Insulation The Structure Index
There are three components of heat loss:
- area of surfaces between hot and cold
- insulation value of the barrier
- temperature difference between the two sides
Getting a visual picture of the home’s construction methods and materials is the first step in determining what can be done to reduce energy usage. Every boundary between conditioned space and the outside, is a potential heat loss (gain) area.
For more information on Heat Loss/Heat Gain Calculations.
Major Building Components
The ceiling/roof area is one of the largest potential heat loss/gain areas in the house. Not because heat rises, but mainly because it is the largest single surface area in the house exposed to outdoor temperatures. Ceilings are also the easiest area to insulate both in new construction and in retrofit because they are open and generally accessible.
Before the energy problems of the 1970’s, it was common for homes built before about 1950 to have no ceiling insulation – even in northern climates. Today, it’s pretty rare to find homes with no ceiling insulation.
Ceiling insulation is also the most important area to insulate to reduce summer heat-gain. The roof is a large surface area that collects a LOT of solar energy. As the attic area heats up, it is the ceiling insulation that reduces the flow of heat into the living space.
Therefore, most standards will recommend at least 12″ or about R-30 as a minimum ceiling insulation for all parts of the country.
For more information about Ceiling Insulation.
Exterior walls are the second largest potential heat loss/gain areas due to their size and exposure to winds. To be effective, wall insulation/construction should be both a thermal barrier and a wind barrier.
Houses built before about 1965, even in northern climates, were not built with wall insulation. It is much more expensive to retro-fit wall insulation, but it is a good investment.
Residential walls typically are built from 2x4s, meaning that they have a cavity of 3 1/2″ for insulation. Houses built by more energy-minded builders in northern climates will have 2×6 construction. Additional R-value can be achieved by using a foam sheeting.
Floors and Crawl Spaces
Floor areas over unheated spaces – such as crawl spaces and garages, should be insulated. Slab on grade houses should be perimeter insulated with rigid foam. There is very little benefit to insulating a floor that is more than 1 foot deeper than the frost line.
If the rest of the house is insulated, and the floors are not, then the floors can represent a high percentage of the house’s heat loss. Otherwise, it tends to get low-priority as compared to the other areas of the house. There is negligible heat gain through floors.
For more information about Floor and Crawl Space Insulation.
Basement walls tend to be the most over-looked area of new home construction insulation even today. It is RARE to find basement walls that have been properly insulated – on the outside – at the time of construction. If the basement has been finished into living space, then a small amount of insulation may get added when the interior walls are finished.
Basements can account for up to 50% of the conductive heat loss in a new built home without basement wall insulation. Many new building Energy Codes are now requiring basement wall insulation for this reason.
Concrete is a very poor insulator. It is typical for basements to have at least 1 foot of wall exposed above ground, and some homes, such as those with walk-out basements or sloped grades, can have much more exposure. The ground does provide some insulating value, but only below the frost line. Therefore, more than half the basement walls – which are very large surface areas, are exposed to below freezing temperatures ( in cold climates).
Because most homes in southern and moderate climates do not have basements, this is a non-issue.
For more information Basement Insulation.
Windows and Doors
Windows and doors typically represent a relatively small surface area so their actual R-value is not really as important as the way they are built and how well they stop air infiltration. For example, it would be better to have a fixed single-pane window, than a loose-fitting triple-pane window.
Windows are also a critical factor in heat gain. Up to 50% of the heat gained by a house can be from solar radiation (sun shine) coming through unrestricted windows. Modern windows have ‘low-E’ and other coatings to help reduce the amount of solar gain.
Modern steel doors with a foam core and magnetic weatherstripping can substantially reduce heat loss from both conduction (R-value) and convection (infiltration) as compared to old wood doors.
Source: Text by Bob Fegan 12/2008; Graphic DOE at www.eere.energy.gov/consumerinfo/energy_savers/insulation.html 7/2003