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. has a higher R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor.
Polyiso doesn't use ozone-depleting blowing agents; it uses pentane.

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-value: 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: , , , ,
23.
Sep 2, 2010 2:08 PM ET

THANKS Mr. Holladay!!
by Jerry

Thank You Mr. Holladay,!! you've made my day!!

Sorry about just the zip code, I shoulda at least added to my post "Arkansas".

I do appreciate,
Jerry


22.
Sep 2, 2010 4:28 AM ET

Response to Jerry
by Martin Holladay

Jerry,
Your question is like a scavenger hunt. You made me do some detective work on your Zip code, but I think I figured it out. You're in Arkansas.

Your plan is a good one.


21.
Sep 1, 2010 7:03 PM ET

Help! with ridgid foam
by Jerry

Hello,

Getting started on a new home (zip code 72854) and my plan is to install at least a 1" polyiso ridgid board with the radiant barrier to the exterior fastened to the 2x6 studs (no osb) with a 1/2" air gap between it and brick veneer, then fill the interior walls with cellouse.

Is the above plan okay for my zone?? If not what do you recommend??

Thanks much,

Jerry Turner


20.
Aug 21, 2010 3:35 AM ET

Response to Anonymous
by Martin Holladay

Anonymous,
If liquid water enters your basement once a year, you need to deal with your water entry problem before you insulate. Period.

The best way to handle it is probably to install an interior French drain -- a trench filled with crushed stone -- on the interior side of your wall footing. The drain pipe in the trench should lead to a sump; the sump should have a sump pump that discharges far from your building, or to a town sewer (if local codes permit).

Then install a dimpled mat on your wall between the concrete wall and your rigid foam insulation.


19.
Aug 20, 2010 8:41 PM ET

Your point is well taken
by Anonymous

Your point is well taken about the relative humidity of the air space, this will change my approach. Thanks for the thoughts, but one more issue I need to resolve before I use this product. What if once a year about 12 ounces(coke can amount) of rain comes over foundation wall between concrete and sill plate and down the concrete wall and onto the XPS. this water leak is a very rare occurence for me (despite grading as best as I can).. In fact the small amount of rain does not even puddle onto the ground it is so small in volume. I will try re-grading, but the XPS is mold resistant/water resistant so this should not be a high risk for mold growth even if I cannot fix this type of leak correct??

Thanks


18.
Aug 20, 2010 3:27 AM ET

Response to Anonymous
by Martin Holladay

Anonymous,
Depending on the amount of condensation, you could still have a problem. If the relative humidity of the air adjacent to the dripping concrete is always high, you could certainly have mold on your framing lumber and fiberglass -- even if the 2x4s aren't touching the concrete.


17.
Aug 19, 2010 8:12 PM ET

basement wall condensation
by Anonymous

thanks for answering my question, but I guess my thought is, so what if some basement wall condensation occurs on the concrete. The walls(wood and fiberglass which can harbor mold) are 12 inches away and would stay dry. the mold cannot grow in/on the framed walls if it doesn't come in contact with the condensation/water on the concrete wall. some water on concete can't harm anything. curious for your thoughts/experience. thanks


16.
Aug 18, 2010 5:06 PM ET

Response to Anonymous
by Martin Holladay

Anonymous,
Yes, you need rigid insulation against the concrete to prevent condensation. If you have only fiberglass batts -- even fiberglass batts that are 12 inches away -- there's nothing to stop the interior humidity from reaching the cold concrete. Fiberglass batts are vapor permeable and air permeable.

By the way, you don't need a 12-inch air gap between your insulated concrete wall and your studs. One or 2 inches is fine. Ideally, all of your required R-value will be provided by your rigid foam, and your 2x4 stud bays will be uninsulated.


