Safety Measures for Stormy Windows

July 01, 2005

A version of this article appears in the July/August 2005 issue of Home Energy Magazine.

Q. I am trying to find out how much fresh air is required to be introduced to an apartment by means of a forced-air furnace. I understand that the manufacturer will provide a spec with his furnace. However, I would like to know if there is some sort of chart showing how much fresh air to introduce in homes in the state of Washington.
If we don’t introduce enough, we run the risk of poor indoor air quality. If we introduce too much fresh air, are we going to decrease the efficiency of the furnace?

       A.The furnace itself will not introduce air into your home, but an air inlet in the return duct will. I assume that this is what you are talking about, because your state code requires ventilation air to be distributed, and using your duct system is the easiest way to do this.
        The state of Washington has some specific air flow requirements: 0.35 ACH or 15 CFM per person. Sometimes the number of people is taken by default to be the number of bedrooms plus one. A typical 1,600 ft2 house has a volume of 12,800 ft3. To achieve an air change rate of 0.35 per hour requires about 75 CFM (or about 4.6 CFM per 100 ft2 of floor area). A two-bedroom house would require about 60 CFM of ventilation air.
        Another point of reference is ASHRAE Standard 62.2 “Ventilation for Acceptable Indoor Air Quality.” This standard says that you should have 1 CFM for every 100 ft2 of floor space plus 7.5 CFM per person (this is half the Washington state code requirement). So a 1,600 ft2 house with three occupants needs 1,600/100 + 7.5 X 3 = 16 + 22.5 = 38.5 CFM of continuous ventilation.
        Not introducing enough fresh air, as you wrote, could harm indoor air quality. Introducing too much fresh air will not much change furnace efficiency directly, but it costs money and energy to condition the fresh air, so this contributes to your heating bill.

Safety Measures for Stormy Windows

        Q. In the sidebar “Better Glass for Coastal Homes” the (HE Mar/Apr ’05, p. 25) the use of shatter-resistant film was briefly mentioned as an option in retrofitting options for impact-resistant windows.Will shatter-resistant film really offer the storm protection needed during high-wind events?

       A. This is not a simple subject, but let me try to explain it.
        In single-family homes, and many multifamily homes and condominiums, the existing, non-impact-rated windows andor glass doors generally cannot be retrofitted to meet current impact standards and building codes. They can only be replaced or covered with impact-resistant products to be brought up to current code.
        The use of safety and security films does reduce flying glass shards if the window glass is broken during a hurricane. Unfortunately, this is perhaps the least of your concerns. The truth is that some of the testing that has been done on safety and security films for storm protection is being misrepresented as meeting the impact standards for windows installed in the lower 30 ft of buildings. In these cases, the impact standards all call for large-missile testing and rating for a product to be used. The large missile is a 2 X 4 piece of lumber ranging in weight from 4 to 9 lb, depending on the design wind speed of the location where the product is to be used.
        Many of the safety and security films are tested for small missile impacts, which consist of steel ball bearings striking the glass or product. These tests are reserved for windows that are located 30 or more ft above the ground only, where large-missile impacts are unlikely. In addition, the glass that is frequently used in the window film tests is 3/16 inch thick tempered glass installed in a commercial window frame—not what you usually find in most homes.
        Therefore, the test conditions are not applicable for the windows installed in our homes, because the wrong test is being performed on a different type of window and glass. When homeowners have safety and security films installed on their windows, they may get a false sense of security and protection from storms.
        Installation methods vary from installer to installer, and while some very specific installations may perform in a given storm, none of them meet the large-missile test standards that are required by the Florida Building Code.
        The bottom line is this: To meet the current code in Florida in the Windborne debris region, there are two options for existing buildings. First option: The glazed openings could be protected with an impact resistant and rated product. Second option: The entire unit—frame and glass—would have to be replaced. Installation must be per the manufacturer's installation instructions and the product approval test report. That also means the building structure (rough opening) must be substantial enough for the new fenestration to be anchored adequately.