Anne Rowe – January 25, 2016

We have the knowledge and materials to build outstanding new homes—but the fact is most New Zealanders are going to continue to live in our existing housing stock.

We can retrofit older homes in order to provide better health and energy efficiency, and really we should be doing so. These goals should rank higher when homeowners are planning additions or renovations.

Spending money on better insulation is not as immediately appealing as a flash marble bench top in the kitchen. The interior comfort that you experience in a well insulated, well ventilated house is not immediately apparent. However, spend a winter in a draughty, damp villa and these qualities are sorely missed.

Chris and I raised our family in a Wellington villa built in 1907. Like many people, I find these gracious old homes deeply appealing. The native timber joinery and stained glass windows are particularly beautiful. But living here has not been without its challenges.

A series of improvements have been undertaken since 2002. Here’s a run down of what we’ve done to make the house a warmer, drier and less costly place to live and work.

• ’02: We renovated the house at the rear and insulated those walls and ceilings as part of the building works.
• ’07: We installed gas central heating, distributed using very old cast iron radiators salvaged from a building site. These have the benefit of retaining their heat long after the heating has been switched off.
• ’08: A double layer of insulation was fitted in the original part of the house: the first layer between the joists and a second layer rolled out over the joists.
• ’09: A solar hot water 30 tube system was installed to heat a 270 litre water tank.
• ’12: A land drain was dug at the rear to divert the water we had discovered ponding under the house! This was costly but had to be done and the reduction of the damp feeling in the rear of the house was immediately noticeable. The following winter’s heating bill was also much smaller than the previous.
• While the house was being re-piled, we laid down black plastic to stop the damp rising. This is a cheap and efficient way to prevent cold and damp radiating from the bare earth under a house.
• When we put in the new floor, a double layer of insulation was fitted underneath. In the areas that couldn’t be accessed from beneath the house, we fitted insulation panels between the floor joists from above, a tricky job.

We’ve also installed solar hot water and a grid-tied a 2kW PV panel system (we can produce a maximum of 15 kWhr on a good day).

The house is definitely cosier and warmer in winter than previously but it’s still not as comfortable as it could be. In 2016 we used our Passive House blower door tester to ascertain the air tightness of the house. The results showed that, under the standard Passive House testing regime, the air inside the house changed ten times in an hour, or once every six minutes. This is putting a significant heating demand on the house in winter.

In order to reduce this to an acceptable level of airtightness, which we believe to be in the order of three air changes per hour, we initiated a series of air tightness measures. The first one was to replace open recessed light fittings with closed LED type. As a temporary measure, draught proofing strips were applied to all of the openable windows. Extract fans were fitted with back draught dampers.

Our next project is to retrofit double-glazing and brush seals to our existing sash windows. This will be expensive. However, anecdotally, I have heard it makes a very big difference to the comfort levels in a room.

I hope this helpful. Your comments and questions are welcome.

Fact File

• Built 1907 in Kelburn. 4/5 bedrooms. Weatherboard exterior, tin roof, timber piles. Some original sash windows replaced with casements in 1961.
• Wall insulation: Greenstuf polyester 90mm thick R2.5
• Floor insulation: NOVAfloor underfloor insulation R1.4
• Ceiling insulation: Ecofleece R2.6 wall and ceiling 140mm loft (double insulated; ie 280mm)
• Solar hot water: Apricus 30 tube evacuated tube system with 270 litre duplex stainless steel tank
• PV: 2.0kW Canadian Solar CS6P panel with Enasolar inverter

With double-glazing now required in new homes, awareness is growing of its value and more older homes are being retrofitted with double-glazing. It’s not a cheap improvement and sadly, many people are making bad choices that won’t perform well.

There are many different types of double glazing. The better performing ones:

• Are argon-filled. Argon is a gas that is less conductive than air. It is present between the two panes of glass.
• Have a low-e coating. Short for ‘low-emissivity’, this is an extremely thin metal-oxide layer that reflects long-wave radiant heat from inside back into the house, keeping things cosier in winter.
• Have spacers separating the panes of glass that do not conduct heat effectively. They may be made out of plastic or stainless steel — not aluminium.
• Have good quality seals.

Compared to standard double-glazing, argon fill will reduce heat loss by 3-9 per cent, and low-e coating by 20-30 per cent.

But it’s not just about the glass—so don’t rely on R-values measured through the centre of the pane when you are comparing different windows. Ask for the ‘whole window’ R-value, that accounts for the performance of the glazing, the frame and their relative proportions. (Also be aware that if you read information from European sources, these will quote U-values. This is the inverse of the R-value, so the lower the U-value the better.)

Avoid solid aluminium frames

The frame is crucially important. If you are replacing the whole window sash, the worst option is an aluminium frame that is not thermally broken. Aluminium conducts heat very effectively: so while your expensive double-glazed glass is much better at keeping heat in, heat is going to pour through the frame that holds it.

It’s not just about wasting the money you’ve spent heating. Because of the difference in temperature between the outside and inside of the aluminium, a lot of condensation is likely to form on the inside of the window frame. Condensation means damper air and that can cause significant health problems.

Better frame options

There are good alternatives to solid aluminium, some more affordable than others. At the top-end, there are brilliant European made units designed to last 80 years that have outstanding performance (and look amazing).

Wooden frames are naturally thermally broken. Wooden/aluminium composites are also available. These typically have a solid timber core, timber inside and aluminium outside, with a range of colours to choose from.

Aluminium frames can of course be thermally broken — several ways of achieving this are detailed here, with drawings. These reduce issues with condensation but still conduct more heat than other frame materials.

And uPVC frames are almost certainly cheaper and will perform better thermally — like timber, it reduces heat loss through the frame by about 40 per cent. Some uPVC products have had bad press in the past; certainly it must be constructed with New Zealand conditions in mind given the strength of UV light here and our climatic variations. With modern specifications and manufacturing, you can expect a 30-year lifetime.

Period joinery

If you have period timber joinery, you likely want to keep the existing sash. Specialist joiners can remove the timber sash and fit a double-glazed unit into the timber. But old joinery typically doesn’t seal very tightly, so be aware that you’ll lose heat and have draughts through the cracks around the windows.

Other considerations

Because window frames are the weakest point in any building’s thermal envelope (the outside shell of the building), larger windows will perform better than the same specifications in a smaller window because there is more glass area relative to the frame.

Fixed windows will also perform better than ones that open. The ability to open windows on opposite sides of the house is important, because it creates cross-ventilation that can very effectively cool a house in summer. But consider having more fixed windows on the south (cold) side of the house.

Remember too that our climates vary considerably and in much of the South Island, triple glazing is called for.

More reading

See how heat is lost through aluminium glazing.
ECCA has developed an ENERGY STAR rating for windows to help compare different products.
Baffled by talk of mullions, transoms, sashes and casements? There’s an excellent glossary on BRANZ’s Renovate website to help you identify different window components.
Renall Windows in Carterton manufacture European style wooden windows.
Eco Windows import top of the line European made windows (but no longer manufacture in NZ).

Chris Rowe is a Director of CORA Associates and a certified Passive House Designer.