Windows 102 - Ratios and Coefficients

When it comes to solar heat gain through windows: ratios and coefficients are very important. Don’t worry this isn’t a maths lesson! (See last week's post - Windows 101.)

Glazing to Floor Area Ratio

Solar passive design does not mean the entire northern façade of a house should be glass. Well, not in the Canberra climate and not with standard construction methods.

The glass to floor area ratio should be tested carefully during the design process because, there comes a point, when the thermal benefits of soaking up free heat from winter sunshine are outweighed by the heat lost through the windows as soon as the sun goes down. PLUS, even if you’ve designed a northern eave that prevents the sun striking the glass during the hotter months (so minimising radiant heat gain), you will still experience much greater conductive heat gain through the glass than if it were wall. Remember that even a good quality PVC or timber-framed double glazed window transfers heat 6 times faster than a well-insulated standard brick and plasterboard wall (see Windows 101).

Using thermal performance simulation software (BERS Pro), we optimise glazing areas in our homes, and the eave size and position, to ensure we find the right balance between winter heat gain and loss and to ensure we don’t create summer ovens that are dependent on air-conditioning. Relative to most new homes, our houses have low glazing areas yet they are extremely light-filled and well-connected (visually and physically) to their surroundings. We think very carefully about where the windows go to maximise light, warmth and connection to the outdoors without compromising thermal comfort and energy efficiency.

Here’s an example using our Stray Leaf house. It has its long axis facing almost directly north so has great solar passive design potential, however, the majority of the northern side is still insulated wall.

Detailed modelling in thermal performance simulation software (of the home’s exact construction and orientation and using decades of hourly weather data and exact solar position specific to the Canberra region over a whole year) shows that the glazing to floor area ratio (G:FA) we have specified for the northern rooms (in this case, between 30 and 40%) gives optimal thermal performance across a year in Canberra for this design. Any increase to the window sizes, increases both the predicted winter heating load and the summer cooling load.

In the kitchen/dining space if we increase the glazing to cover most of the northern wall (G:FA = 65%), as is common in many new homes, the predicted heating requirements of the space increase by 20% and the predicted cooling requirements nearly double (190% increase).

Glass is great to look at, and through, but thermally it’s a very big hole in a wall! It is also far more expensive than any wall cladding system. Think very carefully about the size and location of windows – it can save you a significant amount of money upfront, during the build, and during the lifetime of the home as you heat and cool your house to remain comfortable and healthy.

Solar Heat Gain Coefficient (SHGC)

“SHGC measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it transmits.” (AWA website)

In the Canberra climate, heating dominates our energy use so we really want to maximise the passive radiant gain over winter, while minimizing the conductive loss. This means we want a high SHGC but a low U value. For the sorts of windows we tend to specifiy at Light House (for biggest bang for buck with good design in the Canberra climate) a high SHGC is around 0.4 – 0.5, and a low U value is between 2 and 3. Different types of double glazed windows can give very different numbers to these but remember its best to have the SHGC not going lower, while you don’t want U values getting higher (See Windows 101 for a reminder on U-value versus R-value).

You may have heard of lowE coatings or films that can be applied to windows to improve their thermal performance. Be sure to check you are not being sold a product that might improve the conductive properties (ie reduce the U value) but greatly reduce your home’s ability to soak up free heat from the sun over winter. What works well on the NSW coast is usually not the best option for Canberra!

Dressings. It doesn’t seem right to move from maths to fashion... and I think I’ve ranted enough for today. Window dressings (inside and out) can wait until next week and lesson 103. Until then, enjoy the warm sunshine.