Framing Double Windows

Framing double windows on your home will both help to keep it warm in the winter yet cool in the summer, whilst also greatly reducing your fuel bills. The simple fact of the matter is that glass is an excellent conductor of heat, allowing the heat in warm air to pass through it.

You can prevent that heat loss by adding a second, inner, pane of glass, creating an air space between the two sheets of glass. This layer of air prevents the heat from being transferred to the outer pane of glass, helping to keep your property warm. Using wood, framing double windows is no different to framing a regular single pane of glass window. However, fitting the glass into the double frame is different.

Window Details

Window Cross Section DetailsYour window needs to meet certain construction standards and codes. One of the conventions builders follow is to make the tops of the windows align with door openings. To accomplish this, the bottom side of the window’s head (the top part of the frame) should be at a distance of 6 foot eight inches from the finished floor surface.

The distance from the floor to the top of the sill (the bottom part of the frame) will depend on the size of the window glass, and the particular room and architectural design. Typically, this distance will be 1 foot for a living room, about 2 foot six inches for a dining room, and about 3 foot six inches for a kitchen.

A good portion of the actual construction of a window is hidden from view, so sectional views are used to illustrate the details of a window and how they are assembled. Details of construction vary among manufacturers and window models, therefore, when framing for a window, refer to the manufacturer’s product literature for the actual specifications, and examples of how the window will interface with the wall structure.

Window Framing

Openings in the wall of a house, whether balloon or platform framing, call for a permanent horizontal support beam at the top of the opening, called a header. The header is required for directly carrying the structural load which the which vertical studs in the space would have carried.

Your local building code will specify the size required for the header; it varies depending on the width of the opening. A good header can be built by sandwiching 3/8 inch plywood between two pieces of 2 inch dimensional lumber. For larger windows spanning more than 8 feet however, a pre-manufactured laminated beam member is stronger, more durable and less trouble.

Each end of the header beam is supported by vertical members called jack studs and king studs. A rough sill at the bottom of the opening does not carry structural loads but helps to anchor the window unit. The sill sits on cripple stud vertical members. The actual size of the rough opening should be 1 inch wider and one half inch taller than the window unit to be installed, including jambs; this provides room for adjustment during installation.

Removing an Old Window Frame

With a hammer and masonry chisel chop away the rendering from around the window jambs to expose the existing wooden frame. With a saw cut through the old window frame about 4 inches above the sill on both sides. You should now be able to lever out the old frame uprights and its head with a crowbar. Make a cut through the center of the window sill and window board, then again lever them out with the crowbar, you should find that the jamb studs will also come out at this point.

Fitting a Double Window Frame

Place the new frame into position and check that it is upright and level with a spirit level. At the top and bottom of the new frame, secure it into position using wedges. Before proceeding any further check that any casements will open properly, this will be a test as to whether or not it is correctly leveled and upright.

If you’re satisfied all is well, either hammer in 4 inch masonry nails or screw the double window frame into position; you need to make at least one fixing about four inches from each of the corners along the uprights and frame head.

Mix some render and tidy up the area surrounding the frame, when this is dry you can fit the new window sill. Apply an adhesive to the tongue and grooved edge and push it into position, then fix it securely with screws. On the inside a window-board can be cut and either nailed or screwed into place. Finally add any window handles and locks.

Glazing Double Windows

The one thing you mustn’t do here is use putty in a double window frame. The glass in double window frames needs fitting with special glazing strip systems. These are time consuming to use as well as awkward to work with if you’ve not been trained to use them.

Talk to your double window frame supplier or DIY store and they’ll almost certainly be able to offer you window units that are already glazed and just require fitting to your new double window frame. Pre-glazed double window units for casements and pivot window units are always the easiest to fit for the DIY/home enthusiast.

Double window frames can also be purchased in PVCu, Steel or Aluminum. PVCu and Aluminum ones are extremely low maintenance and do not require being regularly treating or painted. However, both Steel and Aluminum can be a source of heat loss from inside your property and so are probably best avoided. PVCu is an excellent material for double window frames, but lacks the natural look of wood; which is of course the sustainable option for double window frames.