Insulation Systems with Good "Whole Wall" R-Values

Claudette Reichel  |  7/17/2008 2:44:54 AM

Figure 1, Radiant Heat Barrier Under Roof

Figure 2, Cellulose in Wall

Figure 3, Drainable Wall

Firgure 4, Raised Top Plate

Figure 5, Three Stud Corner with Drywall Clips

Figure 6, T-Wall Options that Allow Insulation

R-value is standardized measure of a material’s resistance to heat flow — the higher the R-value, the greater the insulating ability. However, the actual R-value of a wall or ceiling can be a lot less than the R-value of the insulation material in it, depending on the installation. For the ENERGY STAR label, homes are inspected at the open wall stage (or tested with an infrared camera) to receive full credit for proper insulation installation.

Whole wall R-value is a concept that takes into account the gaps in insulation (at windows, doors, studs, voids, etc.). Although homeowners may not be able to determine their whole wall R-values easily, the concept is helpful in comparing the actual energy performance of options. Insulation should surround all conditioned space (except a slab on grade in the Deep South) with as few gaps as possible.

Recommended Insulation Material R-values: Recommended R-values are based upon cost analysis of insulating areas of the building envelope in relation to the benefits in a climate region. As energy costs rise, so do the R-value recommendations. Recommended insulation levels for steel may not perform as well as recommended levels in wood-frame buildings, but the cost of achieving the same performance would outweigh the benefit. Also note that the R-value measurement standard assumes no air leakage through the insulation. Air currents in standard density fibrous batts and loose fill materials reduce their insulating value.

Installation: Compressing (squeezing) insulation erodes its R-value and should be avoided. Batt and roll insulation should be slit and trimmed to fit around wiring, electrical boxes, etc. If using paper-faced batts, staple the paper on the ends, not inside, the studs. Insulation should be trimmed to fit into voids around rough openings, chimneys, etc. Even a 2% gap makes a difference in the rating of a home, and the compression of insulation can make it almost useless. In climate zones 1 and 2, unfaced friction-fit batts are recommended.

Radiant barriers: A radiant barrier under the roof decking (foil side down) can block 95% of the roof’s radiant heat. This is most beneficial when attic insulation levels are R-19 or less, or when the air conditioner or ducts are located in the attic. A radiant barrier may produce little energy savings when the roofing already has a heat reflective coating or is a light-colored metal or tile or when the HVAC is within the conditioned space and the recommended level of attic insulation is installed. Certain low-e paints and coatings can perform as radiant barriers, although they are typically less effective at blocking heat than foil materials.

Attics with ridge and soffit vents are ideal for radiant barriers, maintaining an air flow and allowing built-up heat to escape. Proper installation is critical. Detailed information about radiant barriers is available from the Florida Solar Energy Center at www.fsec.ucf.edu and the U.S. Dept. of Energy at www.orhl.gov(Figure 1, Radiant Heat Barrier)

Foil-faced sheathing or a radiant barrier housewrap behind brick veneer walls can provide a worthwhile radiant barrier benefit, particularly on west and east sides. It can help block the heat radiated by the sun-heated bricks, with the added benefits of a vapor barrier that can resist water vapor diffusion into the wall from the high vapor pressure drive of a rain-wetted, sun-heated brick veneer.

A radiant barrier in either location should have a shiny side facing a vented air space of at least one inch and an emissivity rating of 0.05 or less. In climate zone 2, foil faced plywood or OSB sheathing can be used, yet a perforated foil is suggested to allow some drying to the outside in cold weather. In cooler climate zones, the foil should be on an insulating material to prevent moisture condensation problems; a foil-faced rigid foam insulating sheathing is a good choice.


Characteristics of Insulation Materials

Type of Material

R-value/ inch of Thickness

Other Characteristics

Fiberglass, mineral wool

Inexpensive. R-value eroded by air currents, compression. If flooded, must replace. Noncombustible.

- batts, rolls (low-high density)

2.9 - 4.3

Kraft paper good seasonal vapor retarder. Suitable for do-it-yourselfer.

- loose fill, “Blown In Blanket” (BIB)

2.3 – 3.1

Good coverage (BIB for walls, roof).

Cellulose

Less air movement & erosion of R-value. Sound absorbing. Increases moisture buffer capacity of wall. Borate fire retardant may deter insects, fungi. If flooded, must replace.

- loose fill

3.4 - 3.7

Good coverage. Can settle & lose R-value.

- damp spray

3.5 – 3.8

Good coverage, performance. Let dry before installing drywall. Stabilized to prevent settling.

Rigid foam boards

As sheathing, good seasonal vapor retarder and reduces thermal bridging. If sealed, can provide exterior air barrier. Preferred for steel framing, masonry construction. Can withstand flooding.

- expanded polystyrene (EPS, “beadboard”)

4.0

Semi-permeable without skin. Least expensive type of rigid foam. Available foil-faced (for radiant barrier).

- extruded polystyrene (XPS)

5.0

Stronger than others. Low permeability; impermeable with skin.

