Why Installing Windows & Doors Correctly is so Important

Window & Door openings are breaches in a building envelope that need to be carefully managed in order to prevent moisture damage and air leakage in homes as well as to save energy. Water leaks from around windows and doors are the visible result of a poor fitting or sealing job, but that's far from where it ends. If there are air leaks from around windows and doors, not only is a home inefficient - ie. It's losing energy - but it's susceptible to condensation forming around these areas creating moisture that can be hidden within the wall framing or insulation and causing problems later.

Moisture & more specifically humidity is the number one cause of degredation in North American homes, because as timber is a hygroscopic material, once the lumber in a typical house wall reaches a humidity level greater than 15% the chances of insect or mold attack increase significantly. This should be kept in mind when installing windows & doors in stud walls and it's a little more complicated than sticking on flashing or basic tape, liberally applying the silicone sealant or squirting in the ubiquitous can of "not so" Great Stuff to make a connection that is neither airtight or healthy - with the foam residue and waste chemicals produced left to bioaccumulate too.

why not to spray foam windows with not so great stuff
Why not to install windows and doors with spray foam...

When installing windows and doors in a home, it's essential to put comprehensive strategies in place to manage moisture in all forms, being air leakage, condensation on windows which can be hard to stop, wind-driven rain & snow, as well as a straight-forward downpour, in addition to being as Eco-Friendly as possible in the process.

Preventing Water Leaks from Windows & Doors

One thing is certain, water only runs downhill - unless that is it has a reason to do otherwise. When installing windows and doors the first basic rule is to "think like water" - ensure that all openings are flashed and taped appropriately and to code so that water that wants to run downhill can do so rather than accumulate and that it is directed towards the exterior of the wall. Depending on local climatic conditions, you then have to seal around openings to allow for capillary action combined with wind pressure or by sealing for wind driven snow. What we've generally found is that if a homes structure is comprehensively Air-Sealed then it's generally speaking water-sealed too. For the best tips on water sealing windows and doors see below, and be sure to watch the EcoHome "How to Seal Windows & Doors correctly" video Guide at the bottom of the page. 

Stopping Air Leakage around Doors & Windows

High Efficiency buildings like LEED certified, Passive House, ZNE, Passive Solar, Zero Energy Homes or even Positive Energy Homes all rely on the building envelope having a high level of insulation, and a managed level of Air Transfer Rate, preferably via an HRV or ERV unit to control the energy losses from essential ventilation. You can read more about Air Permeability of homes here, but to put it simply let's quote from the US Department of Energy when they state "the average home has the equivalent of a 2ft square hole in it." The next time someone in your home complains of feeling a draught or if your utility bill is burning a hole in your pocket, if your doors and windows weren't properly sealed, remember that 2ft2 hole in your outside wall!

We're not helped in North America either because most homes are stick-built and as wood is a living material it does move with changes in the weather and over time. Succesfully sealing windows against air-leaks relies on flexible seals being fitted between the frames and the framing, often called Air Sealing System Tapes, which are part of an effective Exterior insulation and finish system (EIFS). We should also remember that doors and windows by their very nature move if not fitted tightly, though that doesn't necessarily mean fastened tightly. Some level of flexibility is essential, and many products on the market also help with water sealing too - effectively killing 3 birds with one stone - but as with most building materials it's important to be using the right product in the right application. Most specifically, the moment you're installing high performance windows and doors then you really need to budget sufficient rolls of vapor open, exterior air sealing fleece-backed tape with full-surface adhesive into the equation. See the Top Tips and video below for specific details, and consider subscribing to EcoHome as a free member as we often have member-only offers on Eco-Friendly EIFS products.

How to Install Windows & Doors Correctly

The following window and door installation tips will improve any building structure, whether a new home build, or a DIY home renovation. All the same, as EcoHome is all about LEED, Passive House, ZNE, Passive Solar or Zero Energy Green Home Construction, we can say definitively that the tips below become even more pertinent the higher-performance the home is designed to be - and especially important if using the latest generation suspended-film insulating-glass glazing units and windows.  

Window flashing detail in a high-performance home with exterior insulation  © Glen Sakuth
  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.
  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.
  4. 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 insulation (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.
  5. 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.
  6. 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. 
  7. Use High Performance insulating window bucks instead of regular window bucks made of ordinary lumber - especially when the wall assembly includes a continuous external rigid insulation layer.

How window placement effects thermal bridging

Thermal bridging around windows and doors results in loss of energy, cold spots, condensation points and potential wall damage. To demonstrate the importance of window and door placement relative to the wall's exterior, we ran some thermal transmission tests with the following results.   

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 - in cold climates this translates to more heat in your house and more money in your pocket, or in warm climates, more unwanted heat outside and a lower load on the aircon unit, also saving money.

The images below simulate an exterior temperature of 0 °C and an interior temperature of 20 °C, with a triple pane window. In the first diagram the window is installed flush to the exterior of the insulated wall, with a heat flow of 7.9W. The second diagram 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 flush mounted window frame © Denis Boyer for Ecohome

Thermal bridging through window frames
Thermal bridging through inset mounted window frame © 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 North American or 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.

Correct Choice & Fitment of Doors & Windows makes a measurable difference

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 in cold climates, or north side in warm climates) can make life in your home noticeably more pleasant, save you money and keep wall structures in better condition, longer. To discover how to choose the best windows for your house, see here or see here to know which type of window frame to choose between wood, aluminum or PVC / Vinyl 

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Top Tips: For Well Fitted & Sealed Windows & Why It's Important:

  • Attention to Detail is essential when fitting windows and doors - for all the reasons above
  • Use the Right Specialist Materials as using the best materials available ensures good sealing that lasts
  • If we were talking about sealing a Boat we wouldn't have to stress the point, trust us, without properly installed and sealed windows and doors, your home's performance is also sunk!

Further reading for water & air sealing & building envelope permeability in homes: