A Room by Room Heat Audit Before Replacing Windows in Austin
A house in Austin does not heat evenly. The rooms facing west cook from three in the afternoon until sundown, the north bedroom stays easy, and a kitchen with a big sliding window can run noticeably warmer than the hall beside it. Before any window gets replaced, a room by room audit tells which openings carry the load and which can be left with simpler glass. The steps below follow that audit in the order it is usually done.
Why audit room by room instead of ordering one glass for the whole house?
One spec across every window is easy to order and wasteful in practice. A strong solar coating on a shaded north window dims the room for no gain, and a basic coating on a west picture window leaves the sofa baking. Austin makes the gap wide, with highs of ninety degrees or more on well over a hundred days a year and a house orientation that can vary from one lot to the next.
The audit takes an afternoon with a notebook, a hand thermometer and a phone. Each window gets a line: direction, size, shade, and what happens in the room around it at three, four and five o'clock.
What goes into the audit?
Five things, in this order.
- Direction the wall faces, read with a compass app rather than guessed from the street.
- Glass area of the opening, since a wide slider takes in far more sun than a narrow double hung.
- Shade from trees, a porch roof, a neighbor's house or a deep eave.
- Surface temperature of the glass and the sill in the late afternoon, taken with a hand thermometer.
- What the room is used for, because a home office with a screen needs glare control that a hallway does not.
How do the rooms usually sort out?
The table below shows the pattern that shows up on most Austin houses. It is a starting point, and each row gets adjusted by what the afternoon walk shows.
| Wall and room | What the sun does | Glass priority | Frame note |
|---|---|---|---|
| West living room or den | Flat, hot sun from mid-afternoon to dusk | Lowest shgc, strongest soft coat low e | Light colored vinyl window or fiberglass window |
| South kitchen or dining room | High summer sun, low winter sun under the eave | Low shgc, overhang helps | Casement seals tighter than a slider |
| East bedroom | Bright, mild morning sun | Moderate shgc, clear glass | Double hung is fine |
| North study or hall | Gentle, even light | Lower u factor, lighter coating | Wood window or composite frame suits it |
| Bay window or bow window | Several faces at once | Spec each face separately | Check the weight of the head and seat |
Which number on the label matters in which room?
The shgc, short for solar heat gain coefficient, leads on west and south walls. A lower figure means less of the sun's heat gets in. The u factor leads where the wall sees little sun and the winter chill matters more, which means north rooms and any room over a garage. A freeze arrives roughly every two years, so a very low u factor is a modest priority overall, and it is worth chasing only where the wall needs it.
Visible transmittance comes third. It shows how much daylight passes, and a strong coating can lower it. A west den with a television wants low glare, while a north study that depends on daylight does better with a clearer glass.
Does the frame change the audit?
Yes, though less than the glass. A dark clad window on a west wall gets hot to the touch and passes some of that heat inward. An aluminum window with no thermal break moves heat through the frame in both directions, and that shows up as a warm sill in July and a cold sill in a freeze. Vinyl, fiberglass and composite frames keep a steadier surface temperature, and a wood window with a clad exterior sits between them.
Older houses in Clarksville and Hyde Park often have narrow openings in thick walls. Those can limit the frame depth, which matters if a triple pane is on the list, since three panes need a deeper sash than two.
What about the gap around the window?
An audit that looks only at glass misses the largest leak. The gap between the frame and the wall framing, plus the casing and the sill pan, decides how much hot air walks in around the unit. A hand held near the trim on a windy afternoon often feels moving air even on a new window. Shims, foam, sealant and flashing tape all belong in the notes.
Clay soils east of the Balcones Fault shift with wet and dry seasons, and that slow movement racks an older frame out of square. A window that binds or a sash that sticks in only one season points to that. Squareness gets checked on every opening before a new unit goes in, because a new window in an out of square opening seals badly no matter how good the glass is.
How do neighborhoods and lots change the picture?
Mueller and the newer subdivisions have young trees and little shade, so most walls take the full sun. Tarrytown and Travis Heights have large old trees that shade some walls through July and leave others open. A lot near Shoal Creek or Barton Springs may sit in a hollow with more humidity and some natural shade. Hill country lots west of the escarpment sit higher with more open sky, and limestone walls there hold heat into the evening.
These differences are why two houses on the same street can need different glass on the same compass side. The audit records the lot, not just the orientation.
How should the results turn into a window list?
Sort the notebook into three groups. Group one holds the hot windows, usually west and south with large glass areas, and gets the strongest low e coating and the lowest shgc. Group two holds the average rooms, which take a balanced coating with a good u factor. Group three holds the shaded and north windows, which get the lightest coating and the best u factor the wall can use.
Each group then gets a frame choice, a gas fill and a spacer. An argon fill and a warm edge spacer are the usual pairing across all three groups, since the added detail is small compared with the result. Having only three glass specs keeps the order simple while still matching each room to its sun.
What can go wrong if the audit is skipped?
The common failures are all mismatches. A house gets one coating everywhere and the north rooms feel dim. A west wall gets the same basic glass as the east wall and the afternoon heat stays. A sliding glass door gets a window spec that is too light for the load, and the frame bows in the first hot summer. Each of these is easy to avoid on paper and hard to fix after the install.
A written audit also gives the installer something concrete to follow, and it makes later seal failure or fogging easier to trace to a specific unit and wall.
When is the best time to do the audit?
A clear day in late summer gives the clearest picture, with the sun at its strongest and the heat load obvious. A second pass on a cold morning after a front shows the winter side, with the cold spots and the condensation lines. Both passes together take a few hours and replace a great deal of guesswork.
The audit turns a vague complaint about a hot house into a short list of windows with a reason attached to each, and that list drives every later choice of glass, frame and install order.