Superior Insulation for Energy-Efficient Homes: R-Value, Thermal Bridging and Mineral Wool

What Is Superior Insulation?

The main purpose of insulation is to reduce heat loss in winter and keep excess heat out in summer. Superior insulation is not a single product but a way of building the whole envelope, based on four principles:

• Generous insulation levels in the walls, roof and floor, set for the climate and at least at the level of the local code.
• Continuous insulation over studs, joists and junctions, so heat cannot bypass the insulation through the framing.
• An airtight envelope, with an air barrier whose joints are overlapped and taped, and continued where walls meet floors and the roof.
• Well-insulated windows and doors, because they are the weakest parts of the envelope.

How much is enough depends on the climate. For houses in the northern United States, building scientist Joe Lstiburek of Building Science Corporation has proposed a rule of thumb, the “10-20-40-60 rule”: R-10 under the slab, R-20 on foundation walls, R-40 in the walls and R-60 in the roof, as Green Building Advisor reports. In warmer climate zones, the energy code asks for less, as the table below shows.

What Is R-Value?

R-value measures a material’s resistance to heat flow: the higher the R-value, the better it insulates, according to the U.S. Department of Energy. It depends on the type of insulation, its thickness and its density. Canada uses the metric RSI value (R-value × 0.176), and Europe mainly uses the U-value, in W/m²K. The U-value is the inverse of RSI and measures how much heat passes through, so for U-values lower is better.

One point is often missed: the R-value of the insulation is not the R-value of the wall. Heat flows more easily through studs, joists and other building materials, so the overall R-value of a wall or ceiling is lower than that of the insulation between the framing. This is why continuous insulation matters.

Minimum R-Values by Climate Zone (2021 IECC)

The 2021 International Energy Conservation Code (IECC) sets prescriptive minimums for new homes in the United States, by climate zone. Many states and cities adopt it with amendments, and Canada has its own codes, so the local code always decides.

Thermal Bridging and Continuous Insulation

A thermal bridge is a path through the envelope where heat bypasses the insulation. In a wood-frame wall, the studs, plates, headers and rim joists are the main thermal bridges: wood insulates much less than the insulation between the studs, so heat escapes through every stud. Junctions, where walls meet the roof, the floor or each other, and the edges around windows are other weak points.

Continuous insulation (ci) is a layer of exterior insulation, usually rigid boards fixed outside the sheathing, that runs uninterrupted over the framing. It covers the thermal bridges, and in cold climates it also keeps the sheathing warmer, which lowers the risk of condensation inside the wall. The 2021 IECC wall options for climate zones 4 to 8 reflect this: most of them combine insulation between the studs with continuous exterior insulation, for example R-20 + R-5ci.

Thermal bridges can rarely be eliminated, but careful detailing at junctions, corners and around windows reduces them.

Airtightness and the Blower Door Test

Insulation slows the flow of heat through materials; airtightness stops heat from escaping with leaking air. Leaking air also carries moisture into the walls, where it can condense. An airtight envelope depends on a continuous air barrier: membranes and boards whose joints are overlapped and taped, and that continue across junctions and around windows and doors.

Airtightness is measured with a blower door test. A calibrated fan mounted in an exterior door changes the air pressure in the house by 50 pascals and measures how much air leaks through the envelope. The result is given in air changes per hour at 50 pascals (ACH50). The 2021 IECC limits leakage to 5.0 ACH50 under any compliance path, and its prescriptive path requires 3.0 ACH50 or less in climate zones 3 to 8.

Vapor control is a separate question. The right vapor retarder depends on the climate and the wall assembly, so it is specified for each project’s climate and checked by a local professional. As the Insulation Institute notes, no insulation product alone can prevent moisture problems in a home.

A home this airtight needs a planned supply of fresh air, so we design airtight homes with balanced ventilation with heat recovery. Read more in our article on home ventilation and HRV vs ERV.

Windows and Doors

Windows are the weakest part of any envelope, and large windows are a hallmark of Scandinavian design. A window that meets the 2021 IECC maximum U-factor of 0.30 for climate zones 3 to 8 insulates to only about R-3.3, far less than the insulation in the surrounding wall. The lower the U-factor, the less heat the window loses, and for colder climates the U.S. Department of Energy suggests considering gas-filled windows with low-e coatings.

