Rigid Foam Insulation

Versatile and Effective, Rigid Foam Can Be Used in Walls, Roofs, and Foundations

UPDATED 8/9/2012

Expanded Polystyrene

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. is cheap and effective
EPS is the least expensive and most vapor-permeable of the three types of rigid foam.

One inch of EPS has a permeance of 2.0 to 5.8 perms, making it a semi-permeable material.

R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. : 3.6 to 4.2 per in., depending on density

EXPANDED POLYSTYRENE

Extruded Polystyrene

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. is versatile, tough, and waterproof
Because of its high compressive strength and water resistance, XPS is often used below grade to insulate slabs and foundation walls.

One inch of XPS has a permeance of 1.1, while 2 inches have a permeance of 0.55, making XPS a semi-impermeable material.

R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. : R-5 per in.

EXTRUDED POLYSTYRENE

Polyisocyanurate

PolyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. is the most environmentally benign
Polyiso doesn't use ozone-depleting blowing agents; it uses water.

Because it can absorb water, polyiso is not recommended for use under slabs or on the exterior of foundation walls. The foil facing, however, makes it an excellent exterior drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope. for above-grade walls, as long as seams are taped.

R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. : R-6 to R-6.5 per in. (lower in cold temperatures)

POLYISOCYANURATE

CODE NOTES

Cover your foam inside and out
Section R316.4 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. requires interior rigid foam to be "separated from the interior of a building by an approved thermal barrier of minimum 0.5 inch gypsum wallboard or an approved finish material equivalent to a thermal barrier material..." The only exceptions are listed in sections R314.5 and R314.6. These two sections note that 1 inch or more of masonry or concrete material is an adequate thermal barrier; so is 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. consisting of 15/32-in. or thicker plywood or OSB. Sections R314.5.3 and R314.5.4 explain that when rigid foam is installed in attics or crawlspaces "where the space is entered only for service of utilities," then the foam can be protected by an ignition barrier, which is a less-stringent requirement than a thermal barrier. Acceptable ignition barriers include 1/4-in. plywood, 3/8-in. drywall, or sheet metal.

Section N1101.6.1 of the IRC requires that exterior foam be covered with a "...rigid, opaque, and weather-resistant protective covering to prevent the degradation of the insulation's thermal performance." The covering also has to extend at least 6 in. below grade.

Section N1101.8 of the IRC requires that you post the R-values of the insulation products you use on a house. These R-values should be posted "on or in the electrical distribution panel."

FOAM IS RECYCLABLE

But rarely recycled
Polystyrene foams are thermoplastics, which means they can be melted and recycled into new products at the end of their service life (although very little of the foam is actually recycled).
Read more in this GBA news story.

PolyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. is a thermoset plastic, which can’t be recycled.

Bracing Foam-Sheathed Walls

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. can replace plywood or OSB on a framed wall as long as the builder ensures that the walls are adequately braced to prevent racking.

The four most common methods are:
1. Diagonal 1x4 let-in braces
2. Diagonal L-profile steel strapping like Simpson RCWB
3. Inset shear panels
4. A few strategically placed pieces of OSB—usually installed at corners.

A useful guide to bracing foam-sheathed walls, "IRC Wall Bracing," is published by the Foam Sheathing Coalition.

IS POLYSTYRENE MORE POISON THAN IT'S WORTH?

A recent feature in Environmental Building News has raised concerns over the negative environmental effects of brominated flame retardants. One of these flame-retardant chemicals, HBCD, is added to 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. and 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. insulation. Because HBCD is bioaccumulative and potentially toxic, some green builders avoid the use of XPS or EPS where better substitutes exist.

For more information, see "Polystyrene Insulation: Does It Belong in a Green Building?"

ALTERNATIVES TO RIGID FOAM

Builders looking for a rigid insulation product that isn't made from plastic or petroleum might be interested in investigating cork insulation panels or semi-rigid mineral wool panels.

Although cork and mineral-wool panels usually cost more than rigid foam and probably aren't available at your local hardware store or lumberyard, either product can be used on the exterior side of wall sheathing, just like rigid foam.

ABOUT RIGID FOAM INSULATION

Rigid foam can be used in walls, roofs, and foundations, for retrofits or new construction. Most varieties of foam have a higher R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch than fiberglass, cotton, or cellulose.

There are three main types of rigid foam insulation: expanded polystyrene (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.), extruded polystyrene (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.), and polyisocyanurate. Key differences are R-value per inch, water resistance, compressive strength, permeability to water vapor, facings, and of course, cost. All foam insulation products are petroleum-derived.

Rigid foam sheets are sold in several thicknesses; most lumberyards carry insulation ranging from 1/2 in. to 2 in. thick. Thicker sheets (up to 6 in. thick) are usually available by special order.

If you are looking for a substitute for rigid foam insulation that isn't made of plastic or petroleum, you might want to consider using cork insulation panels or semi-rigid mineral wool panels.

Rigid foam can solve 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. problems
Because a stud connects the inside of a house to the outside, it can act as a bridge for heat to escape (studs have lower R-values than insulation). Thermal bridging through the studs significantly degrades the thermal performance of the wall. In all climates, exterior foam sheathing improves a wall's performance.

