On those frigid winter days when gusts pound the windows and the only song that comes to mind is “Baby, It’s Cold Outside,” the term “R value” can be provocative.
In construction speak, R value is how well your insulating material — whether pink or yellow fiberglass batting, cellulose, rock wool or spray foam — is doing to keep out the cold.
“It’s the resistance to heat flow,” said Peter d’Entremont, an architect with Knight Associates Architects in Blue Hill. “The higher the R value, the greater the effectiveness of the insulation.”
But that, he said, is only part of the story.
First, Insulation 101.
Heat flow has three basic mechanisms — conduction, convection and radiation.
Conduction is the way heat moves through materials. Visualize a spoon in a hot cup of coffee with the heat moving up the spoon into your hand.
Convection is the circular motion that happens when warmer air or liquid — which has faster moving molecules, making it less dense — rises, while the cooler air or liquid drops down.
Radiant heat travels in a straight line and heats anything solid in its path that absorbs its energy.
In winter, heat in a home flows directly from all heated living spaces to adjacent unheated attics, garages, basements and even to the outdoors.
Heat flow also can move indirectly through interior ceilings, walls and floors, wherever there is a difference in temperature.
The largest heat loss in modern, well insulated construction comes from air changes — opening the doors and windows.
The better the insulation, the lesser the heat flow, the lower the energy costs.
The next question becomes what insulation materials have the best R value and which are most effective overall in keeping the heat in and the cold out.
Top of the list, said d’Entremont, is closed cell spray foam insulation. It also is the most expensive.
“Closed cell foam tackles the three methods of heat transfer rather well,” he said.
Closed cell spray foam insulation is composed of isocyanate and polyol resin and sprayed onto roof tiles, concrete slabs, into wall cavities or through holes drilled into a cavity of a finished wall.
It has an R value of R-5 to R-6 per inch. In contrast, blown-in fiberglass has an R value of R-2 to R-4 per inch.
Other insulating materials are good at resisting conduction and convection, but have no reflective, or radiant, attributes unless they contain a reflective foil, d’Entremont said.
Falling into this category are open cell foam insulation, fiberglass, rock wool, which is made up of spun fibers from rocks, and cellulose, which is chopped waste paper treated with fire retardant.
Which is not to discount the value of the insulation materials listed above.
“The ability to stop infiltration is as important as R value,” d’Entremont said. “They stop the air well depending on their density.”
Another issue in deciding on insulation is controlling moisture within a home. Not doing so can lead to mold.
D’Entremont said the goal should be to control moisture, not stop it.
Moisture can come from simply breathing, cooking, taking showers.
Warm air holds more moisture than cold air. If the moisture is allowed to cool then it condenses into liquid. If warm enough, the condensation leads to mold.
The worst case scenario, d’Entremont said, is mold growing inside a wall that can’t be seen but that rots the outer sheathing and structure.
“The tighter we get our houses, the more important is moisture control,” he said. “So now there is more ventilation. You don’t want a house to be as dry as a desert or as humid as the tropics.”
Many builders are now looking at “robust” walls with insulation that stops the vapor from passing through, but also gives the insulation material a chance to dry out.
“There is no perfect answer,” he said.
One newer approach to insulating a home, ‘net-zero’ provides most of the home’s energy needs naturally through solar and other alternative energy sources.
Once homeowners select a type of insulation and have it installed, d’Entremont suggests having a blower door test done before Sheetrocking to detect any residual air infiltration.
A blower door is a powerful fan that mounts into the frame of an exterior door. The fan pulls air out of the house, lowering the air pressure inside. The higher outside air pressure then flows in through all unsealed cracks and openings. The auditors may use a smoke pencil to detect air leaks.
Another option is what is known as a hybrid system combining spray foam with an overlay of fiberglass.
“It’s called ‘flash and fill,’” said d’Entremont. “That’s been a promising approach.”
Yet another option is to simply build thicker roofs and walls in order to add more insulation.
D’Entremont shared a chart for a hypothetical, 2,400-square-foot house with a glass area equal to 10 percent of the floor area, or 240-square-feet.
“That’s a normal amount of glass for a house with some nice views and/or a sunny orientation,” he said.
The chart shows the distribution of heat loss over one year with the building insulated to the current energy code.
The greatest loss of heat, he said, is glass, followed by subsurface losses through an insulated basement slab and insulated foundation walls.
“The house is assumed to be quite tight so the next largest losses are through the walls and roof,” d’Entremont said. “Ironically, those losses combine for a bit over a third of the heat loss, yet most of our attention is usually focused on them.”
He said the best strategy in this case would be address the quality and quantity of windows plus the amount of insulation in the foundation.
The gold standard is net-zero energy use with a combination of good air control and insulation combined with a renewable energy source.
He used as an example one house the firm built with a small thermal collector for hot water on the house roof and a fairly large photovoltaic collector on the garage roof.
The house is heated and cooled with a modestly sized ground source heat pump. All the energy for heating, lights and appliances is produced at the house.
“This is the basic strategy for all net-zero houses,” d’Entremont said.
“Insulate well, a tight house, good ventilation, an efficient heat source, all of which drive the energy budget to a minimum,” d’Entremont said. “Then you supply the energy needed with solar [or wind] power that is usually grid connected so that you can bank your power generation from month to month.”