Insulating Roofs, Walls, and Floors

Choose the Right Insulation for Each Part of the House

UPDATED 4/12/2013

Bird's-Eye View

Most homes need more than one type of insulation

In most climates, insulation is needed on all sides of a house: under the slab or lowest conditioned floor, at the basement walls, at the above-grade walls, and at the ceiling or roof.

Because different insulation materials are good at doing different things, it makes sense to choose insulation based on the job it has to do. Heat flow is only part of the equation; moisture, air leakage, and drying potential are also important considerations.

See below for:
GRADING INSTALLATION QUALITY
ABOUT INSULATING FOUNDATIONS
ABOUT INSULATING ABOVE-GRADE WALLS
ABOUT INSULATING FLAT CEILINGS WITH ATTICS ABOVE
ABOUT INSULATING ROOFS AND SLOPED CEILINGS


Foundations

Basements, crawlspaces, and slabs need the least amount of insulation

It's important to insulate the below-grade portions of a house, unless these spaces are entirely outside the home's thermal envelope.

Air leakage is usually not a major issue for below-grade insulation. Moisture resistance, however, is critical.

Concrete slabs and concrete foundation walls can be insulated from the inside or the outside. Once a foundation has been backfilled, however, options narrow, and interior insulation is usually the only option.

Most foundations are insulated with one of three types of foam: XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest., or closed-cell spray polyurethane. Polyisocyanurate can be used to insulate the interior side of basement walls.

ABOUT INSULATING FOUNDATIONS


Walls

Above-grade walls are limited by cavity thickness

Almost any type of insulation can be used to insulate a wall. If cavity insulation is installed—for example, fiberglass batts, sprayed fiberglass, cellulose, or spray polyurethane foam—the maximum R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. will depend on the wall thickness.

You can improve the performance of any type of cavity insulation with one or more layers of exterior foam.

ABOUT INSULATING ABOVE-GRADE WALLS


Flat Ceilings

Flat ceilings are the easiest and cheapest to insulate

While there are many possible ways to insulate a flat ceiling—that is, the floor of an unconditioned attic—it's hard to beat the advantages of cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection.. Cellulose is inexpensive, effective, and green. And because it's cheap, you can afford to pile it on.

Because of the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season., air sealing is vital to the performance of insulation. Ceilings must be carefully detailed to limit exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. of heated air.

ABOUT INSULATING FLAT CEILINGS


Sloped Ceilings and Roofs

Sloped ceilings and roofs can make room for more living space

Insulating sloped ceilings and roofs can be a challenge, especially if the rafters aren't deep enough to fit the required amount of insulation. One solution to the shallow-rafter problem is to install a continuous layer of rigid foam insulation beneath the rafters.

A layer of rigid foam insulation—whether installed on the interior side of the rafters or above the roof sheathing—is also an effective way to address the problem of thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the rafters.

Designers of homes with insulated sloped ceilings have to settle two questions:

  • Where will the insulation go?
  • Will the roof be vented?

There is no right answer to these questions. As long as durable details are included, insulation can go above or below the roof sheathing; vented and unvented roofs can both perform well.

For more information on insulating sloped ceilings, see:
How to Build an Insulated Cathedral Ceiling and

  • Creating a Conditioned Attic.
  • ABOUT INSULATING ROOFS


    RELATED ENCYCLOPEDIA ARTICLES

    Insulation Overview

    Insulation Choices

    Batt and Blanket Insulation

    Blown-In or Loose-Fill Insulation

    Rigid Foam Insulation

    Spray Foam Insulation

    The Building Envelope

    Air Barriers

    OTHER CONSIDERATIONS

    Vegetative roofs

    Vegetative (green) roofs can contribute to the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of a roof assembly, although not as much as many people think. The R-value of soil varies, but averages about R-0.12 per in. (R-1.4 per ft.). Although lightweight soil with added perlite or vermiculite has a somewhat higher R-value, it’s safe to say that almost any type of conventional roof insulation will be cheaper per unit of R-value than a vegetative roof.

    Should foil-faced bubble wrap be used under slabs?

    In a word, no. Many bubble-wrap manufacturers have been caught making deceptive marketing claims, so builders should consider any claims with caution.

    It's important to remember:

    • Radiant barriers are ineffective unless they face an air space. Compressed bubble wrap installed between soil and concrete does not contain a meaningful air space.
    • Slabs lose heat to the ground below by conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., not radiation.
    • Before it is compressed under a slab, bubble wrap has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of about R-1 or perhaps R-2—far less than the minimum R-value recommended under a slab.
    • Many foil-faced bubble-wrap products cost more per square foot than 1 in. of XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. foam. Of course, the foam has a much higher R-value.

    ABOUT INSULATING ROOFS, WALLS, AND FLOORS

    It's not unusual for a house to have three or four types of insulation: spray foam, loose fill, rigid foam, and/or batts. Each type has multiple uses, but most also have limitations on where they can be used.

    The best insulation for each location depends on a number of factors, including cost, ease of installation, available space, and the material's resistance to moisture.

    All insulation types perform best when they're installed well. Some (like batts and blankets) can lose significant R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. with even a slightly sloppy installation.

    Grading installation quality
    The Residential Energy Services Network (RESNET), a national association of home-energy raters, long struggled with the question of how to estimate the R-value of walls that vary widely in performance depending on the skill of the insulation installer. Eventually, RESNET developed a useful rating system for insulation installation quality. The system is described in an article published in the January/February 2005 issue of Home Energy magazine, “Insulation Inspections for Home Energy Ratings,” by Bruce Harley. The RESNET rating system recognizes three levels of insulation installation quality: Grade I, Grade II, and Grade III.

    [Click to enlarge]

    “In order to qualify for a Grade I rating, insulation must … fill each cavity side to side and top to bottom, with no substantial gaps or voids around obstructions (that is, blocking or bridging—as seen in the grade II photo below), and it must be split, or fitted tightly, around wiring and other services in the cavity. In general, no exterior sheathing should be visible through gaps in the material,” Harley wrote. “Compression or incomplete fill amounting to 2% or less of the surface area of insulation is acceptable for Grade 1, if the compression or missing fill spaces are less than 30% of the intended fill thickness (that is, 70% or more of the intended insulation thickness is present).”

    [Click to enlarge]

    The standard for a Grade II installation is somewhat lower
    “A Grade II rating represents moderate to frequent defects: gaps around wiring, electrical outlets, plumbing, other intrusions; rounded edges or ‘shoulders,’ larger gaps, or more significant compression. No more than 2% of the surface area of insulation missing is acceptable for Grade II.”

    Grade III installations are the worst
    “A Grade III rating applies to any installation that is worse than Grade II.” For further information on the RESNET grading system—including illustrations of good jobs and sloppy jobs—see “Assessing the Quality of Insulation Installed in New York Energy Star Labeled Homes.”

    ABOUT INSULATING FOUNDATIONS

    Basements

    Because foundations aren't really exposed to vast temperature swings, less insulation is needed there. Insulation in a basement should be chosen to do more than slow the flow of heat through these relatively stable environments; the best choices of basement insulation stop air and water, too. Basement walls and floors can be insulated on the inside or the outside, inside being the easier method for retrofits and outside being easier (in general) for new construction.

    Exterior insulation choices should be moisture tolerant
    Below-grade walls can be insulated on the interior or exterior. If you choose exterior insulation, you can use EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest., XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., closed-cell spray foam, or rigid mineral wool. Because polyisocyanurate can absorb water, it should not be used under a slab or on the outside of a foundation. Polyisocyanurate performs well, however, when used on the inside wall of a basement or crawlspace.

    The most common insulation under slabs is XPS, although EPS also works if its density is adequate and if it is rated for ground contact. If the insulated slab must bear heavy loads, XPS is usually a better choice than EPS.

    Closed-cell spray polyurethane foam can also be used under a slab.

    To insulate a basement wall from the inside, the foam should be applied directly to the concrete, in order to keep moist interior air away from the cool, damp surface and lower the risk of condensation. Any of the following insulation materials can be used on the interior of a basement wall: closed-cell spray polyurethane foam, polyisocyanurate, extruded polystyrene (XPS), or expanded polystyrene (EPS). To meet code requirements for a thermal barrier, the foam will probably need to be protected with a layer of gypsum drywall; fiberglass-faced drywall is more moisture resistant than paper-faced drywall. In most code jurisdictions, Thermax brand polyisocyanurate meets requirements for a code-required thermal barrier without the need for drywall.

    Under no circumstances should fiberglass batts be used to insulate basement walls. Because fiberglass batts are air-permeable, they are unable to prevent moist interior air from contacting colder basement walls. That's why fiberglass-insulated basement walls can easily become damp and moldy.