Installing rigid foam insulation over wall or roof framing reduces this thermal bridging, raises the R-value of the wall or roof assembly, and can eliminate or reduce air leaks. Building scientists Joseph Lstiburek and Peter Baker have reported that 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 exterior walls also helps the framing stay dry by raising the dew point of the surface where water vapor is likely to condense. Remember, though: 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."

Rigid foam also is very useful for insulating the interior or exterior of foundation walls. In this case, the best practice is to apply rigid foam directly to the concrete, keeping air away from the cool and damp surface and lowering the risk of condensation. To meet fire codes, interior foundation foam will probably have to be covered with a layer of gypsum drywall.

Installing foam on a wall is straightforward
Foam wall sheathing is usually attached to studs or OSB sheathing with cap nails. Foam up to 3/4 in. thick can be attached with a pneumatic cap stapler like the Bostitch SB150SLBC.

If the wall will include vertical strapping to create a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. , the foam sheathing can be tacked in place with just a few fasteners, as the strapping will be screwed into the studs through the foam. For more information on installation techniques, see How to Install Rigid Foam Sheathing.

Foam can be attached to a concrete wall with specialty fasteners like Styro Tapit fasteners or adhesives like PL300 Foamboard Adhesive or Handi-Stick Polystyrene Construction Adhesive.

BLOWING AGENTS ARE AN ENVIRONMENTAL ISSUE

PolyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. has the most benign blowing agents, while those used for 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. are the worst
In the foam manufacturing process, a blowing agent creates tiny bubbles in the foam. These bubbles slow the flow of heat the same way tiny air pockets slow the flow of heat in a fiberglass batt.

Extruded polystyrene (XPS) and polyisocyanurate (polyiso) rigid foam insulation used to be made with ozone-depleting CFC blowing agents, but when ozone depletion was identified as a major environmental problem, the CFCs were replaced with HCFCs through international agreement.

Although HCFCs are more environmentally benign than CFCs, they still cause some damage to the earth’s protective ozone layer. HCFCs were eliminated from polyiso insulation in 2003 — polyiso is currently produced with hydrocarbon blowing agents — but they are not scheduled to be totally eliminated from XPS until 2020. Most XPS manufactured in the U.S. is still blown with HCFC-142b; European manufacturers have converted to non-ozone-depleting blowing agents. Only a few specialized XPS insulation materials in the U.S. are currently produced without HCFCs.

The blowing agents currently used to manufacture XPS in the U.S. are hydrofluorocarbons (HFCs) with a high global warming potential. Because the global warming potential of these damaging blowing agents is 1,430 times more potent than carbon dioxide, many green builders avoid the use of XPS. For more information, see Calculating the Global Warming Impact of 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. has long been made with non-ozone-depleting pentane rather than HCFCs or HFCs.

Expanded Polystyrene

Expanded polystyrene (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.), also known as beadboard, is usually white. It is commonly used for coffee cups and coolers. EPS is the most commonly used insulation in structural insulated panels (SIPs) and insulating concrete forms (ICFs).

EPS can be manufactured in different densities. Although higher densities of EPS have a greater compressive strength than lower densities, EPS is never as strong as 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.; EPS is more susceptible to crumbling at the edges and to other job-site damage. That’s why EPS is rarely used for wall 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. .

The R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of EPS depends on its density, with higher-density foams having higher R-values. The R-value range is from about 3.6 to 4.2 per inch.

EPS is the least expensive and most vapor-permeable of the three types of rigid foam; 1 in. of EPS has a permeance of 2.0 to 5.8 perms, making it a semi-permeable material. EPS can absorb more water than either of the other two types, from 2% to 4% by volume; that’s why XPS is usually preferred for below-grade applications. However, high-density EPS can be used below grade.

Extruded polystyrene

Extruded polystyrene (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.) is stronger, smoother, denser, more water-resistant, and more expensive than 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.. It’s also a better thermal insulator, rated at R-5 per in. XPS is less vapor permeable than EPS. One inch of XPS has a permeance of 1.1, while 2 inches have a permeance of 0.55, making XPS a semi-impermeable material.

Because of its high compressive strength and water resistance, XPS is often used below grade to insulate slabs and foundation walls.

The two most common brands of XPS are Dow Styrofoam, which is blue, and Owens Corning Foamular, which is pink.

For green builders, XPS has two major strikes against it: it contains the flame retardant HBCD, and its blowing agents have a very high global warming potential. For these two reasons, most green builders try their best to avoid the use of XPS.

Polyisocyanurate

Polyiso has higher insulating values (R-6 to R-6.5 per in.) than 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 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. at warm temperatures; however, when temperatures drop, the performance of polyiso worsens. For more information on the the cold-weather performance of polyisocyanurate, see In Cold Climates, R-5 Foam Beats R-6.

Because its manufacture doesn’t require the use blowing agents that deplete the ozone layer or contribute to global warming, polyiso board is considered the most benign type of rigid foam from an environmental perspective. However, because it can absorb water, polyiso is not recommended for use under slabs or on the exterior of foundation walls.