15.
Aug 18, 2010 3:56 PM ET

basement insulation
by Anonymous

plan to use XPS for below grade basement wall against concrete slab in zone 5 climate to prevent interior water condensation on cold concrete. My question though, If I have a 2x4 wood framed wall with fiberglass insulation about 12 inches from the concrete wall do I really need rigid foam insulation in the first place as the condensation will not come in conctact with the water sensitive wall??

thanks


14.
Jul 14, 2010 2:48 PM ET

Try two paragraphs down
by Daniel Morrison

Sorry Sheleen, it was 314.4 in the 2006 IRC, but it's 316.4 in the 2009 IRC. I cahnged the sidebar to reflect the 2009 IRC.

The code books are updated every three years.

Thanks,
Dan


13.
Jul 12, 2010 8:58 PM ET

RE: IRC references on rigid insulation
by Sheleen Feldhaus

I have the 2009 IRC and have been trying to find the information on rigid insulation for some time. However, when I referenced what you had above, the entries were not even remotely the same. Do the IRC and IBC come out annually or, like other codes, on a multi-year basis?
If it is multi-year, then where can I find the complete one and what is the difference?


12.
Apr 30, 2010 10:57 AM ET

Second response for Nanette
by Martin Holladay

Nanette,
If you are in zone 5, and you plan to install rigid foam on the exterior side of your wall sheathing, the foam needs to have a minimum R-value of R-5 on a 2x4 wall to minimize the chance of condensation.

You're right that a three-dimensional drainage membrane between the foam and the OSB allows for some "hygric redistribution" in the event that the OSB gets wet, lowering the chance of problems. I don't know whether the ridges in StuccoWrap are robust enough to perform this function. Also, there is a slight downside — some degradation of thermal performance — to decoupling the exterior insulation from the interior insulation with an intervening drainage gap.

If the existing OSB looks dry, and there are no water stains or soft spots, the risks associated with adding polyiso to the exterior are low. If you switch to EPS, however, the wall assembly will be more forgiving. You can achieve any R-value you want with EPS; however, it will be thicker than polyiso of the same R-value.


11.
Apr 30, 2010 8:09 AM ET

follow up question
by Nanette Andersson

Thanks for your response. I’m in zone 5 where the conventional wisdom (not to mention code) for years has been to put a vapor barrier on the interior (warm) side. I was hoping to use polyiso due to its higher R value per inch. My thought was that the stucco wrap, or its cousin drain wrap, would act as an exit for moisture that might occur inside the wall assembly due to the double vapor barrier. Also, wouldn’t the polyiso raise the Dew point temperature sufficiently to reduce the possibility of condensation at the first condensing surface which is the back of the rigid? Adding R-value to these walls in an environmentally acceptable and economic manner is a tough nut to crack – so thanks again for your input.


10.
Apr 29, 2010 7:28 AM ET

Advice to Nanette
by Martin Holladay

Nanette,
You do indeed have a potential "vapor barrier problem," since your interior poly prevents your wall assembly from drying to the interior — increasing the risk associated with adding exterior foam.

Tyvek StuccoWrap is vapor permeable, and will not in any way reduce the danger of a double vapor barrier.

The best approach would be to open up the wall and remove the poly, but that route is expensive and disruptive. And second possibility is to substitute EPS for your exterior polyiso, since EPS is much more vapor-open than polyiso. EPS will allow the wall assembly to dry to the exterior — slowly, to be sure, but much faster than if you had used polyiso.


9.
Apr 29, 2010 7:18 AM ET

Advice to Russell
by Martin Holladay

Russell,
The colder the climate, the more important it is that any rigid foam insulation installed on the exterior side of OSB be thick enough to prevent condensation. The colder the climate, they higher the minimum R-value of the exterior foam.

It is impossible to answer your question without knowing your climate.


