A key part of my professional home inspections here in Ottawa involves a close check of the windows, especially those that have been replaced. You see, there are two approaches to this. These are called ‘full frame’ and ‘inserts’.
The ‘full frame’ method requires the removal of ALL the old window. This includes the complete outside frame of the window. What you are left with is what’s called the rough opening, the entire hole in the wall where the original window was installed.
The ‘window insert’ approach to window replacement involves the removal of only the inner part of the old window, leaving the framing of the previous windows. The new windows are then inserted into the framing of the previous windows.
The preferred approach is ‘full frame’. The ‘window inserts’ type of installation may not be as reliable as removing the entire previous window and installing the new window into the original rough opening. There are a number of reasons for this. First, with the inserts, you are left with two seams that have to be sealed and can potentially leak. These are the seam between the wall and the original outside frame, plus the seam between the original outside frame and the newly inserted window frame. That makes it twice as hard to get a good seal around the window.
The second issue with ‘window inserts’ is the fact that the new window is now smaller than the original one. This means less light and less ventilation. The impact is not overly great. It is generally less than a 10% reduction.
Finally, with ‘window inserts’, it is vital that the old window frames be sound and solid. It is very foolish to install new windows into rotted frames (see the picture). You loose any benefit that the new window can offer when the frame that it is inserted into does not provide solid support or a reliable weather barrier.
So, when you go ‘full frame’ with your window replacement, you get to remove all the old window. This gets rid of any old rot. It also allows you to see into the surrounding wall to see if there is any rot or mould concealed. This lets you do any needed clean-up or repair so that the wall can accept the new window and provide reliable support and performance.
I hear you thinking, if ‘full frame’ window replacement is so much better, then why would anyone go with ‘window inserts’? The short answer is to save time and money. ‘Full frame’ windows have to be custom fabricated to make sure they are sized correctly to properly fit into the rough opening in the wall. This generally costs more. And because they are custom built, they take longer to get. With ‘window inserts’, stock windows are used. These are readily available. They are also mass produced, so they tend to be less costly.
Whenever I do a home inspection, I always check to see what type of window installation has taken place. If I see ‘window inserts’ I take that much extra care to ensure they provide a positive weather seal. I also check the surrounding frame to make sure it is in good condition, and providing reliable support and weather performance.