Polyiso often comes with a foil facing. One inch of foil-faced polyiso has a permeance of 0.03 perm, which is quite low. When used as 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. , foil-faced polyiso creates a vapor barrier on the outside of the building; that means that an interior plastic vapor barrier should never be used on a polyiso-sheathed wall.

Foil-faced polyisocyanurate is more resistant to ignition than unprotected XPS or EPS. For this reason, some (but not all) building inspectors allow foil-faced polyisocyanurate to be left exposed on crawlspace walls or in attics without requiring a layer of drywall as a thermal barrier.

FURTHER RESOURCES

"Overview of Wall Sheathing Options," from the Foam Sheathing Coalition

"Impact Resistance of Advanced Framed Wall Systems with Insulating Sheathing as the Primary Sheathing," from Building Science Corp.

Video: Superinsulating a Home With Rigid Foam

How to Install Rigid Foam Sheathing

Using Rigid Foam As a Water-Resistive Barrier

Avoiding the Global Warming Impact of Insulation

Calculating the Global Warming Impact of Insulation


Image Credits:

  1. Dan Thornton / Fine Homebuilding
Tags: , , , ,
74.
Aug 24, 2011 11:31 AM ET

Response to Minneapolis Disaster
by Martin Holladay, GBA Advisor

Minneapolis,
Q. "Is there any reason I should avoid making my inner layer of two exterior foam board layers that paper-faced insulation?"

A. No.


73.
Aug 24, 2011 11:23 AM ET

paper facing, for the inside layer of two ext layers?
by Minneapolis Disaster, 6B

Quick question:
A nearby lumberyard has a lot of thick RMax with paper facing, and fewer, thinner sheets with foil facing. Is there any reason I should avoid making my inner layer of two exterior foam board layers that paper-faced insulation? I'd like to take advantage of the thick stuff if I can.


72.
Aug 21, 2011 4:05 PM ET

Response to Dennis
by Martin Holladay, GBA Advisor

Dennis,
Q. "If I install rigid foam insulation on the exterior (roof and walls) and re-side and re-roof will that solve my problems?"

A. Yes, as long as you come up with good attachment details for the new foam and new sheathing (if any).


71.
Aug 21, 2011 1:49 PM ET

Foam Board Insulation
by Dennis Brewer

Hey Martin...The room is a Sunroom that is heated and cooled. The panels are three inch thick super insulated sunroom panels. Manufactured with extra-dense one piece polystyrene and sandwiched in aluminum alloy skins and locked together with the added strength of structural I-beams. The "I- beams " are aluminum which conducts the heat and cold causing the thermal bridging problems. If I install rigid foam insulation on the exterior (roof and walls) and re-side and re-roof will that solve my problems?...BTW...tks for the quick response.


70.
Aug 21, 2011 1:03 PM ET

Response to Dennis
by Martin Holladay, GBA Advisor

Dennis,
I'm not familiar with the type of aluminum-faced panels you are describing, but it sounds like you have a serious thermal bridging problem. You can solve the problem with either interior or exterior insulation; your choice. If you only address the ceiling, rather than the ceiling and the walls, you may still have a thermal bridging problem at the perimeter of the ceiling.

Any type of rigid foam -- EPS, XPS, or polyiso -- can be used. Polyiso is the most environmentally friendly of the three. Of course, the thicker the insulation, the better.


69.
Aug 21, 2011 12:23 PM ET

Foam Board Insulation
by Dennis Brewer

I have a "3 season" room made out of 3" thick foam board aluminum panels on the roof (cathedral ceiling) and walls. The are attached together with aluminum framing that conducts heat in the summer and cold during the winter. In the winter I have a condensation problem on the coupling frames and in the summer the coupling frames are hot to the touch. The roof of these panels is shingled with asphalt shingles. Would it be wise to install rigid foam board insulation on the interior side of the room and cover it with a dropped ceiling using fiberglass tiles? Or is there another method that would help my problem like installing the rigid foam board insulation on the exterior and re-roofing? I read that there may be a concern with chemical vapors if I install them on the interior side. My only other option would be to tear the structure down and and build a conventional framed room with is a expensive option. If it is feasible to install the insulation on the interior side, which type should I use? Tks


68.
Aug 1, 2011 11:42 AM ET

Re: response
by M Deangelis

Great, thank you. We'll seal the edges with canned foam.


67.
Aug 1, 2011 6:12 AM ET

Response to M Deangelis
by Martin Holladay, GBA Advisor

M Deangelis,
The plan will work, as long as the edges of the sheets of polyiso are sealed carefully with caulk or canned foam to prevent air from getting around the edges.


66.
Jul 31, 2011 10:51 AM ET

Insulating interior garage wall with polyiso boards
by M Deangelis

Hi,

My husband is preparing to drywall our garage. He wants to put polyiso rigid foam insulation between the wall studs. He is cutting the foam 15" wide by 8' high and pressing it into the stud wall cavity. He has two layers, one is 2" thick, with the foil side facing toward the inside of the house, and then he is putting a second layer of 1" thick polyiso board on top of this, with the foil facing the garage. Then, he was going to drywall over that. Is this ok? I'm just wondering if the warmer, moist air from inside the house will get into the walls and condense as it moves closer to the colder drywall. The garage will not be heated. If this is not a good approach, could you advise the best way to insulate the garage walls? Thank you!