8.
Apr 28, 2010 6:52 PM ET

retro fitting foil faced polyiso with interior plastic vapor bar
by Nanette Andersson

Hi,
Thanks for the good information. I'm trying to retrofit a 1970's residence in a cost effective manner. The existing 2x4 wood stud wall section is as follows, gyp board, interior palstic vapor barrier, R11 fiberglass batt, blackboard sheathing, masonite siding. I would like to remove the siding (which is failing) and add over the blackboard tyvek stucco wrap, then foil faced polyiso rigid, then a 1x nailer, and finally fiber cement siding. Do you think the tyvek stucco wrap will be enough to take care of the double vapor barrier problem? Any other thoughts would be appreciated.


7.
Feb 9, 2010 8:29 PM ET

Hunter H-Shields on attic floor
by Russell G. Namie

I am considering installing 2 inch Hunter H-Shield panels on top of the osb flooring in my attic. I'm looking to enhance the 6 inch Kraft-faced fiberglass which comes just to the top of the 2 X 6 joists. I know that this is unorthodox, but I cannot sacrifice the floor to pile on several inches of fiberglass or cellulose. I contacted the factory and they said it would be o.k. My single concern is that I may be adding a second vapor barrier. They say it will not. Any thoughts would be appreciated.
I'm also waiting for an estimate from a company that will remove the fiberglass and install open-celled foam. Please advise. Thanks RgN


6.
Feb 2, 2010 7:57 PM ET

Paul, your concern is baseless
by Martin Holladay

Paul,
Cold-climate builders routinely install foil-faced polyiso as exterior sheathing without any "mildew" problems -- nor condensation problems, nor mold problems. Joe Lstiburek and Betsy Pettit have done several such houses in Massachusetts within the last few years.

Roofers have been installing polyiso on top of roof sheathing on commercial buildings in very cold climates for many years.

The polyiso warms up the wall cavities, preventing any condensation. As long as there is no interior polyethylene, these wall and ceiling assemblies work very well indeed.


5.
Feb 2, 2010 6:08 PM ET

AP foil faced
by Paul_L

Hi,
I submit that your article on Polyisocyanurate, under my point of view, is not sufficently clear and, thus, can drive to error in insulation project. Indeed, this product in a very good vapor barrior and,, thus, in my opinion, should ABSOLUTLY be intalled on the warm in winter side to avoid any mildew problem in walls. I think that this apply to medium (far north of Florida) to cold climate region.
Paul_L


4.
Feb 1, 2010 5:26 PM ET

It depends
by Martin Holladay

Rolf,
Once you have met code requirements, there is no simple answer to the question, "How much insulation do I need?" I've seen a variety of answers to this question, ranging from "as much as you can afford" to "twice as much as the code minimum."

The more you install, the lower your energy bills will be.


3.
Feb 1, 2010 4:34 PM ET

How much insulation?
by Rolf Miller

I have an old home in Colorado, built with no insulation in the walls. It didn't take much research to figure out that high density, closed cell, spray in place foam was the best way to insulate the walls. I stripped the walls and had them filled. As an engineer I was concerned with heat loss thru the studs. so plan to put up 1" rigid foam before drywall, this would make the total R value in the walls about R-27 to R-31. All the contractors, that have been in the house from window installers, ventillation, to drywallers have said I am going way over board. After reading a few things on this website over the past week I see that I was spot on about some of my concerns, and in fact I am now wondering if I am using enough insulation. Should I put more than 1" of rigid foam on the walls, if so how much?


2.
Feb 1, 2010 11:55 AM ET

Shrinking polyisocyanurate
by Martin Holladay

Ken,
I wrote an article for JLC on this issue in July 2002. The article, "Shrinking Insulation Boards Plague Roofers," is posted online:
http://www.jlconline.com/cgi-local/view.pdf/a7d9b8d94d2cc2fe469544849bf8...

As far as I can tell, the problem was limited to polyisocyanurate manufactured for roofing applications.


1.
Feb 1, 2010 11:43 AM ET

Polyisocyanurate shrinkage
by Ken

Hi
I just cruised thru the article relating to rigid insulation. Were you aware of the shrinkage problems with Polyisocyanurate insulation. Some reports show over an 1" in length and width.

Ken K


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