that meet Prescriptive energy code requirements by using a 'thermally-broken' metal frame (an insulating material separates the outside portion of the metal frame from the inside portion).  These "thermally improved" windows can be more expensive than wood windows.  Most metal frame windows do not meet the standard energy requirements, but are sometimes used in new projects that comply under the Performance energy approach, where a window efficiency "penalty" is offset by another feature that exceeds energy code efficiency requirements, earning an energy "credit" that offsets the window energy "penalty". 

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Low-E glass is specially coated to retard solar heat gain and reduce thermal heat loss.  This glass started to be used routinely by window manufacturers in the late 1990's.  If your windows are dual-pane, but of older vintage, they may not be Low E.  Not having Low E glass will make a modest difference in winter thermal efficiency.  However, if your home experiences hot summer/fall weather, not having Low E glass can make a big difference in indoor comfort and the need to run AC systems - especially when there are large south and west-facing windows (unless they are well-shaded) and skylights. 

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THERMAL INSULATION at ROOFS, WALLS and FLOORS

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When it comes to roof, exterior wall, and exposed floor insulation, the most important part of a residence is the roof.  Roof area often exceeds wall areas; perhaps more importantly, because heat rises, the warmest indoor air rises to the underside of the ceiling; thus there is greater potential heat loss at the ceiling/roof plane than at other exterior surfaces.  If a roof is under-insulated, adding insulation here is a priority.  While an uninsulated floor is not as important as the roof, adding floor insulation (above a crawl space) is relatively easy to do.  Uninsulated walls are of moderate concern (except in very hot and very cold climates), and are difficult to add insulation to (compared to accessible attic spaces and under-floor crawl spaces).  Wall retrofit insulation is installed either by removing the wall's inside or outside finish materials, or, more typically, by blowing insulation into each stud bay by making holes in either the inside or outside of the walls.  An insulation contractor can provide more information on retrofit insulation options and costs.

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More considerations (new construction insulation levels discussed below are energy code Prescriptive type compliance values):

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• CONSTRUCTION DATE:  For existing residences constructed in 1979 or later, when CA's energy code took effect, it can be assumed that there is at least a modest amount of insulation at roofs, exterior walls, and exposed floors.  Residences constructed before this date were not subject to an energy code, so insulation may or may not exist.  From my experience, roof insulation was common by the 1960's, wall insulation and floor insulation common by the 1970's.  That said, even homes built in the 80's and 90's may have less than ideal roof insulation levels.

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• ROOF INSULATION RECOMMENDED AMOUNT:  For new construction, the energy code calls for R-38 insulation in warmer locations (e.g. east of the East Bay hills).  Where there are ducts in attic space, this insulation would be at the ceiling plane, and the code calls for additional insulation at the roof plane. For areas west of the East Bay hills (e.g. Oakland, San Francisco) R-30 insulation is sufficient.  Portions of the South Bay, and portions of the North Bay, are in either R-38 or R-30 required zones.  Where homes have attic space, R-38 is recommended regardless of location because there is little cost difference between R-30 and R-38.  For existing homes, if the existing insulation is at least R-30, it may not pay to improve it.  If the home has a vaulted ceiling roof system, no attic space, and insulation is unknown, consider adding insulation above the roof deck next time the roofing is replaced.

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• EXISTING ATTIC INSULATION R-VALUE?  If the insulation is 'batt' type, the R-value is probably printed on the batt covering.  Whether the insulation is batt, or blown-in, the insulation level is approximately R-4 per inch.  If the attic insulation has no R-value label, simply measure the depth of the insulation near to attic access hatch.  And if the attic access hatch is not insulated, attach some insulation to it.  Thermal energy seeks out the path of least resistance.  Even small uninsulated areas can have relatively large impacts on heat loss / energy use.

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• EXTERIOR WALLS:  For new construction, the energy code typically calls for walls to have both batt insulation between the framing (studs) and continuous insulation outside the studs.  This was a significant change from prior to 2017, when batt insulation alone would usually suffice.  For existing homes, if the walls have insulation, it will not pay to increase the insulation value.  If there is no insulation, I would strongly consider adding blown-in insulation - especially in areas with colder winters and hotter summers (i.e. inland areas).  I previously lived in a 1948 home with no wall insulation.  In the 1990's, we had cellulose insulation blown into the stud bays.  This takes two holes per stud bay, as there is blocking half-way up the wall.  We had the holes made in the outside, which was stucco, so as to not inconvenience us inside.  Admittedly, the stucco patches were noticeable at first.  But with new paint, after some time elapsed, I never noticed the patches anymore.  Other insulation contractors may do a better patching job than what we got. If you are doing a major remodel on the inside, then perhaps making holes on the inside, or stripping off the interior finish in order to install batt insulation, may make sense.

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