The concept of conduction is how we normally think of heat loss/gain in a home. When you put your cold hands around a hot cup of coffee, you’re using the principle of conduction to warm your hands. The transfer of energy via conduction takes place directly through a material from molecule to molecule. The resistance to this heat flow is measured by the R-value in the insulation and other materials that we use to construct the ceiling, walls, and floors of our homes. The higher the combined R-value of the materials we use, the better the resistance to heat gain and loss.
That sounds straight forward and simple at face value, but it can be misleading when we are comparing different construction systems and choices. First of all the ASTM tests used to rate a material’s published R-value are not real world. The tests used tend to favor permeable insulations such as fiberglass over rigid insulation systems, because they DO NOT take into account the real world issues of air and moisture infiltration into the wall and ceiling cavities which can seriously degrade the published “laboratory” R-values of permeable insulation products.
The other misconception about R-values is that they only provide information about the “insulation cavity” and do not take into account the “whole wall”. For example, about 20 to 25% of the area of a typical framed “stick-built” wall is comprised of wood studs (~ R1 per inch) that have no where near the R-value of the actual “insulation”. In addition, the insulation has to fit around pipes and wires and be custom cut where the stud spacing varies from the standard 16” or 24” on center framing. This is why it is difficult to compare a house framed with 2×6’s and R-19 fiberglass insulation to a house “framed” with Structurally Insulated Panels (SIP) with 3.5-inches of R-14.9 expanded polystyrene (EPS) rigid insulation. If we just consider the R-value of the insulation the two systems the 2×6 wall appears to be better. However if we look at the true “whole wall” performance as tested by the Oak Ridge National Laboratory [ORNL], the R-value of the 2×6 system is only 11, and R-value of the SIP system is 13.95.
When we’re talking about conduction in the exterior envelop of a home, we’re basically talking about insulation. And the purpose of insulation is to trap and confine “dead” air. To do that effectively an insulation has to be sufficiently dense and completely fill the insulation cavity or space it is designed to insulate. For illustration purposes the the attached pdf file compares various insulation choices and their strengths and weaknesses. The insulation types described are typically used in stick built construction (which is used in about 80% of homes built in the U.S.) and provides a good decision matrix for that type of construction.
Fiberglass and the other (air) permeable insulation materials dominate stick built construction, but construction is no different than other disciplines and professions and we tend to do a lot of things because “we’ve always done them that way.” So I think it would be helpful if I listed all the things we’ve added over the years that add cost and complication, just to work around the weaknesses of permeable insulation materials:
- Added a plastic moisture barrier on the warm side of the envelope to help keep moisture from getting into the insulation cavity that would cause mold and/or rot
- Added a house wrap like Tyvek on the outside of wall to partially mitigate air infiltration and intrusion. Note: The primary purpose of a house wrap is to provide a moisture barrier, they only reduce air infiltration by a little over 10%.
- Purposely over sized framing members in the wall and ceiling to provide sufficient thickness for code mandated R-values.
- Added vented soffits and roof vents to bleed moisture out of ceilings and attics, again to prevent mold and rot.
- Added rubberized membranes on the roof deck from the eave to beyond 24” of the exterior wall to mitigate the effects and damage of ice dams. Why? Because of the inherently poor insulation at the intersection of the wall and the ceiling provided by fiberglass and other permeable insulations.
- Routinely install larger than necessary heating and cooling systems to make up for poor envelop performance
If you want to build a net zero energy home with a “tight” envelop, avoid permeable insulation materials and use a foam-in-place, closed cell polyurethane insulation with a “green” blowing agent.
Download Residential Insulation Comparison Table [pdf file]
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Sustainable Dwelling 