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State House to Passive House 8 – Step 4 – Let's Go Shopping (for windows and ventilation)

The initial modelling gives a proof of concept. It should give an idea of what type of construction, and insulation levels you need. It will tell you what performance level you need for your windows. This allows you to go shopping for windows.

New Lift and Slide PVC Sliding Doors will be a significant improvement from the previous bifold doors which had visible gaps at the top

Windows are the weakest link. They are the key purchase. They do not have the same insulation level as a wall. This means their thermal performance will have one of the biggest impacts. It is hard to think of a deep retrofit that would not involve an upgrade to the windows. It will be a major cost.

The size of the windows, their placement, and their shading, refined with the modelling are part of that story. The insulation values of the frames and glass are another part. The insulation value of the glass will be higher than the frame. Reducing the amount of frame, or insulating over will make a big improvement. This changes the way you design elevations/fenestration.

They need to be installed in line with the thermal envelope to ensure there thermal break or heat loss around the edges. Commonly a recessed installation is required for timber frame construction. This needs some care to detail correctly.

Recessed Sill Detail

The G value of the glass (sometimes shown as SHGC), then has an effect on the solar amount of solar gain. Its value can make a significant difference to the energy balance of the building.

To finalise your modelling for the pre-construction certification, there will be a refinement of the construction, thermal break modelling, and confirming the MVHR ventilation unit and windows.

There is a wide range of choices. First, you need a window that will be sufficiently airtight for the blower door test/airtightness requirements. The entry doors can be leaky so watch out for them. The units will need multiple seals often with a mechanism. to clamp them tightly closed. There is no point in having high insulation levels if that warm air can just seep out the cracks.

PVC, Timber and Aluminium examples of high performance windows

In my initial modelling, I had a generic PVC window. For an EnerPHit level of building performance/efficiency in a single house or duplex such as mine, you are most likely looking at a timber or PVC window.

A good quality thermal broken aluminium window might be an option, especially for larger buildings. A company that supplied all three recommended aluminium windows for a school due to being hardwearing.

There is then a wide range of performance in different timber, PVC and aluminium windows. PVC is less expensive but can still have good performance. Timber windows are often clad in aluminium on the outside for reduced maintenance. Some PVC windows are also clad in aluminium or have a wrap to give a colour.

There are locally made and imported windows. There are Certified Passive House Windows, that provide an assurance of performance with all the information you need loaded in PHPP. Certified windows do tend to have a premium and are often designed, but always for colder climates. The wider the window, usually the better performance. Double glazing will be enough for some warmer climates in New Zealand but upgrading to triple glazing was only $2-6k extra.

I went out to a dozen window suppliers and received 8 prices. I didn't get to thank everyone for pricing, Apologies if you are reading.

I decide to go with STÄRKE PVC windows. I made my decision based on the balance of the following factors.

  1. Price - I felt PVC was the right price point for this budget-driven project. I am looking for what an average upgrade should be, not the highest performance.

  2. Locally made - I needed windows quickly after the fire. They had a 4-8 week lead in time, which they meet once sizes were confirmed. This gave me the chance to confirm sizes post-demo.

  3. Information - Often I struggled to get all the information I needed for the modelling. STÄRKE had worked with Sustainable Engineering to have all the data readily available. They had a very helpful technical advisor.

  4. Compliance - STÄRKE had testing of recessed installation with a taped air seal and typical details. This got the building consent over the line. Some councils are getting very savvy with their questions on recessed high-performance windows.

New bedroom window with tilt and turn opening sash

There is a wide range of performance and price options with an ever-growing number of suppliers.

European high-performance windows are generally glazed from the inside. It's more secure and can be replaced without expensive scaffolding. It is such a better way of doing things.

For ventilation, there is less of a choice. You want to use a Passive House Certified MVHR unit otherwise you get penalised in the modelling. Wolf, Zehnder and Stiebel Eltron are the main brands that I know of for residential units. Finding someone who can help with the design, installation and commissioning can be a challenge, especially in Wellington. There is often a cost for the design and quote.

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