Builders Share: Window Jamb Extensions

One of the principal reasons for gutting Chez Melendy was to improve its capacity to hold heat, ie. R-factor. There was absolutely no insulation in the entire house. It's amazing that the prior residents were able to keep warm in these northern winters. It's safe to say, they spent a lot of money on fuel. I'll post later on how I went about insulating the house but today I'll write about creating window jamb extensions that became necessary as a result of adding R-factor.

Adding insulation meant increasing the thickness of the walls 2 1/4". This presented the problem where the original window jambs and sill were shy of the new interior wall surface by that amount. In order to mount new casings and sill, I had to make up the difference with extensions.

I decided to attack the problem with extending the sill first. The sill is the thick piece of wood that the window sash rests on. It's mounted at a slight angle so that it will shed water towards the outside. Besides supporting the sash, the sill supports the "stool" which is the finish piece most people call the window sill.

The main problem was finding a way to fasten the extension to the existing sill. Because of the 2-1/4" thickness I needed to find a fastener that was long enough to anchor it securely while at the same time not splitting the wood. At first I tried mounting several thinner pieces of wood to pad out the sill but found this to be time-consuming, and the final result weak and wobbly. However, once I hit on the idea of drilling counter-sink holes half-way through the extension piece and then using woodscrews to secure it I was in business.

This method also worked for securing the jamb extensions. Again I needed a way to anchor one thin piece of wood to another using a fastener that was at least 2" long and not splitting the wood in the process.

I made jamb extensions out of the same pre-primed finish stock that I used for the door jambs. I used a 3/8" bit to drill a counter-sink hole about an inch into the extension piece and then used a 2-1/4" wood screw to anchor it to the jamb.

When it comes time to trim out the casing, I may need to plug the holes I made. Some wood dowels should work for that if needed. It will depend on how I decide to finish the casing.