- polyisocyanurate (ISO board)

5.6 – 6.5

More R-value in less space. Typically foil-faced; so provides vapor barrier & radiant barrier ( blocks heat and vapor drive-from brick veneer)

Spray foam

Excellent air barrier, seals penetrations. Uniquely well suited for insulating unvented attics, band joists, gaps in thermal/air barrier envelopes.

- closed cell, high density (urethanes)

5.6 – 6.3

New types have no CFCs. Cures to semi-rigid, adds strength, floodproof. Semi-permeable. Can withstand flooding.

- open cell, low density (polyicynene)

3.6

Stays flexible. Permeable. Can absorb water but dry quickly.

Radiant barriers

varies with use

Requires vented air space. Most effective under roof deck. Insulation still needed; blocks only radiant heat, not conduction.

Autoclaved aerated concrete (AAC)

1.1 – 1.4

Provides structure & thermal mass benefit. Noncombustible.

Continuous coverage to reduce thermal bridging: Non-insulation materials, even wood framing, create a thermal bridge, allowing greater heat flow through the wall. Materials that are good conductors of heat (such as metal framing) can substantially erode the effectiveness of insulation. Building systems that reduce thermal bridging, provide a continuous “thermal blanket” surrounding the conditioned space and therefore preserve a higher “whole wall R-value” include: using insulating foam sheathing in addition to wall cavity insulation, OVE (24 inches on center), SIPS and ISPS (foam core panels), ICF (foam forms and concrete) and AAC (insulating concrete).

Insulation systems with additional advantages: Some types of insulation systems offer additional benefits, in addition to their R-value, that can make a real difference in overall energy-efficiency. These properties should be factored into your cost-benefit comparison.

  • Kraft paper on fiberglass batts provides a vapor retarder that adapts to the seasons, which is advantageous in mixed climates. Unfaced, friction-fit batts are preferable in the hot-humid climate zone.
  • Loose fill insulations provide good coverage without the extra efforts of slicing to fit around wiring or plumbing and trimming to fill gaps and voids.
  • Blown-in-blanket (BIB) is a loose fill material held in place (in walls or cathedral ceilings) by netting, providing good coverage in irregular spaces, nooks and around obstructions.
  • High density damp spray cellulose in walls and attics provides good coverage like other loose fill products, less air flow through the insulation so it retains its rated R-value, increases the water storage buffer capacity of the wall and provides sound insulation. Its borate fire retardant used in many brands also has insect- and fungi-resistant properties. (Figure 2, Cellulose in Wall)
  • Rigid foam can be used as exterior sheathing, in SIPS, ISPS, ICF and other systems. It provides high R-value per inch, full coverage without gaps or compressions, a vapor retarder that can adapt to the seasons (see Durable/Moisture Control segment) and, when sealed, an air flow retarder. Rigid foams (especially closed cell extruded polystyrene) can withstand being submerged in water are well suited to wall assemblies designed to be cleaned and may not need replacement in the event of flooding (a “drainable, dryable wall”).  (Figure 3, Drainable Wall)
  • Spray foams create very effective air sealing, which can justify the higher cost. They are especially well suited to insulate and air seal unvented conditioned attics, band joists, irregular and hard to access spaces, holes and voids.
  • High density (closed cell) foam is considered flood resistant. It can be used to partially fill wall cavities to achieve R-13 or higher while allowing space to drain, flush and dry wall cavities after flooding. (With flood and fungi-resistant exterior sheathing and interior wallboard such as paperless, moisture resistant drywall and treated sheathing, this “drainable, dryable wall” system may withstand flooding and reduce the need for gutting and replacement of materials if mold can be controlled).

Framing details: Advanced framing details provide a much more continuous insulation system than traditional framing techniques and prevent insulation voids that can result in moisture problems.  (Figure 4, Raised Top Plate)

Attic insulation should extend over the top of exterior walls without blocking air flow from the soffit vents. This can be done with a raised top plate or raised heel truss to increase the roof height at the eave. In hurricane zones, check to make sure this raised top plate assembly is wind code compliant.

Exterior walls with headers over door and window openings should be framed to include insulation in the headers; double header lumber can be placed on the exterior side to leave space for rigid insulation on the interior side.. Various methods have been developed to provide continuous insulation in exterior wall corners and at T-walls (where an interior wall intersects an exterior wall), which are normally blocked by framing lumber. (Figure 5, Three-Stud Corner with Drywall Clips), (Figure 6, T-Wall Options that Allow Insulation)

Doors and Windows: The biggest insulation gaps in the thermal envelope are windows and doors. Choose ENERGY STAR labeled units.

Insulating and sealing attic access doors over conditioned space is particularly important. If possible, locate the attic access in an unconditioned space and include a lock on it for security. Various types of attic access door covers are now commercially available to cut air leakage and provide some insulation.

Insulated doors have a skin and insulating foam core, creating a much higher R-value (R 7 to R 13) than a solid wood door (about R-4). Steel skin doors are economical, durable and efficient. Fiberglass skin insulated doors can be stained and have the look of genuine wood, with many beautiful styles suitable for front entry doors.

Glass in both windows and doors should be insulated (two layers with an air space) and low-e with framing of wood, vinyl, composite material or metal with a thermal break. (See Windows section)

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