65.
Jul 23, 2011 5:48 AM ET

Response to John Byers
by Martin Holladay, GBA Advisor

John,
Your question -- "how can one prevent condensation on the warm (interior) side of the barrier?" -- shows a fundamental misunderstanding. Condensation does not occur on warm surfaces; it occurs on cold surfaces.

In general, if polyisocyanurate sheathing is properly specified -- that is, if it is thick enough -- its use will prevent condensation, because the interior side of the polyiso will always be warm (above the dew point). Here's an article discussing the issue: Calculating the Minimum Thickness of Rigid Foam Sheathing.

Concerning your second question -- "will the felt/siding exterior be a sufficient substrate for spray foam if my designer gets his way?" -- it's impossible to answer without a site visit. It's really not a great idea to install spray foam from the interior if the house has no wall sheathing, but in a remodeling situation, it's sometimes necessary to choose the "least-bad" path. You need to be sure that foam won't ooze through cracks and appear on the exterior, through the siding, and you need to consider whether the spray foam will make future siding repairs difficult.


64.
Jul 22, 2011 10:33 PM ET

foil faced foam as a moisture barrier
by John Byers

Hello. Excellent forum. Thanks a lot!
I'm remodeling and residing a house that has 2x4 framing with fiberglass batts but NO sheathing under the current siding, only a layer of builders felt between framing and siding. I'm arguing with my remodeling designer who wants me to remove the batts when I expose the framing and apply spray foam. I want to use foil backed rigid foam sheathing. My designer says that the rigid sheathing will act as a moisture barrier and cause condensation problems in the wall. I've read that the polyiso foams with foil back are indeed a moisture barrier, so how can one prevent condensation on the warm (interior) side of the barrier? Also, will the felt/siding exterior be a sufficient substrate for spray foam if my designer gets his way?


63.
Jul 19, 2011 6:28 AM ET

Response to Sherri Thurman
by Martin Holladay, GBA Advisor

Sherri,
If you are building a new house, and if you don't know whether or not to include OSB on your walls, it's time to call in the professionals. Your team needs an architect or an engineer; it sounds like you don't have either. They can answer your questions about wall sheathing options.

If your builder wants more information in attaching siding to vertical furring strips over foam, here's an article with more information: Fastening Furring Strips to a Foam-Sheathed Wall.


62.
Jul 18, 2011 11:08 PM ET

Ridgid foam insulation
by Sherri Thurman

We are building a new home in Zone 2 - South Texas. We are planning on using the ridgid foam insulation with concrete siding - our contractor is concerned about the strengh of the foam insulation walls - carrying the concrete siding. Should we also use OSB and if so when should it be applied?


61.
Jul 5, 2011 1:11 PM ET

Edited Jul 5, 2011 1:20 PM ET.

Response to John Mikkelson
by Martin Holladay, GBA Advisor

John,
1. Installing interior EPS is OK.

2. Your chosen thickness -- only 1/2 inch -- has negligible R-value (about R-1.8), so it's hardly worth installing. Two inches of polyiso (R-13) would be much better.

3. Why insulate between the studs and rafters with fiberglass batts -- the worst performing insulation available? If you can afford to install new dormers, you can afford better insulation.

4. Tyvek is not a vapor barrier; it is vapor permeable.


60.
Jul 5, 2011 1:02 PM ET

EPS on inside of fiberglass batts?
by John Mikkelson

Hoping someone here can give me a thumbs-up or thumbs-down on this... We are renovating a part of our 100-year old house in Albany, Oregon. Fairly mild climate, with lots of rain in the winter. The space we are remodeling is a formerly converted attic space that we have been using (and plan to continue to use) as a bedroom, though the low (78") ceiling dictated by the current roof framing means it isn't officially "habitable space." That's where we find a hidden blessing, in my opinion. Because of this classification, we will not be held to code standards for insulation and can install just what will work, no more, no less. The room, like our entire house, had little (ceiling) or no (wall) insulation, so whatever we do will be a massive improvement in comfort. Because we will not be raising the roof ($$) I want to maximize what we can do with what we have -- 4' high knee walls I can thicken into the room, 2x4 rafters (9:12 pitch) I plan to sister with 2x6s, and rafter ties (collar ties?) above which we have 10" or more space for higher R-values down the center of the room. I like the idea of installing a 1/2" seam-taped layer of EPS on the inside of the room with 1/2" drywall over it, both layers covering whatever thickness of fiberglass will fill stud / rafter cavities (leaving vent space in rafters, of course!) Is there a downside here? I see elimination of thermal bridging (+), slight increase in R-value (+) and minimal lowering of ceiling height (+). I'm just wondering about moisture "capture" I guess. This room is sided at the gable end with aluminum over the original wood clapboards; where we plan to add dormers, it will be fibercement clapboards over OSB. I would normally plan to install Tyvek under the siding, but in this case would that be two vapor barriers working against each other in the walls? Same with the batts - install with paper face or remove so I don't have paper against EPS? Perhaps I'm overthinking this, but the more I read (here and elsewhere) it seems literally impossible to overthink insulation and/or water / vapor / condensation issues! Thanks...


