The amount of heat loss in a typical home through the building exterior (excluding windows and doors) is about 40% of the total heat loss. The table below shows typical insulation values of various building components that, when used in unison, achieve the necessary level of super insulation required to meet Passive House standards…
In what climate areas are these insulation values applicable?
These insulation values are climate specific, and in this case are applicable to DOE climate zone 5 as seen in the map below. Climate zone 5 is predominantly a heating climate — the insulation levels should generally increase for more northerly climates and decrease for more southerly climates, but only until the point that cooling loads become dominant over heating loads.
What are the available insulation options?
The following table lists the R-values per inch of various insulation options, and the necessary wall thickness in inches of each option in order to meet Passive House standards for climate zone 5.
Elimination of Thermal Bridges
Thermal bridges are thermal "short circuits" in the various components of the external building envelope. They are areas that are very conductive to heat flow in an otherwise highly insulated building exterior. Prime examples are structural framing members and joints where different construction materials intersect. Besides reducing the building's overall insulation value, thermal bridges are potential locations for condensation, which can lead to mold growth and moisture damage. Elimination of thermal bridges is therefore critical to maintaining building air quality and durability, as well as to achieving the necessary levels of super insulation required for Passive House performance.
What wall systems can achieve the necessary Passive House performance standards, and also eliminate thermal bridges?
Because of the significant thickness of insulation required, none of the common residential wall systems can be used "as-is" to meet Passive House standards, but all of the systems can be modified in some way to be appropriate. Obviously, no single-stud wall system alone using 2x4's or 2x6's is applicable because of insufficient insulation levels and thermal bridging. However, a double-wall system of the proper thickness filled with fiberglass, cellulose, or any of the spray foams would suffice (thermal bridging would also be eliminated). Similarly, TJI joists of the proper depth used as wall studs and filled with insulation would also suffice, provided that thermal bridging was accounted for. Typical ICF (insulated concrete forms) and SIP (structural insulated panel) systems using EPS or XPS do account for thermal bridging, but neither of these systems alone are attractive because products with the thicknesses of foam required (10-12 inches) have limited availability and are very costly. Thinner, standard ICF and SIP products are viable if combined with extra foam or an added stud wall filled with insulation to boost the R-value to the required levels. Similarly, straw-bale wall systems alone do not achieve the necessary R-values, but when combined with an added stud wall filled with insulation become viable. The straw bales also eliminate thermal bridging from the stud wall.
What wall systems do the HarvestBuild Associates recommend, and why?
The HarvestBuild Associates prefer to specify natural building materials whenever possible because of their superior health and environmental benefits. We therefore recommend our Naturally Passive House concept using either our Edge-Plus straw-bale wall system or our Lath-Plus straw/clay wall system. Our Edge-Plus system features an exterior stud wall filled with cellulose insulation and an inner straw-bale wall on-edge, yielding R-values as high as 66. Our Lath-Plus wall system is a typical double-stud wall, but with straw/clay filling the inner stud wall and cellulose filling the outer stud wall and cavity between walls. R-values of 50-60 or greater are easily obtained by varying the thickness of the cavity between inner and outer stud walls.
How do the Naturally Passive House wall systems compare in price to other Passive House wall systems?
For similar R-values, both the Edge-Plus straw-bale wall system and Lath-Plus straw/clay wall system are comparable in price to other stud-framed wall systems with insulation between the framing members. Both, however, are less expensive than any of the wall systems using just foam as insulation.
The Edge-Plus and Lath-Plus wall systems form the basis of the Naturally Passive House concept, and of the many possible Passive House wall systems, are the only two with an emphasis on natural materials. Both of these systems are comparable in price to, or less expensive than, other Passive House wall systems, and both offer significant health and environmental benefits that the other systems do not.