Triple-pane windows add a third pane and a second gas-filled cavity. This lowers the U-factor further and keeps the inner pane warmer, which improves comfort next to large windows. Whether double or triple glazing is right depends on the climate, the window area and the budget. Our Solaris plan, for example, specifies black aluminum windows with Low-E argon-filled glass and insulated steel exterior doors.

Mineral Wool vs Fiberglass

Mineral wool is our preferred insulation material where it is available. According to the Insulation Institute, mineral wool is made from rock (stone wool) or blast-furnace slag (slag wool), and fiberglass from glass. Both are made by melting the raw materials and spinning them into fibers, and both are sold as batts, rolls and loose fill. On the main performance points, the Insulation Institute rates them alike:

• Thermal performance: any material with the same R-value insulates equally well if it is installed and sealed correctly.
• Fire: both are naturally non-combustible. Stone wool has a very high melting point, over 1,000 °C (about 1,830 °F), according to ROCKWOOL.
• Sound: both have a Noise Reduction Coefficient (NRC) of 1, the top of the scale, which measures how much sound a material absorbs, and the same comparison lists both at STC 43, the rating for how much sound passes through a wall. Keeping outdoor noise out depends more on mass, airtightness and windows.

We prefer mineral wool where it is available; stone wool in particular is non-combustible, with a melting point over 1,000 °C. Where it is not available, fiberglass with the same R-value performs the same thermally, as long as it fills the cavity completely without gaps or compression.

Savings, Comfort and Long-Term Value

For a typical existing home, ENERGY STAR estimates that air sealing and adding insulation can save an average of 15% on heating and cooling costs, or 11% on total energy costs. In a new home, the envelope is designed from the start, which lowers the heating and cooling loads that the equipment is sized for.

The comfort benefit is easy to feel: warmer interior surfaces, fewer drafts and more even temperatures from room to room. Many buyers also value energy efficiency, which can support a home’s long-term value.

How We Design the Building Envelope

We design homes from Brazil to Northern Europe and Canada, so we do not use one fixed specification. The insulation levels, the continuous insulation and the windows are adapted to each project’s site, climate and local code, and a locally licensed professional confirms the specification against the local energy code.

As an example, these are the main insulation layers in the study plan for our Solaris Signature Plan, a single-story home on a slab-on-grade foundation:

• Exterior walls: 2×6 wood studs at 16 in on center with R-21 (RSI 3.7) mineral wool between the studs, plus 1 in of R-5 (RSI 0.88) continuous rigid insulation outside the sheathing. The cladding is fixed on furring strips over a weather-resistive barrier.
• Flat roof: trusses at 24 in on center insulated with R-49 (RSI 8.6) blown-in mineral wool, under elastomeric roofing membranes.
• Slab floor: a polyethylene vapor barrier with sealed joints directly under the 4 in concrete slab, and 4 in of R-20 (RSI 3.5) extruded polystyrene (XPS) under the entire floor area, on compacted granular fill.
• Interior walls: mineral wool in the insulated partitions, which helps reduce sound between rooms.

This is the base specification of the study plan, and the R-values are for the insulation layers only; the whole-wall value is lower because of the framing. Compared with the 2021 IECC prescriptive tables, the walls match the R-20 + R-5ci option for climate zones 4 to 8, the R-49 roof matches the table for zones 2 and 3, and R-20 under the whole slab goes beyond the R-10 slab-edge requirement. For each site, we adapt the layers to the local climate zone and code: for a Solaris build in climate zones 4 to 8, for example, we increase the roof insulation to R-60 and bring all other layers in line with local requirements.

Two of our case studies show this adaptation in practice. For a forest site in a cold Northern European climate, our Solaris case study uses aerated concrete block walls wrapped in continuous rigid foam insulation, an insulated flat roof and triple glazing, and reaches energy class A in the design-stage energy calculation. For a Nordic home in Brazil, the goal shifts from keeping heat in to keeping heat out: continuous exterior insulation, a thicker insulation layer and a light reflective membrane on the flat roof, and double-glazed low-e windows.

Insulation is only one part of the energy picture. Solaris also specifies an energy recovery ventilator (ERV), two high-efficiency split-system heat pumps and solar panels on the flat roof; read more in the benefits of energy-efficient Scandinavian homes. Each of our Signature Plans, such as Aurora and Solaris, shows a projected HERS Index for several US climate zones. For a home designed around your own site, see our custom home design service.