    For more information, see How to Insulate a Basement Wall.

    Crawlspaces

    Although some builders insulate the floor above a crawlspace (the crawlspace ceiling), most building scientists recommend building a sealed, insulated crawlspace that includes wall insulation. It usually requires less insulation (and involves fewer tricky details) to cover a short wall around the perimeter than the whole floor.

    Sealed crawlspaces should be built and insulated exactly like basements.

    Of course, a well-detailed insulated crawlspace needs more than just insulation. Among the other critical details are careful air-sealing of the rim-joist area (click photo to enlarge) and (if the crawlspace has a dirt floor) installation of a ground cover.

    Slabs on grade

    Some builders insulate slab perimeters without insulating under the slab. In all but the warmest climates, however, it's better to install a continuous layer of EPS, XPS, or spray polyuyrethane foam under the entire slab. Some builders modify an ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure. for use as a form for the slab that includes insulation.

    If the home has in-floor radiant heat, it's especially important to include a thick layer of foam directly under the entire slab. Experts disagree on exactly how much foam to add, but they all agree that at least some is a good idea. Engineer John Straube of Building Science Corp. says that after about 4 in.—perhaps 6 in. if the slab includes radiant heat—the money is better spent elsewhere. However, PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. builders sometimes install up to 14 in. of sub-slab insulation.

    Soil has a measurable R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. , so it can insulate the bottom of the slab from the exterior air to some extent. But soil is also a nearly infinite heat sinkWhere heat is dumped by an air conditioner or by a heat pump used in cooling mode; usually the outdoor air or ground. See air-source heat pump and ground-source heat pump.. The average soil temperature varies depending on the climate and the soil depth; however, if the soil has an average temperature of 55°F and the interior of a house has an average temperature of 72°F, heat will always want to flow from the warm side of the slab toward the soil. That's why it's important to insulate under a slab.

    ABOUT INSULATING ABOVE-GRADE WALLS

    The strategy adopted for insulating a home's above-grade walls depends on the wall construction used.

    • Walls built from SIPs or ICFs already include insulation.
    • Concrete-block (CMUConcrete masonry unit. Precast concrete block used to build walls. CMUs have hollow cores that can be filled with concrete onsite for additional reinforcement. The use of stronger, more lightweight types of concrete such as autoclaved aerated concrete (AAC) is becoming increasingly popular in CMU manufacture. ) walls are best insulated from the exterior with rigid foam or spray polyurethane foam.
    • Wood-framed walls can be insulated with cavity insulation (fiberglass batts, sprayed-in-place fiberglass, cellulose, or spray polyurethane foam), on the interior (with rigid foam board), on the exterior (with rigid foam board or spray polyurethane foam), or with a combination of approaches (for example, some cavity insulation and exterior foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. ).
    • Existing buildings with load-bearing brick walls are a special case; the careless installation of insulation on the interior of such walls can permanently damage the building. For more information, see Insulating Old Brick Buildings.

    Thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel.
    The effective R-value of a framed wall assembly with cavity insulation is always less than the R-value of the insulation alone, as thermal bridging through the studs degrades the performance of the wall. Thermal bridging can be reduced, and the thickness of the wall increased, by:

    • adding foam sheathing to the exterior of the wall;
    • adding a layer of rigid foam under the interior drywall; or
    • building a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. with staggered studs.

    Foam sheathing
    The performance of any wood-framed wall will be improved by installing exterior rigid foam sheathing; the usual choices are XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. or polyisocyanurate. Although EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. can be used, it is more fragile than the other two options.

    Adding foam insulation to the outside of a wall affects the wall's ability to dry out when it gets wet. Different types of foam insulation have different permeance ratings, but after a few inches they're all pretty impermeable to moisture. Most foam-sheathed walls are designed to dry to the inside. This means that interior plastic vapor barriers should never be used on foam-sheathed walls.

    According to Joseph Lstiburek and Peter Baker of Building Science Corp. (see link below), adding 1 in. of R-5 insulation to a 2x6 wall insulated with fiberglass batts increases the effective R-value of the wall from 14.4 to 19.4, a 35% gain with only a 15% increase in wall thickness.

    Adding 2 in. of foam raises the R-value from 14.4 to 23.8, an improvement of 65%. A layer of insulating foam on the outside of walls also reduces the risk of condensation by raising the dew point of the surface where water vapor is likely to collect.

