Components of the Building Project 2 – Roof/Ceilings and Walls (including Glazing)

Incorporating passive design principles is the most cost effective way to achieve good thermal performance of your home. Passive design considerations apply equally with regard to your roof, ceilings, walls and glazing as it does with the rest of your home. These include:

• Location and size of windows and shading to let the sunshine in when the temperature is cold and exclude the sun when it is hot. Radiant heat passes through glass and is absorbed by building elements and furnishings, which re-radiates. Re-radiated heat has a different wavelength and cannot pass back-out through glass as easily. In most climates, trapping radiant heat in winter is desirable but it is to be avoided in summer.

• Shading and Orientation are keys to the passive performance of any dwelling. Direct sun can generate the same heat as a single bar radiator over each square metre of surface area. Shading can block up to 90% of this heat.

• Use of thermal mass to store the sun’s heat and provide night time warmth in cold conditions.

• Location of windows and doors to allow natural cooling by cross ventilation.

• Ensuring effective seals to openings around windows and doors to minimise unwanted draughts. Air leakage equates to 15 – 25 percent of winter heat loss in a building.

It is acknowledged that the implementation of passive solar design principles can be challenging on some sites. For example, winter sun might be blocked by neighbouring buildings. Or the best views may be to the south or west, requiring windows/doors with poor orientation. In these instances, selecting wall finish or glazing with improved thermal performance is critical in order to compensate for aspects of the building design that are detrimental to its thermal performance.

Roof/Ceilings

Traditionally, for metal roofs, sarking (woollen blanket on reflective foil) will be required. The woollen blanket has the added benefit of reducing the noise from any expansion/contraction of the metal roof. With tile roofing, the use of sarking (reflective foil only) would be the norm. The use of air-cell insulation under both roofs could assist in reducing heat conductivity.

Additionally, the installation of whirly birds/roof vents will assist in releasing heat from the roof cavity keeping the dwelling cooler in summer. Further, as mentioned in the September 2012 Newsletter article on Components of the Building Project, the warmer air in the roof cavity/ceiling space can be redirected down through the Cupolex domes under the slab providing cost effective under slab heating.

Installation batts in the ceiling with an R-rating (Resistance to Heat Flow) of 2.5 plus should be the minimum target for most dwellings in the Sydney and immediate surrounding areas.

With effective passive design elements in place, balanced with the above R-value properties of the roof/ceilings, your dwelling will provide you with year-round comfortable living conditions.

Walls

The minimum Total R-value for walls (Resistance to Heat Flow) for dwellings in Sydney and immediate surrounding areas should be between R1.9 to R2.2. These minima are based on the dwelling having appropriate passive design elements in place. The R-value for some of the common wall construction elements are:

This graph indicates that traditional building practices are not always the best and additional correctly placed insulation would be required to match newer energy efficient wall building systems. That said, even with the best wall and roof systems, care is needed to ensure that appropriate cross ventilation has been included in the design of the dwelling to provide optimal living conditions.

Glazing

Windows in a typical insulated home can account for more heat gain or loss than any other element in the building fabric. In summer, heat gain through an unshaded window can be 100 times greater than through the same area of insulated wall.

Heat flows through glazed elements such as windows, glass doors or fixed glass panels are determined by the combined effect of the glass, frame and seals. Heat flows through glazed systems in several ways:

• Conduction. This is the movement of heat energy through the glass and frame materials from the air on the warmer side to the air on the colder side.

• Convection. This is the movement of heat energy by air that passes over the surface of the glazing unit taking heat away from the glass and frame.

• Radiation. This is the heat that is transmitted as electromagnetic waves until they are reflected or absorbed by materials.

With glazing, the value of heat flow is expressed as its U-value. The lower the U-value, the less heat is transmitted.

When looking at the U-value of your windows/doors, the important consideration is the combined U-value of the glass and frame (known as the system U-value) rather than the individual U-values for the glass and frame. The U-value (remembering the lower the U-value the better) of more common windows/doors follow:

The ability to conduct heat can also be expressed as its opposite – the ability to resist conducted heat flow – represented by R-values. (R-values are used to describe insulating properties in many other building materials. The higher the R-value, the less heat is conducted.) U-values and R-values can easily be converted. R-value=1/U-value, U-value = 1/R-value.

For example, a window system with a U-value of 5.5 will have an R-value of 0.18 (ie 1/5.5). When compared to the Resistance to Heat Flow R-value of 4.22 for Hebel Wall Panelling, the low R-value for the window system highlights the care needed when considering the window/door systems (including glazing) for your dwelling.

Summary

Efficient passive design is paramount.

Things to consider in passive design include:

• Shading,

• Orientation,

• Roof design,

• Window position,

• Window size and orientation,

• Ventilation, and

• Finally, how you live.

Being able to visualise all these elements before anything is constructed is always difficult even for those trained in design.

That said, no one knows your place or your requirements as well as you do. You may find it useful to prepare a Visual Diary/Scrap Book to collect images, articles, photos, examples of what you like and don’t like, notes on where the sun rises and sets, records on which windows get sun and shade, notes on the orientation you want to optimise the view, what are the areas of your home where you spend most time etc.

The more information you can provide to your designer can only help to achieve the optimal passive design possible. In turn with the correct design, the lower the construction costs will be to achieve the best possible energy efficient outcome.

If you would like assistance in reviewing your design ideas before approaching your designer, please feel free to contact DirectBuild at (02) 9713 7428 or bwilkie@directbuild.com.au – we would be pleased to assist.

Reference: Your Home Design for Lifestyle and the Future Technical Manual Fourth Edition.