59.
Jun 15, 2011 1:28 PM ET

Second response to Francis
by Martin Holladay, GBA Advisor

Francis,
Your plan will work.


58.
Jun 15, 2011 11:19 AM ET

Thanks, Martin
by Francis Matott

Thank you for the advice, Martin. I'm definitely doing under the slab, and at this point am considering using at least 2" rigid on the exterior at the base of the foundation walls, (to insulate the edges of the slab), but leaving it below grade, and then adding at least 3" of rigid from top to bottom on the interior to achieve the R15 requirement. In that case, the foundation will be inside 2 layers of rigid below grade, with only interior rigid above grade -- do you think that's a viable option?

Thanks again!


57.
Jun 15, 2011 5:20 AM ET

Edited Jun 15, 2011 5:21 AM ET.

Response to Francis
by Martin Holladay, GBA Advisor

Francis,
It's very important that you install a continuous horizontal layer of rigid foam under any slab with radiant heat. In your climate, I strongly suggest that you use at least 4 inches of rigid foam. You won't regret it. Either EPS (if you choose the right density) or XPS will work; EPS is more environmentally friendly.

There is no single answer to the question of whether to install basement wall insulation on the interior or the exterior. Both approaches have advantages and drawbacks. If you insulate on the interior with rigid foam, it's easier to avoid a thermal bridge at the connection between the sub-slab foam and the wall foam.

Whatever you do, don't ever use fiberglass batts to insulate a below-grade basement wall.


56.
Jun 14, 2011 7:54 PM ET

New Construction Advice
by Francis Matott

Our foundation will be poured in just a few days here in zone 6, (northern NH), and I'm getting a lot of different opinions on how we should insulate. I will have a radiant slab heated basement, and NH requires that I insulate the walls to R-15 from footing to sill. Our options are 3" of rigid on the outside, or any number of options on the inside, (our builders prefer Dow Thermax), or a combination of both. I don't like the idea of exposed insulation on the exterior, nor do I like having my structure over some concrete and some insulation, (I'd rather have it over all concrete). But the world is our oyster since it's new construction -- does anyone care to offer their preferred insulating system in this case?


55.
Apr 1, 2011 1:06 PM ET

Response to Edward Blanco
by Martin Holladay, GBA Advisor

Edward,
You can order EPS foam in any thickness you want, from 1/2 inch thick to 3 feet thick. If you choose the right density, you could get R-19 in 5 inches of EPS. One supplier of EPS who would be happy to sell you 5-inch-thick EPS is:
Branch River Plastics
15 Thurber Blvd.
Smithfield, RI 02917  
401-232-0270
Fax: 401-231-3434
www.branchriver.com

But why do you have to achieve R-19 in a single thickness? It's usually better to install rigid foam in multiple layers, so you can stagger the seams.


54.
Apr 1, 2011 12:56 PM ET

Rigid R-19
by Edward Blanco

I currently have a project that is asking for a rigid insulation that is equal to an R-19 value. I have done research and have not been able to come up with a rigid insulation that has a R-19 value. Do you have any suggestions. It would be greatly appreciated.


53.
Mar 14, 2011 5:34 AM ET

Edited Mar 14, 2011 5:34 AM ET.

Response to Carl
by Martin Holladay, GBA Advisor

Carl,
Q. "Am I missing something?"

A. No, you are not missing anything. But as I'm sure you know, every local code inspector makes the final determination of whether any proposed detail is code-compliant. Most code officials agree with your analysis, but some do not. Be sure to run it by your local official to be sure everyone is satisfied with your proposal.


52.
Mar 13, 2011 9:29 PM ET

Thermal barrier over foam
by Carl Mezoff

Martin,

We have the common situation of how to make a crawl space warm by closing up the block vents and applying rigid insulation to the interior face of the CMU walls. (We will comply with the ventilation requirement with a small amount of interior air per exception 4 of IRC 408.2.) I believe that Dow Thermax foam insulation can be installed to the interior crawl space walls without a supplemental covering (as would be mandated by your sidebar and IRC 314.1.2), because it is claimed to have a foil face that has been tested to meet ASTM E 84 for flame spread and smoke developed.

It seems like an ideal way to achieve a warm crawl space without having to apply gypsum board over the foam. (The ICC legacy report number is NER-681.) Am I missing something?

Carl M.


51.
Mar 4, 2011 5:04 AM ET

Edited Mar 14, 2011 5:31 AM ET.

Response to R Olson
by Martin Holladay, GBA Advisor

R Olson,
Here are two articles that answer your questions:

Calculating the Minimum Thickness of Rigid Foam Sheathing

Are Dew-Point Calculations Really Necessary?


50.
Mar 3, 2011 11:48 PM ET

Edited Mar 3, 2011 11:49 PM ET.

Wisconsin Dells, WI
by R Olson

Hello,

I am building a new home and have been trying to determine the cost effective amount of foam to use on the exterior. The home is near Wisconsin Dells, WI. I was planning to build a 2x6 wall (R-22 Cellulose) with continuous OSB & 1.5" XPS foam on the exterior. Would this be an acceptable system for our climate? Do I risk condesation on the interior surface of the OSB?

