Dense-pack blown-in insulation has two advantages over the more common batt insulation. First, dense-pack naturally fills all gaps and cracks, while hand-cutting batts inevitably leaves voids and squeezing batts into odd spaces leads to compression. Voids and compression increase unnecessary heat loss. Fill walls and floors with dense-pack fiberglass or cellulose to get the necessary insulating value. Dense pack fiberglass insulation or cellulose insulation is considerably less expensive than closed cell spray foam and uses techniques all builders know. Loose fill fiberglass insulation has an R-value of about 3 per inch, while dense pack fiberglass has an R-value of about 4.2 per inch. For example, Owens Corning ProPink L77 has an R-value of R 4.25 per inch. Blown-in cellulose is a good natural, recycled and more sustainable alternative to fiberglass. However, it must be installed at the proper density (3.5 pounds per cubic foot) to avoid settling and it should be protected from moisture with an effective moisture barrier.
Closed-Cell Spray Foam
Closed-cell spray foam insulation has some important advantages. It can achieve the same insulating value in a 6″ wall, as a 12″ wall filled with fiberglass or cellulose and would therefore allow for about 6″ more of additional living space on each side of the house. Closed cell foam, also called high-density foam, is impermeable to water vapor, making it a good choice for unvented attics or crawl spaces. Most importantly, it greatly improves airtightness. However, at current prices for equivalent R-values, it is about two to three times more expensive than dense-pack fiberglass blown in between double offset studs walls and it can have some negative global warming and environmental impact. Depending on local costs and climate, fiberglass or cellulose may be better choices for your overall insulation approach. However in other cases, the unique qualities of spray-foam make it ideal for solving thermal problems or reducing moisture risks at specific locations in the building shell, such as sealing and insulating rim joists in two-story construction.
Rigid Foam Board
Rigid foam board insulation can be used as a reasonable alternative to blown-in fiberglass or cellulose in limited spaces where more R-value is needed. To effectively use rigid foam board, wall, ceiling and floor assemblies should be designed so they can be used most cost effectively. Good applications for rigid insulation include:
- on the exterior of standard walls where added R-value is needed
- above roof sheathing as part of an unvented vaulted ceiling to gain adequate insulation value near the eaves of a low-slope roof instead of raised-heel trusses
- in locations where plumbing or ducts must be placed too close to outside wall sheathing
While there may be some truth to the idea that floors lose less heat than walls or ceilings it’s still essential to the goal of net zero energy consumption to insulate them well. This means attaining roughly the same R-value in floors as in other building components. There are several options for insulating floors:
Crawl Space: In colder climates, the most cost-effective floor system is a ventilated crawl space. Installing 12″ I-joists and blowing in dense pack insulation will bring floors to around R-45. It might be tempting to reduce cost by choosing batt insulation but the many wires and pipes present in most floors make it difficult to install well. In this case, the structural subfloor serves as the air barrier. Most builders choose to carefully seal the perimeter of each floor sheet with construction adhesive. Crawl spaces require foundation vents. It’s common for these vents to be cut through the rim joist where they displace insulation and promote air intrusion into the insulated space. It’s better to block out crawl space vents in the foundation wall where they will not interfere with insulation. If the foundation wall is mostly below grade, a well can be installed.
Insulated Slab: Slab-on-grade floors tend to have fewer air leaks than wood-frame floors, although plumbing penetrations must be sealed. In colder climates, achieving the necessary R-value below a slab floor requires between 8 and 10 inches of expensive extruded polystyrene or high-density expanded polystyrene insulation. Care should be taken to install the same thickness of insulation around the perimeter where heat loss is greatest. Warmer climates may need much less, or even zero, insulation depending on local conditions, making a slab more economical in such climates. Learn more about insulated slabs here.
Insulated Basement: With full basements, below grade walls would ideally be insulated on the exterior to bring the thermal mass of the concrete wall into the building’s thermal boundary. The easiest way to do this would be to build the basement wall with insulating concrete forms. This is likely to be the most expensive option. Alternatively, place two-inch layers of expanded polystyrene against the concrete – staggering the joints) – and then build a 2×4” frame wall with R-21 batt insulation to get a total of about R-38 in the basement wall. Depending on design requirements, it may be possible to insulate the floor above the basement and declare the lower space unconditioned.
HRV/ERV Ducts and Insulation
It may be tempting to run ventilation ducts from HRVs/ERVs through attic spaces or wall cavities where they can interfere with the insulation. The simplest and least costly solution is to add more insulation over the ducts. But a better approach is to design the home so that the ducts are contained within the conditioned space. This can be done with soffits, false ceilings, or insulated airtight chases. Some projects bring the entire attic into the thermal boundary by insulating the roof. A similar approach can be used with an unvented crawl space, although this can be more challenging. Any of these solutions should be integrated at the design phase and analyzed for cost-effectiveness.