Sash windows open by sliding one or both panels up and down, so they never swing into a room, walkway, or patio. Whether you choose aluminum or vinyl, single hung or double hung, getting three things right will decide how well they perform for years: accurate measurements, careful installation, and simple regular care.
This guide walks you through all three, step by step.
How to Measure for Sash Windows
Accurate measurements are the most important step, because custom windows are built to your exact sizes. A few millimeters can be the difference between a snug fit and a gap you have to fill.
Tools You’ll Need
- Steel tape measure
- Level
- Pencil and notepad
- Step ladder (for upper windows)
Replacement or New Construction?
For a replacement window, you measure inside the existing frame, and the new window fits into it. For new construction or a full-frame replacement, you measure the rough opening in the wall, with the old frame removed.
Step-by-Step Measuring
- Measure the width in three places. Measure from side to side at the top, middle, and bottom of the opening. Record the smallest number.
- Measure the height in three places. Measure from the top to the sill at the left, center, and right. Record the smallest number.
- Check that the opening is square. Measure both diagonals, corner to corner. If they differ by more than about 6 mm (1/4 in), the opening is out of square, so mention it when ordering.
- Measure the depth. Measure from the inside edge of the frame to the outside edge. This confirms the new window will sit properly in the wall.
- Note the sill and surroundings. Note the sill angle, any trim, and anything nearby that could get in the way, like pipes, outlets, or cladding.
- Write it down clearly. Record width × height for each window, labeled by room, such as “Kitchen – left.”
Measuring Tips
- Always use the smallest width and height, never the largest. A window that’s slightly small can be shimmed and sealed, but one that’s too big won’t fit.
- Measure every window separately, even ones that look the same.
- Measure in the same units throughout (millimeters or inches) and say which units you used.
- Not sure? Ask us for a professional measuring visit before you order.
How to Install Sash Windows
A good installation keeps your sash windows level, square, and weather-tight, so they slide smoothly and seal properly. If you’re comfortable with basic carpentry, you can follow these steps. For large windows, upper floors, or structural changes, we recommend professional installation.
Tools and Materials
- Level and tape measure
- Drill/driver and screws (suited to your wall type)
- Shims (plastic or composite)
- Low-expansion window foam
- Exterior-grade silicone sealant and caulk gun
- Flashing tape
- Utility knife, pry bar, and safety gear
Step-by-Step Installation
- Check the new window. Before removing anything, confirm the size, color, glass, and opening direction match your order, and check for any shipping damage.
- Remove the old window. Take out the old sashes and frame carefully. Clean the opening of old sealant, nails, and debris.
- Prepare the opening. Make sure the sill is level and solid. Apply flashing tape to the sill and sides to keep water out of the wall.
- Dry-fit the window. Place the window in the opening to check the fit. You should have a small, even gap all around for shims and foam.
- Set, level, and shim. Center the window and place shims at the corners and behind the fixing points. Check that it’s level, plumb, and square, comparing both diagonals.
- Fix the frame. Screw through the frame at the fixing points into the wall. Don’t overtighten, as this can bow the frame and stop the sashes from sliding.
- Test the sashes. Before sealing, slide each sash fully up and down and lock it. It should move smoothly and close evenly. Adjust shims if it sticks.
- Insulate the gap. Fill the gap between frame and wall with low-expansion foam. Avoid regular expanding foam, which can push the frame out of shape.
- Seal inside and out. Run a neat bead of exterior silicone around the outside of the frame. Finish the inside with sealant or trim.
- Final check. Test the sashes and locks again, remove protective film, and clean the glass and frame.
Aluminum vs. Vinyl: Installation Notes
- Aluminum: Use fixings and sealants compatible with aluminum. Avoid direct contact with materials that can cause corrosion, such as treated lumber or some metals.
- Vinyl: Vinyl expands and contracts with temperature, so leave the recommended expansion gap and don’t overtighten screws.
Common Mistakes to Avoid
- Installing the frame out of level or square, which makes sashes stick or leak
- Overtightening screws and bowing the frame
- Skipping the flashing tape at the sill
- Using high-expansion foam
- Sealing over the weep holes (drainage holes) at the bottom of the frame
Sash Window Care & Maintenance
Aluminum and vinyl sash windows need very little upkeep. A few minutes of care every few months keeps the sashes sliding smoothly, the seals tight, and the glass clear.
Cleaning the Glass
Clean with a soft cloth or squeegee and mild glass cleaner or soapy water. On double-hung windows with tilt-in sashes, unlock the sash, release the tilt latches, and tilt it inward to reach the outside glass safely from indoors. Avoid abrasive pads, razor blades, and ammonia-based cleaners on coated (Low-E) glass.
Cleaning the Frames
Wipe the frames with warm water and a little mild soap, then dry them. Don’t use solvents, bleach, or scouring powders, which can damage powder-coated aluminum or dull vinyl.
Keeping the Tracks Clear
Dirt in the side tracks is the most common reason sashes stick. Vacuum the tracks with a brush attachment, then wipe them clean. Make sure the weep holes at the bottom of the frame are clear so rainwater can drain out.
Lubricating Moving Parts
Once or twice a year, apply a silicone-based spray (not oil or grease) to the tracks, balances, and locks. Oil attracts dirt and can make sliding worse over time.
Checking Balances, Locks, and Seals
- Balances: If a sash won’t stay up or drops on its own, the balance may need adjusting or replacing.
- Locks: Check that locks engage fully and that screws are tight.
- Seals and weatherstripping: Look for gaps, cracks, or flattened seals. Replace damaged weatherstripping to stop drafts.
- Exterior sealant: Inspect the sealant around the frame and reseal any cracked or missing sections.
Aluminum vs. Vinyl: Care Notes
- Aluminum: In coastal areas or near salt air, wash the frames more often to prevent salt buildup and protect the powder coating.
- Vinyl: Never paint vinyl frames, and keep grills and heat sources away from them.
Seasonal Maintenance Checklist
- Spring: Clean glass, frames, and tracks; clear weep holes; check seals after winter
- Summer: Lubricate tracks and locks; check that sashes slide smoothly
- Fall: Inspect weatherstripping and exterior sealant before cold weather
- Winter: Wipe away condensation; check for drafts around sashes and frame
Frequently Asked Questions
What’s the difference between single hung and double hung sash windows? On a single hung window, only the bottom sash slides; the top is fixed. On a double hung window, both sashes slide, giving better ventilation and easier cleaning.
Can I install sash windows myself? Yes, if you have basic carpentry skills and the right tools. For large or upper-floor windows, or if the opening needs structural work, professional installation is safer and protects your warranty.
Why is my sash window hard to open? Usually it’s dirt in the tracks or a lack of lubrication. Clean the tracks and apply silicone spray. If it still sticks, the frame may be out of square or the balance may need adjusting.
How often should I maintain my sash windows? Clean the glass and frames every few months, clean and lubricate the tracks once or twice a year, and check seals each fall.
Need Help With Your Sash Windows?
A1 Doors & Windows makes custom aluminum and vinyl sash windows to your exact measurements. Not sure about measuring or installation? Contact us for a free estimate and our team will help you get it right.