I would go to 2"XPS, but with our system, the costs really jump from 1.5" to 2". Also, if it is a good idea, I will use a Smart Vapor Barrier on the interior to allow some drying that direction because the 1.5" of XPS will limit so much to the exterior.

Thanks for the other input on this page so far!


49.
Jan 21, 2011 6:03 AM ET

Response to Ed
by Martin Holladay, GBA Advisor

Ed,
You won't get much insulation in a 3/4-in. gap, so you'll need to thicken up your wall to get significant insulation there.

If you are willing to lose interior space by thickening your wall, a wide variety of insulation materials could be used. Polyisocyanurate has one of the highest R-values per inch.

If you choose to use rigid foam insulation, these products are all vapor retarders or vapor barriers, so no further vapor retarder is required.


48.
Jan 20, 2011 10:48 PM ET

Rigid Foam Insulation
by ed

We have a third floor condo in Chicago with Cinder block-Firring strip Wall construction. Since firring strips are used in place of 2x4 studs I believe there is 3/4"-1 1/2" wall cavity for insulation and fiberglass bats are probably not going to be of any use What is the best product for insulating the room. Also would I need a vapor barrier when using rigid foam board?
Thanks in advance.


47.
Jan 4, 2011 3:12 PM ET

Second response to CHH
by Martin Holladay, GBA Advisor

CHH,
My response is unchanged. The foil-faced product is fine.


46.
Jan 4, 2011 2:34 PM ET

Interior Foundation Wall - Rigid Foam Insulation ... Again
by CHH

Martin,

Thank you for your prompt response ... very helpful.

" ... I'm not sure what kind of leakage you are talking about -- air leakage or water leakage...."
Water leakage.

" ... Once you have solved the problem -- by re-grading your yard, ..."
Agreed ... already underway.

" ... you can certainly use polyisocyanurate to insulate the interior of the wall ..."
The Dow product is comprised of three-ply poly/aluminum foil facers laminated to the polyiso foam core. I presume that additional piece of information will not change your answer. Correct?

Thank you again ... appreciate your time


45.
Jan 4, 2011 12:29 PM ET

Response to CCH
by Martin Holladay, GBA Advisor

CCH,
You wrote, "Outside leakage to the basement? Once in a blue moon."

I'm not sure what kind of leakage you are talking about -- air leakage or water leakage. But if your basement wall is leaking water into the interior of the basement -- even if it only happens "once in a blue moon" -- you have to correct that problem before insulating your basement wall.

Once you have solved the problem -- by re-grading your yard, installing roof gutters and downspouts, or by correcting a problem with your footing drains -- you can proceed to insulate your basement walls.

If your basement wall is perfectly dry, and if it never leaks water (even once in a blue moon), you can certainly use polyisocyanurate to insulate the interior of the wall.


44.
Jan 4, 2011 11:42 AM ET

Interior Foundation Wall - Rigid Foam Insulation
by CHH

Good Morning,

Polyisocyanurate (i.e. Dow Super Tuff) is the only rigid foam product carried in stock by two local home center retailers (i.e. national chains). Is it an acceptable product to apply to the interior foundation wall? (i.e. poured-in-place concrete below grade). The foam board will be taped and covered with 2"x4" framing, fiberglass batts and drywall.

Outside leakage to the basement? Once in a blue moon.

Your thoughts?

Thank you ... appreciate any guidance.


43.
Dec 22, 2010 5:31 AM ET

Response to Rob's latest post
by Martin Holladay, GBA Advisor

Rob,
If your sister wants accurate information on cellulose, she should talk to an insulation contractor who specializes in cellulose.

If she is talking to contractors who usually install fiberglass batts or spray foam, she is unlikely to be getting accurate information about cellulose.


42.
Dec 21, 2010 6:10 PM ET

Follow-up re: Martin's cellulose recommendation
by Rob Dickinson

Hi Martin,

Thanks for the response. I was worried about the "foam sandwich" issue as well, and agree with your recommendation about furring out thicker walls and insulating with dense-pack cellulose.

One reason my sister has been nervous about cellulose is that local insulation contractors have been decidedly anti-cellulose in their discussions with her. They have made some claims which I tend to think are totally bogus, but which I don't have sufficient info to conclusively debunk. For example, one insulation guy suggested that the fire retardants manufactured into the cellulose insulation only last for 7 years. Everything I read suggests that cellulose, especially the dense-pack nature of its installation, increases fire resistance. I've never heard anything before about the fire retardants added to cellulose dissipating or losing their effectiveness over time. Sounds totally bogus to me, and my suspicion is that some insulation companies believe they can make more money faster by slapping in the FG batts than doing a professional job dense packing cellulose.

Another thing they told my sister was that the cellulose would create mold issues, especially in the humid South. Again, my reading says this is a non-issue, but the professionals make this out to be scary.

Am I right to tell her that there is nothing to those claims?

Rob


41.
Dec 20, 2010 4:28 PM ET

Response to Rob Dickinson
by Martin Holladay, GBA Advisor

Rob Dickinson,
Most experts advise against creating a "foam sandwich." It probably wouldn't be a good idea to sandwich your plank sheathing between two layers of rigid foam. Although some limited drying is possible through 1 in. of XPS, any drying would be extremely slow.