    Thick foam sheathing is safer than thin foam sheathing. To learn more about determining a safe thickness for exterior foam, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    ABOUT INSULATING FLAT CEILINGS WITH ATTICS ABOVE

    Flat ceilings under unconditioned attics can be insulated with fiberglass batts, blown fiberglass, or blown cellulose, but cellulose works best—especially in very cold temperatures when convective loops can degrade the performance of fiberglass. Regardless of the type of insulation used, more is always better, and it's usually an inexpensive upgrade as space is less of a limiting factor than it would be for walls.

    Spray polyurethane foam can also be used to insulate a flat ceiling, although at a much higher cost than cellulose. An advantage of spray foam is that it air-seals as it insulates. With all types of attic insulation, air-sealing before insulating is almost more important than type and depth of insulation.

    Attic-floor insulation should extend over the top plates of perimeter walls. To provide enough room for the necessary depth of attic insulation, be sure to specify raised-heel roof trusses (click drawing to enlarge).

    Locating insulation at the attic floor has several advantages over locating insulation along the slope of the roof:

    • It's cheaper, easier, and faster to install thick insulation at the attic floor.
    • Unconditioned attics are easier to vent than insulated rafter bays.
    • It's easier to detect and pinpoint roof leaks when the attic is unconditioned.

    ABOUT INSULATING LOW-SLOPE (FLAT) ROOFS

    There are several approaches to insulating low-slope (flat) roofs. This type of roof assembly can be either vented or unvented. Insulation can be installed above the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , below the roof sheathing, or in both locations.

    However, unless the insulation installer understands a few building science principles, it's easy for this type of roof to develop moisture problems — so it's a good idea to study up on the peculiarities of low-slope roofs before proceeding. For more information, see Insulating Low-Slope Residential Roofs.

    ABOUT INSULATING ROOFS AND SLOPED CEILINGS

    Sloped ceilings and roofs can be insulated from above (by installing rigid foam on top of the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. ), by installing insulation between the rafters, from below (by installing rigid foam under the rafters), or by a combination of some or all three of these insulation methods. Any of these methods will work. Although installing insulation on top of the roof sheathing is more foolproof, it's also less common.

    EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest., XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., or polyisocyanurate foam can be installed above roof sheathing. Two or more layers of rigid foam with staggered seams can be topped with eave-to-ridge 2x4s to create vent channels, followed by a second layer of roof sheathing. Exterior insulation like this with staggered seams disrupts conductive heat flow through the framing assembly.

    Installing insulation in rafter bays is risky, as interior moisture can migrate through the insulation (either by diffusion or by piggybacking with exfiltrating air) and contact the cold roof sheathing, leading to condensation. This problem can be prevented by using closed-cell spray polyurethane foam, with or without a ventilation channel under the roof sheathing.

    For more information on insulating sloped ceilings, see:

    ABOUT RETROFITTING INSULATION

    Although adding insulation to an existing home is always more challenging than insulating a new home, weatherization contractors have developed many cost-effective methods of improving existing insulation levels.

    It's important to manage any moisture problems in a home before engaging in air-tightening measures or insulation improvements. Inspect the home to identify any leaks or high-moisture areas, and be sure that the home is equipped with adequate mechanical ventilation.

    Among the tried-and-true methods used by experienced weatherization workers:

    • To insulate a basement floor, install a continuous layer of XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. foam on top of the concrete. Top the foam with 2x4 sleepers and a plywood subfloor. If a low ceiling makes every inch critical, the sleepers can be omitted; in that case the plywood subfloor should be mechanically fastened through the foam to the concrete.
    • Basement or crawlspace walls can be insulated with interior XPS, EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest., or closed-cell spray polyurethane foam. The foam should be protected with a thermal barrier (for example, 1/2-in. drywall).
    • Above-grade frame walls can be insulated by blowing dense-packed cellulose into stud cavities through holes drilled through the siding. When insulation is complete, the holes are plugged.
    • If siding is being replaced, rigid foam or spray polyurethane foam can be installed on top of the exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . Exterior foam retrofit jobs require considerable trim work around windows and doors, however.
    • Load-bearing brick walls are tricky to insulate; proceed with caution. For more information, see Insulating Old Brick Buildings.
    • Flat ceilings under unconditioned attics are usually easy to insulate with blown-in cellulose.
    • Improving the insulation over a sloped ceiling is often easier from the exterior than the interior. Rigid foam insulation can be added above the roof sheathing in conjunction with new roofing.

