21 of the 44 Ways to Create Your Sustainable Home series here on the podcast. We’re continuing through Section Five: The Building Envelope.

In my experience, as people go to design their new build or renovation, it can be all about maximising the indoor / outdoor connection, and having a home that feels light-filled and expansive. 

And so, the glass windows and doors you choose, in their size, arrangement, design, functionality and type, all become really important decisions in achieving this in your home. And, they’re likely to take a big chunk of your building budget.

However, glass windows and doors are also where the majority of uncontrolled heat gain and heat loss happens in a home. They sit at the intersection of the envelope performance issues we’re covering in this section: thermal performance, ventilation, airtightness, and moisture management. 

And yet, most homeowners haven’t been given the information they need to make deliberate choices about them.

Way #21 is: Review Your Window Selection.

The Australian Government’s YourHome guide states that, when it comes to single-glazed windows which are conventionally used in Australian residential construction, up to 40% of a home’s heating energy can be lost through windows, and up to 87% of a home’s heat can be gained through windows in summer.

That’s a huge amount of energy lost and gained through one part of your home, and when so many are focussing on maximising their indoor / outdoor connection and often including a lot of glazing in their home, that’s a big challenge to the ongoing thermal performance of your home.

Every glass window and door cuts a hole in your building envelope, and hence, creates a change to how that building envelope can perform.

So your windows and doors have to do a lot of jobs, in protecting and sheltering you, and in helping improve the thermal comfort, energy efficiency and performance of your home.

When it comes to the thermal performance of windows, two numbers can tell you how well a window system will actually perform: the U value, and the SHGC or Solar Heat Gain Coefficient.

The U value measures conduction, which is how readily a material conducts or passes heat through itself. To quote YourHome: ‘The U value for windows (expressed as Uw), describes the conduction of the whole window (glass and frame together). 

The lower the U value, the greater a window’s resistance to heat flow and the better its insulating value.’ U value is the inverse of R value. The higher the R value, the more insulative something is. The lower the U value, the more thermally resistive something is.

The SHGC, or Solar Heat Gain Coefficient, measures how much solar radiation passes through the window into your home. It’s expressed as a number between 0 and 1. The lower the SHGC, the less solar heat is transmitted. 

In practice, because this is about solar radiation transmitted through the window, the angle of the sun will impact this figure. So the same window can have different solar gain depending on its position, its geographic location and the season.

In lots of locations around the world, it’s actually more economical to use double glazing than single glazing. The industry in those locations, even in temperate climates, has recognised it’s a better choice, and manufacturers now produce it as standard. 

It’s still super common in Australia to use single-glazed windows in new builds. So, when you’re doing your energy modelling, test what it does to your home’s performance to switch to double glazing, whatever climate you’re in. Get the data, and then you can make a decision of the value it represents to you.

The frame is part of the window system and significantly affects the overall U value rating. 

Standard aluminium is the most common and economical choice, but it’s an excellent conductor of heat. A thermal break, which separates the exterior and interior sections of the frame with a low-conductivity polymer, substantially improves aluminium frame performance. Timber and uPVC frames are naturally better insulators than aluminium. 

Lots of HOME Method members are specifying uPVC windows, especially tilt and turn types, including honeycomb blinds, and getting a really fantastic outcome thermally.

Airtightness can vary between windows too.

Windows can be a significant point of air infiltration in the building envelope, and different opening types perform very differently. 

Awning and casement windows have compression seals, so they create a good airtight seal when closed. Louvred windows have glass blades closing against each other, and are not a great performer for airtightness. Different window opening types also provide very different amounts of airflow, so ventilation and airtightness both need to be considered together when specifying your windows.

In this Episode, I cover:

• The two thermal performance numbers that matter, U value and SHGC, and what they actually tell you about a window system
• How different frame materials (aluminium, thermally broken aluminium, timber, uPVC, fibreglass and composite) compare on Uw value
• How single, double and triple glazing compare, including the role of argon fill, low-e coatings and tinted glass
• How to think about SHGC by orientation and climate, including why north-facing windows (in the southern hemisphere) do different work to east, west and south
• Why it’s worth considering double glazing and how it compares
• Why frame and installation both matter for airtightness, including the window-to-wall junction and gap-filler choice
• How different opening types (louvred, casement, sliding, double-hung, awning, fixed) compare for both airtightness and ventilation
• The questions to ask your designer, energy assessor, builder and window supplier
• And why window substitutions on site can derail your energy efficiency certificate at completion

Plus a whole lot more.

LISTEN TO THE EPISODE NOW.

RESOURCES:

Season 8 Episode 5 with Tracey Gramlick, Australian Window Association (AWA) >>> Choosing Energy Efficient Glass Windows and Doors

UA blog on what to consider before you buy >>> 7 questions to ask before choosing windows and doors for your home

UA blog on glass, frames, and performance ratings >>> A helpful guide to glass window and door selection for your home

Episode 402 >>> 44 Ways #02: Design for the Breezes + Wind at Your Site

Episode 419 >>> 44 Ways #19: Build Tight, Ventilate Right

A demonstration of a tilt-and-turn window and other insights into uPVC windows >>> uPVC windows video

Australian Government’s sustainable design guide on glazing >>> YourHome: Glazing

Window energy ratings scheme >>> Australian Window Association (AWA)

Window energy ratings scheme in the US >>> National Fenestration Rating Council (NFRC)

A free guide to simplifying sustainability across research, planning, design, materials, and building, from Amelia Lee’s 30+ years in residential design and construction >>> 44 Ways to Create a Sustainable Home

Access the support and guidance you need to be confident and empowered when renovating and building your family home inside my signature online program >>> HOME Method