I would install cellulose insulation in the wall cavities. If you install horizontal 2x4 strapping on the interior side of the walls, you can create a thicker cavity to hold more insulation. Remember -- don't install any interior polyethylene.


40.
Dec 20, 2010 4:18 PM ET

Interior rigid foam insulation
by Rob Dickinson

First - Thanks Martin and others for such consistently useful and well-informed advice! I've received numerous helpful responses that are ensuring my own project is properly detailed.

This question is for my sister's place in Chapel Hill, North Carolina, which is a different climate than I am familiar with. It's climate zone 4. Hot and humid in the summer and cold in the winter.

We're considering what the best insulation would be to use for the interior wall cavities. The house is built with 2x4 framing, approx. 24" on center, but very inconsistent stud spacing. There is a 1" layer of XPS on the exterior of the studs. There is planking sheathing in some places and not others, and some 1/4 plywood in some places and not others. No clear vapor barrier anywhere or housewrap, etc.

I doubt the 1" XPS is applied perfectly or taped or sealed or anything attempting to create a tight air-seal.

The house is gutted, but there is little budget for extensive retrofits for insulation. Spray foam options are probably overly expensive.

We could use cellulose or FG batts on the interior. For higher R-value, though, we are considering fitting polyiso rigid boards in the exterior wall cavities and trigger foaming the edges. It's more labor intensive, but would get us much higher R-value for these thin walls. And given the spacing of the studs is so inconsistent, i.e. not uniform 24" OC, FG batts would probably have more gaps than usual.

Would it be a moisture problem to combine the vapor barrier properties of rigid polyiso boards in the stud cavities with the 1" XPS on the exterior? At least the 1" XPS is reasonably permeable.

I am also wondering whether to recommend furring out the interior walls with crosshatch walls or somethjng like that to create additional space for blown-in cellulose.

Any advice would be greatly appreciated!

Rob


39.
Sep 29, 2010 4:10 PM ET

Response to Wayne
by Martin Holladay, GBA Advisor

Wayne,
Q. "Will this lead to a condensation problem?"

A. No, as long as you install the foam insulation in an airtight manner. You want an interior air barrier, so that warm, interior air is unable to reach the cold wall sheathing.


38.
Sep 29, 2010 3:41 PM ET

Insulating from inside
by Wayne

I have 2x6 framed wall on top of a stepped ICF foundation which is 12 inches thick. The 2x6 wall is close to flush with the outside of the ICF. I propose to insulate the framed wall from the inside with rigid foam. Will this lead to a condensation problem?


37.
Sep 20, 2010 11:29 AM ET

Response to Anonymous
by Martin Holladay, GBA Advisor

Anonymous,
Q. "In concrete block residential construction, it is a common practice to place foil-faced rigid insulation board directly against the interior face of the block, and then nail 1x furring strips over to support the drywall. This gives a 3/4" air gap for the reflective insulation, but does it not also create a vapor barrier on the inside of the building?"

A. Yes.

Q. "What would be an acceptable alternate?"

A. There are many ways to insulate a wall, but you haven't convinced me that there is any problem with using polyiso (or any other rigid foam insulation) on the interior of a concrete block wall. What's the problem?

The concrete block wall can easily dry to the exterior. (A concrete block wall is also invulnerable to moisture -- it won't rot.) The interior strapping and gypsum wallboard are at interior conditions and stay dry. What's the problem? Where's the moisture you're worried about?

If you prefer, a concrete block wall can be insulated with rigid foam insulation on the exterior. Either way works fine.


36.
Sep 20, 2010 11:07 AM ET

Rigid insulation in hot humid climate
by Anonymous

In concrete block residential construction, it is a common practice to place foils faced rigid insulation board directly against the interior face of the block, and then nail 1x furring strips over to support the drywall. This gives a 3/4" air gap for the reflective insulation, but does it not also create a vapor barrier on the inside of the building? Most rigid insulation and foil products have a very low vapor permeability. What would be an acceptable alternate?


35.
Sep 18, 2010 1:01 PM ET

Response to Jerry
by Martin Holladay, GBA Advisor

Jerry,
Re-post your brick tie question on our Q&A page. The brick masons who can help answer your question will never find your question on this encyclopedia page.

Q&A page:
http://www.greenbuildingadvisor.com/qa


34.
Sep 18, 2010 9:28 AM ET

Brick ties w/3" polyiso?
by Jerry

Mornin',

Martin, I have found a source of 3" polyiso board that does NOT have a foil/radiant barrier can it still be used for exterior brick veneered walls using a 1" air space, and I get very puzzled looks from suppliers when I ask about what is needed and how are the brick ties attached to the building without causing major leaking? Is there made such a brick tie available, if so where?

Thanks again,

Jerry Turner


33.
Sep 11, 2010 3:21 PM ET

Much thanks!
by Michael

Martin,
I appreciate your advice (especially on a Saturday), and will take it.
Much thanks,
Michael

PS: I was initially thinking you were suggesting 2X4s "on edge" over the polyiso; later realized you likely intended my laying them flat--hence my question about "nailing".


