Window openings are breaches in your building envelope that need to be carefully managed in order to prevent moisture damage and air leakage. If you keep that in mind when installing windows, you can put strategies in place to manage moisture in the form of air leakage, condensation and wind driven rain.

1- Angled bottom sills: by building your sills on a slight angle out, any water that finds its way in will be directed to the exterior.

2- Window flashings: after wrapping weather barriers into place and taping them down, install window flashings that will safely direct any incidental water safely onto your weather barrier/drainage plane.

Window flashing © Glen Sakuth

3. Don't attach windows by putting screws through the shims. Put the screw either just above or below, then remove the shims when the window is secured. Shims will offer little insulation value, and are most likely wet when purchased and installed, so as they dry they will shrink and allow air to leak out and in.

3- Block supports: windows will often come with wooden support blocks on the bottom, or they are usually added in the form of shims or small blocks of plywood or OSB sheathing. They are necessary to allow a bottom space for insulation. But they are only necessary during installation - once windows are screwed in place, they are supported and the blocks can be removed.

If you are using spray foam do either side of the blocks, and when it sets remove the blocks and insulate in those empty spaces. Wooden blocks will create a thermal bridge under your window, and a cold spot that will create condensation. You could also replace the wooden blocks with foam or rigid mineral wool and eliminate the problem all together.

We have found using mineral wool (Roxul Comfortboard) makes leveling and placing windows quite easy. It is somewhat compressible and it holds its R value when compressed, and it isn't harmed by moisture the way fiberglass insulation can be.

4- Create a durable air seal: low-expanding spray foam is commonly used around windows, which works fine for its insulation value but it is not flexible enough to withstand the normal shifting and still provide an air seal. Adding a bead of acoustic seal, caulking or taping window frames to rough openings can be a quick and easy step that will further reduce any risk of air leakage between window frames and rough openings.

5- Inset windows: position windows towards the center of the rough opening rather than installing them flush to the outside. Windows that are installed at the exterior point of openings have a lot more thermal bridging around the frames. Windows in the centre of the opening reduce the risk of condensation by keeping window perimeters warmer.

The added benefit that isn't always considered is comfort. Thermal bridging and poor quality windows can make it quite unpleasant to be near exterior walls on the really cold days. 

Thermal bridging due to window placement:

Images like the following can often be hard to read without knowing what you're looking for, but the short story is this - when coloured lines are closer together the heat flow is more dramatic.

A thicker coloured line indicates a longer time taken for heat to move through it. Or in layman terms - more heat in your house and more money in your pocket.

The images below simulate an exterior temperature of 0 °C and an interior temperature of  20 °C, with a triple pane window. On the left the window is installed flush to the exterior of the insulated wall, with a heat flow of 7.9W. On the right indicates the same conditions but with the window inset into the rough opening, leading to a heat flow of 7.6W.

Thermal bridging through window frames
Thermal bridging through window frames © Denis Boyer for Ecohome

Thermal bridging through window frames
Thermal bridging through window frames © Denis Boyer for Ecohome


The above simulation is of just one part of one window, and with an assumed heat differential between inside and out of only 20 degrees Celsius.

What this means in real terms: multiply that amount of heat loss to match the average house size with the average amount of windows over the average Canadian winter and you could be looking at something like an additional $20-$30 on your heating bill, not because of the windows you purchased but simply because of where you placed them.

So for fun, keep multiplying -  if you can find 10 more places in your design where a slight modification can eliminate a seemingly insignificant thermal bridge, you just trimmed hundreds off of your annual heating bill. Unchecked thermal bridging in a building envelope is like death by a thousand cuts. One won't kill you but they start to add up.

And if you really want to split hairs, having your window inset by no more than 1/3 will offer you that reduction in thermal bridging but maximize interior passive heat gain, which is slightly reduced the further to the inside that you place your window.  

Windows can really make or break the comfort and efficiency of your home. Choosing the right windows and installing them for maximum performance and durability (on the south side as much as possible) can make life in your home noticeably more pleasant and save you money.