    After air-sealing and insulation work is complete, the renovated home should be tested for radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles.. Radon levels often increase after a home has been weatherized.

    If a house is undergoing extensive remodeling, it's worth considering a deep energy retrofit.

    FURTHER RESOURCES

    Video: Superinsulating a Home With Rigid Foam

    How to Insulate a Basement Wall

    Insulating Low-Slope Residential Roofs

    How to Build an Insulated Cathedral Ceiling

    Creating a Conditioned Attic

    How to Install Cellulose Insulation

    Installing Fiberglass Right

    Insulating Old Brick Buildings

    For more info on roof venting, see the Green Building Encyclopedia page on Attics


    Image Credits:

    1. Fine Homebuilding 160
    2. Roe A. Osborn/Fine Homebuilding #170
    3. Brian Pontolilo/Fine Homebuilding
    4. CIMA
    5. Alex Cheimets
    Tags: , , , , , , , ,
    5.
    Mon, 07/02/2012 - 11:34

    Edited Tue, 07/03/2012 - 08:03.

    Roof insulation
    by Mike McGann

    Martin,
    Great article on insulating cathedral ceilings. I have an existing cathedral roof, unvented. I want to remove the shingles, repair sheathing as needed, install 2 1/2" on rigid polyiso., 1/2" CDX followed by shingles. There is blown in insulation between the rafters. The polyiso. gives me the proper R-value for my Zone 4 location. I am finding that shingle manufacturers are up to date with this method. Any thoughts?
    Thanks,
    Mike


    4.
    Tue, 09/14/2010 - 04:36

    Response to Matt
    by Martin Holladay, GBA Advisor

    Matt,
    1. The best place to post questions is on our Q&A page:
    http://www.greenbuildingadvisor.com/qa

    2. Opinions differ on the matter you raise. However, I believe strongly that fiberglass batts should never be used to insulate a basement wall. The only insulation products I recommend for basement walls are closed-cell spray polyurethane foam or rigid foam insulation.

    If you insist on installing fiberglass batts, then it would be best to choose the unfaced batts.


    3.
    Mon, 09/13/2010 - 22:00

    Basement wall assembly, with closed cell, then fiberglass
    by Matt

    We built stud walls in basement, held 2-3" off the wall. Then sprayed 2"-3" of closed cell foam on these foundation/ basement walls. We were going to leave the stud cavities open, but figured we might as well insulate the cavities now. I would think that paper-less fiberglass would be the best solution for these cavities (affordable, and ok, now that moisture is managed, and that stud cavities are isolated from each other).

    I assume that we would want to allow for maximum inward drying potential (within the cavity), right. So I think i want to use paperless, for less likelihood of mold, though I am concerned that the paperless bats may be hard to keep in place.

    All the details I am finding on this site list the fiberglass as optional but it is not clear to me weather or not having paper on the fiberglass is a good or bad thing, when in this assembly.

    Also I was pushing for the paper less drywall, but GP site lists the mold resistance as the same as that of their mold guard (green board)?

    Any feed back would be greatly appreciated.

    Thanks.


    2.
    Fri, 08/27/2010 - 04:53

    Response to Roger Lin
    by Martin Holladay, GBA Advisor

    Roger,
    It looks like your question has been posted twice. In addition to posting it on this page, you've also posted it on our Q&A page:
    http://www.greenbuildingadvisor.com/community/forum/passivhaus/18064/per...

    Anyone interested in following this topic should probably click on the above link to the Q&A discussion.


    1.
    Thu, 08/26/2010 - 20:38

    Perlite
    by Roger Lin

    I am wondering about whether it may be okay to use perlite as insulation under a basement slab. I am planning a Passive House and under the current plan, I would use 4 inches EPS. According to passive house, the EPS would also be under the footings. I am a little concerned about the structural stability of EPS under the concrete footings. I am considering using perlite granules (R-2.7/in), maybe about 6 inches of it underneath the entire foundation. Are there structural issues to be concerned with?

    Another application is adding perlite into the concrete mix that is to be poured into the ICF forms. In the original plan, I planned to add 3 inches of EPS on the outside of the ICFs, but this makes my walls about 15 inches thick. I am wondering if I can add enough perlite in the concrete to bring the R-value of the concrete to a level that I can skip the additional EPS and again without sacrificing strength.

    Any comments or advice?


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