32.
Sep 11, 2010 7:47 AM ET

Response to Michael's latest question
by Martin Holladay, GBA Advisor

Michael,
1. Concerning the R-value of the roof insulation: the first thing you should do is check local code requirements. I know that California's Title 24 differs from codes used elsewhere in the U.S. You need to find out (1) Whether the code applies to your retrofit work, and (2) even if the code does not apply, what the code calls for in your climate zone. Personally, I wouldn't want to install less than the minimum code requirement for ceiling insulation -- and I would probably want to install more.

2. Two layers of thin foam is better than one layer of thick foam, because the staggered seams reduce air leakage and heat loss at the seams between the foam panels.

3. Sure, you can use 1x3s or 1x4s instead of 2x4s.

4. Most installers find it easier to screw straight down through the foam, hoping to hit the rafters. Trying to install the screws at an angle would only make the task of hitting the rafters more difficult.


31.
Sep 10, 2010 4:47 PM ET

Martin, Thanks for the
by Michael

Martin,
Thanks for the information. Just to clarify; You appear to be suggesting that I add sleepers on top of the insulation, which raises two questions: 1) What r-value and thickness do you recommend I go for? One local insulating contractor told me, "More than 2" is a waste of money in our climate. I ask, in part, to learn about your thinking about the TWO LAYERS (versus 1). I had originally planned on 3-3.5". The other part of my question is with regards to the roof dimensional profile. If I select for R-30, I am looking at almost 5" of insulation, right?; plus the sleepers and the roof--so the fascia might be 10" or more tall. Might I use 1X4s as sleepers instead to reduce the roof profile and still provide enough vent space to prevent mold (via a 3/4" cavity)? Lastly, in terms of attaching the sleepers and insulation, you recommend that I screw-down the sleepers through the insulation. How do you recommend I do this--screw "top-down" through the full thickness of the sleeper, or "toe-nail" the screws? Since my roof will be attached to sleepers, which approach is strongest?
Michael


29.
Sep 10, 2010 4:09 PM ET

Response to Michael
by Martin Holladay, GBA Advisor

Michael,
If I were you, I would install two layers of polyiso with staggered seams, and then 2x4 sleepers, eaves to ridge, screwed through the foam to the underlying rafters. That creates your vent channels. Although the vent channels aren't strictly necessary, the 2x4s help secure the foam.

Then install your top layer of roof sheathing, your underlayment, and your roofing.

I don't think you need any housewrap.


28.
Sep 10, 2010 3:14 PM ET

Above deck roof insulation
by Michael

I plan to re-roof my house in the next few weeks, and have been given conflicting advice. I live on California's Central Coast (Monterey, about 100 miles south of SF).. Part of my house has open-beam ceiling, and that is where I plan to install above deck polyiso. Here are several questions:
1. My building official says that i don't need to ventilate this area, if I don't leave any air gaps. What is your recommendation--vent or not?
2. A house a couple of streets away recently went up with 2-layers of polyiso, and the roof sheathing nailed directly through the insulation to the 2/6 roof decking/ceiling. This affords the most simple configuration for laying the polyiso I suspect (minimal cutting), but does it make a weak nailing due to long nail exposure? And as my roof decking underneath the insulation is only 1X (rather than the 2X of the house nearby), I wonder if the nailing would be stronger if I used 2X4 sleepers above deck to provide a nailing surface? Does doing this affect the ventilation issue?
3. I saw a roof detail in a recent issue of FHB that included house wrap above the deck and before the insulation layer, what are your thoughts?
4. Winters are mild here, but it is moist all summer due to fog (think SF). My thinking was to provide 3-3.5" of polyiso between 2X4 sleepers, and provide venting (just to be safe) via a ridge and soffit vent system. Do I need a house wrap between the insulation and existing roof sheathing? What are your thoughts about sleepers and venting?
Thanks in advance for your time.

Michael


27.
Sep 7, 2010 7:04 PM ET

Foil facing an air space
by Martin Holladay, GBA Advisor

M,
When facing an air space, aluminum foil acts as a radiant barrier. It can raise the R-value of the air space from R-1 to R-2 or R-3.


26.
Sep 7, 2010 4:47 PM ET

"However, if one side of the
by M

"However, if one side of the polyiso faces an airspace, that side should have foil facing"

What advantage does having the foil side face the airspace achieve?

In my case the polyiso will be left an 1'' shallow in the slant cavity to create ventilation path up to the ridge. I assume then foil side would be pointed towards the exterior.

Is their a max thickness from perm perspective that should not be exceeded when using polyiso in a slant wall application?

Per an earlier comment regarding concerns about shrinkage when using Polyisocyanurate. Is it correct to assume that Super TUFF-R would not be prone as it uses a foil facing


25.
Sep 7, 2010 10:56 AM ET

Response to M
by Martin Holladay, GBA Advisor

M,
Yes, you can stack two layers of polyisocyanurate on top of each other. It doesn't really matter what kind of facing the polyiso has. However, if one side of the polyiso faces an airspace, that side should have foil facing.


24.
Sep 7, 2010 9:49 AM ET

Polyiso foam board
by M

Can Polyiso be layered? I need apprx 2.5''. Can I layer 2 sheets on top of each other? Is there a particular side that should face the interior? It will be used in a slant cavity above the kneewall


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!