Windows (Part I)

How many times did we see on Grand Design or other home build programs the stories of things going wrong with the windows/doors. Now after going through the process hopefully the following may help other self builders.


First of all the technical detail. The most important element of the windows is the glass. Some important functions-

  1. Capture heat (free energy) from the sun in the winter/autumn/spring to heat the house. (Called the g value)
  2. When the heat is captured or created in the house minimise the loss through the window (Called the Ug value for the glazing. )The Uw or U-value usually includes the whole window (including the frame ) but be aware that some window company’s may quote only the glazing value rather than the glazing and frame.
  3. Minimise cold air draughts (cold air descending at the window surface) that one may feel if one sits near a window (this is caused by the glazing not being able to keep the differential temperature between the inside and outside below 3 to 3.5 degrees Celsius. (eliminated by triple glaze systems)
  4. Maintain a sense of light in the room (the light level can be typically reduced by 30% (for triple glazed systems) when one tries to balance the above factors.
  5. Ensure that west facing and south orientation windows are correctly shaded (or by means of special glass) in the summer/autumn so that the house will not overheat .

The type of glass used in a self build can reduce the amount of insulation required in the house. One has to balance the g value ( g value represents the maximum amount of solar energy passing through the glass and 0.0 or 0% represents a window with no solar energy transmittance-if glass had a 53% = 0.53  g factor it would let 53% of the solar heat through. ) with the Ug value which represents the heat loss over a surface area in W (m2/k).

The glass that works in Germany may not necessarily work in Ireland. One needs to balance these two values to suit your house and orientation in the PHPP software.

Another important factor is the glazing spacer used between the panes of glass. Most high quality windows will use a thermally broken spacer to ensure that the minimum amount of heat is lost through the glass spacer. One can see this by viewing the colour of the spacer -if is it silver/metallic  then it more than likely has a high heat loss. If it is black it more than likely is a highly insulated spacer.

Window Spacer (Black)









Practical Choice

Options are available in general to have the windows opening out or opening in (with or without tilt and turn).  Tilt and turn mean that blinds and curtains need to be taken into account.

I believe there are only two or three manufacturers of sliding doors that are airtight. Other options are folding units.

  • Review the type of hinges (galvanized, steel, brass etc) and the handles specified (shapes).
  • Some opening out windows/doors have the option of a locking system  to ensure that a breeze will not affect the ventilation or damage them when left open.
  • Establish which doors need external key locks for entry .
  • Establish how many sets of keys you will receive
  • There is an option to have wooden or PVC windows clad with aluminium (evaluate which is more suitable in your environment such as being beside the sea versus fully sheltered).
  • Ensure the Ral colour touch up paint kit is available for small scratches and knocks that happen on site.
  • Establish if you want alarm contacts pre-installed.

Cost Choices

  • A window with minimum openings (more energy efficient) will be significantly cheaper than a window with multiple openings (less energy efficient).
  • A window that stays within the manufactures standard sizes and truck delivery size is going to be cheaper.
  • A window that has a non standard shape is going to be more expensive.
  • Sliding/Folding mechanisms and making them airtight is more expensive.
  • Establish if one can use glazing without a frame in your design (roughly 50% cheaper).
  • A certified passive house window will be more expensive. See link to certified components- Passive House Institute Certified Components

 Importance of Installing Windows Correctly

The frame that holds the glass and those used in passive houses will have an insulator such as cork or other insulation material separating the inside of the frame from the outside climate in order to reduce the heat loss. An equally important detail of a window is how it is going to be installed. This can account for over a third of the heat loss if it is just placed in an opening and secured with a steel band/bracket and then foam filled around the edges. So in real terms money spent on a high performance window/door can be negated completely by installing it poorly.

Below is a video I came across for guidance on installing windows  in a timber frame build (Ireland) with a breathable insulation on the outside and a sketch of an externally insulated block work building later on in the video. There are a number of videos in this series.

In my case I installed the windows in a wooden frame on a ventilated facade. As wood is a fairly good insulator (thermal conductivity of approximately 0.13w/(mk) ) I took the extra step of providing a better insulator around the reveal in order to improve the installation method and reduce the heat loss on the frame as the external cladding is vented with cement board. I am in the process of doing up the thermal bridge calculation using the free software Therm to calculate the actual linear heat loss (Thermal Bridge psi value denoted by the symbol Ψ).  The other type of heat loss is known as the U value and is a measure of surface area in watts per m2 per degree change (W/m2/K).

Where the window or doors were installed on concrete I installed Compacfoam  (rigid insulation) under the window/door and I will insulate and provide an airtight seal up to this material.

Window Frame Mounted on Compacfoam (insulator) in order to minimise the heat loss against the concrete floor.










Glazing (with no frames)

I installed some glazing in the structure of the building without the frames in order to reduce costs. The timber frame manufacturer, Matthew O Malley Timber Limited, rebated the openings and I then taped and sealed the glazing. There were 12 glazing units of 2.4 metres by 0.9 metres approximately.

I installed security tape in the rebate. I experimented with other security tape but found the following tape to be better-Closed cell polyolefin foam tape which conform to BS 7950 Manual Glazing Test from tapes direct in the UK.

Glazing openings with rebate in timber.












As the structure of our home is made with gluelam this helps to minimise the movement in a timber frame build to facilate installing glazing without a frame. I am not sure if glazing can be installed directly in a standard timber frame build.

Other important factors to consider are:

  • air-tightness (the normal passive certified window will have two or three seals mounted in the frame) .








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