A government report and three short terms of weatherization

Posted Wednesday, October 24, 2012 in Sustainable Maine

A government report and three short terms of weatherization

by Paul Kando

According to the Energy Information Administration’s just released Short-Term Energy and Winter Fuels Outlook, heating costs will be higher this year than last, both because of higher energy prices and more sustained, colder weather anticipated. If forecasts are accurate, this will be the most expensive winter ever for heating oil users.  Heating costs for fuel oil user could increase as much as 32 percent from last year. Propane and electricity prices are forecast to drop some. Still total expenditures projected for all fuels will rise.

Unless it is your habit to read government reports, you are probably unaware of all this. Yet sticking with the status quo means higher and higher fuel bills year after year. But should the prospect of higher costs motivate us to improve the energy efficiency of our homes, the benefits are likely to extend far beyond personal savings. If we schedule an energy audit, followed by efficiency improvements, everybody wins: the homeowner saves money, the citizen helps    reduce oil dependency and the sending away 78 cents of every dollar spend on oil, depriving the local economy. The investor locks in a guaranteed return through annual savings. And, as a bonus, we will improve our comfort.

These columns aim to demystify the process. Here are three terms begging for clarification:

“Thermal bridges”: All materials have their own unique value as insulators or conductors. The common way to define these properties are the U value, a measure of heat transmission; and the R value, denoting the ability to resist heat transmission. Mathematically U and R values are reciprocals, i.e. R=1/U and U=1/R.  Windows are usually characterized in terms of U values, building materials and components in terms of R values. R values, unlike U values, are additive – making it possible to express the thermal resistance of an assembly (like a wall) as the sum of its component R values.

Most American single family houses are wood framed structures. The framing members (wall studs, window and door headers, floor and ceiling joists, etc.) and the inside and outside sheathing (boards, plywood, sheetrock) enclose cavities filled with insulation. Insulation R values range from R-3.2 to R-7.5 per inch thickness. In contrast the R value of softwood framing is only about R-1.4 per inch. Therefore, while the insulated portion of a wall assembly may have a value of R-13 to R-21, at the studs and other framing members the assembly’s value may only be  R-7 to R- 9.8 –about half that of the insulated sections. For this reason, framing members of a cavity-insulated building are considered to be “thermal bridges”. Other thermal bridges in a building include window and door frames.

“Wind washing”: Air filters of furnaces and air conditioners are usually made of glass fibers. They are easily washed and/or are inexpensive enough to be disposable. The glass fibers of these filters are similar to the fibers that make up fiberglass insulation - fiberglass batts are heavy duty air filters. This is why unfaced fiberglass insulation, no matter how thick, is no substitute for air sealing.

In the attic fiberglass batts are good insulators but only when properly installed. If air currents – a characteristic of ventilated attics – can enter and “blow through” part of the fiberglass layer, the  insulating value (based on trapped air) is degraded. E.g., a 12" R-38 fiberglass batt penetrated by air currents to a debt of 2" has an R value of only of only R-32, one penetrated 4" deep less than  R-26.

While it can also inside leaky walls, wind washing is a problem mostly in attics. The cure is to keep the wind out. In attics, this may be done by an air barrier (like a board) surrounding the edges of the fiberglass layer.  To prevent wind washing by air currents penetrating from above, it is best to have a layer of cellulose (loose fill or batt) atop the fiberglass.     

“Dew point temperature” is the temperature at which condensation of moisture contained cooling air occurs. Since the warmer the air the more moisture it can hold, we speak of relative humidity – the percentage of the maximum amount of water vapor the  air could hold at a given temperature. As the air cools, the moisture condenses or precipitates. Rain and snow are precipitation, so is morning dew. The dew point corresponds to a given temperature and relative humidity. For example, 80ºF air at 60% relative humidity corresponds to a dew point temperature of ~67ºF. In other words, condensation will occur on any surface having a temperature of 67ºF or cooler.

That cool surface may be window or it may be inside a wall penetrated by the warm moist air through leaks, such as unsealed electric outlets. When this happens, the condensate is likely to rot-damage the structure. It can also cause mold and mildew, not to mention large heat loss as the condensing water vapor gives up the energy it absorbed when it vaporized in the first place.

It is therefore critically important that warm moist air (of any temperature) is prevented from entering a wall cavity.  Failing that, the interior surface of the outer sheathing of your walls must never be allowed cool down to the dew point temperature. This is why air sealing is so important.

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