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Thermal and Moisture Control Layers

Tips and tricks to choosing the continuous exterior insulation that fits the needs of the job

Posted on Feb 21 2017 by michael maines

Editor's note: This post originally was published as part of the ProHOME series at Fine Homebuilding magazine. Michael Maines is a Certified Passive HouseA 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. Consultant and residential designer based in Palermo, Maine.

When installed over an otherwise conventionally framed wall (or a wall built using advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope.), continuous exterior insulation, also known as insulating 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. , is a great way to improve whole-wall R-values and to keep the framing and sheathing dry. There are some tricks to choosing size and type of materials, though.

Depending on the situation, I use different wall systems. For low-energy new construction, the builders I work with often prefer either double-stud walls with cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection., or Larson-truss type outriggers with infill insulation over a standard stud wall. Both can work well, but they have their downsides, including cost, wall thickness, and water-vapor management. Many builders, including Mike Guertin, prefer continuous exterior insulation. Since builders seem to have strong opinions on which wall type they prefer, and I think they can all work fine if designed and detailed well, I worked with Mike to design a durable, efficient wall detail at the ProHOME.

The wall is framed with 2x6s 24 inches on center and insulated with fiberglass batts. The R-value of fiberglass batts is typically a bit more than that of cellulose and a bit less than mineral wool, but they’re all pretty similar. I have to say that fiberglass batts are rarely my first choice for insulation, but they are readily available and inexpensive, and in the right conditions — installed tightly in an airtight cavity with limited temperature differential across the batts — they can perform as intended. (The common assumption that they should not be compressed is wrong; if you compress them, their R-value per inch increases, up to a point.)

It’s important to have at least one really good air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. somewhere in the wall assembly. It’s OK to have more than one air barrier, but it’s never OK to have more than one vapor barrier. I prefer to use vapor retarders rather than vapor barriers, as I’ll describe below. Air barriers and vapor barriers (or vapor retarders) can be one product that does double duty, or they can be dedicated layers. For the ProHOME, we are using one air barrier and two vapor retarders (the sheathing is acting as both a vapor retarder and the principal air barrier).

Detailing for Climate Zone 5

To meet code requirements for limiting vapor transmission by diffusion into the wall in Climate Zone 5, we needed a Class 1 or Class 2 vapor retarder on the interior, but with at least R-7.5 exterior insulation, the code allows a Class 3 vapor retarder. (Class 1 is 0.1 perm or less; Class 2 is from 0.1 to 1.0 perms; Class 3 is from 1.0 to 10.0 perms. Over 10 perms is considered vapor-open.)

CertainTeed is now selling kraft-faced batts with its vapor-variable MemBrain vapor retarder adhered to the kraft paper. The MemBrain normally blocks most water vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. as a Class 2 vapor retarder, but when the relative humidity reaches about 60% (when bad things can happen), it opens up to be vapor-open, allowing the wall to dry to the interior. With airtight sheathing, vapor movement by diffusion is not considered to be a major issue by prominent building scientists, but the MemBrain gives us insurance.

If we needed a functional air barrier at the interior, as some builders try to do with poly sheeting, then MemBrain in sheet form would be a better choice. Because we are using Huber's Zip System oriented strand board (OSB) sheathing as the airtight layer for the walls, we don’t need to worry about the interior of the walls being perfectly airtight, although there would be no harm if it were a second airtight layer.

Zip System sheathing is denser and more airtight than regular OSB (I learned that the hard way on a Passive House project), and Zip’s tape system is now familiar to most contractors. Like other OSB products, though, it is more sensitive to moisture than materials such as plywood or solid lumber. That doesn’t mean it’s a bad product; just that moisture control needs to be considered when using any OSB.

The phenolic coating on Zip sheds liquid water effectively and is open to water vapor. Huber does not publish data for the actual vapor permeance of its sheathing, but standard OSB is usually between 1.0 and 2.0 perms, with some third-party testing finding that Zip is below 1.0 perm. The permeability of OSB and plywood sheathing varies with relative humidity, so it’s hard to pin down. In any case, the Zip panel can be assumed to be borderline between a Class 2 and Class 3 vapor retarder, meaning that it will allow water vapor to pass through, albeit at a slow rate. If the sheathing temperature is below the dew point of the air, the water vapor will condense out of the air and onto the surface of the OSB. CapillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. action will then draw the moisture into the OSB.

Exterior insulation protects the sheathing

To reduce the chances of the OSB becoming saturated, our main defense is to keep air from flowing through the assembly, which we’ve already covered. The next defense is to keep the sheathing above the dew point by installing continuous exterior insulation. One of the key details of a wall with exterior insulation in Climate Zones 3 and higher is to make sure the proportion of exterior to interior R-value is high enough to prevent condensation most of the year. The exterior insulation keeps the sheathing warmer than the outdoor air, but insulation slows heat flow in both directions, so the cavity insulation keeps indoor heat from reaching the sheathing.

The 2012 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. has a handy way to make sure the sheathing stays above the dew point most of the year: table R402.1.1, which shows the minimum R-value of cavity and exterior insulation. [Editor's note: Strictly speaking, the previous sentence is inaccurate. For more information on this topic, see Comments #10 and #11, below, as well as a article titled The 2012 Code Encourages Risky Wall Strategies.] For Rhode Island (Zone 5), it’s 13 + 5. In other words, a minimum of R-13 cavity insulation with a minimum of R-5 exterior insulation. What’s not clear from that table is that if you increase the levels of insulation, it’s very important to keep the proportional relationship the same, or to err on the side of additional exterior insulation. You can’t just keep code-minimum R-5 exterior insulation and bump the cavity insulation up to R-21; if you do, the sheathing will be too cold during a typical winter and condensation will occur.

For Zone 5, the relationship is 38% exterior to 62% cavity. We have R-21 cavity, which means that we need at least R-8 exterior insulation to control condensation. Table R702.7.1 supports this math, requiring at least R-7.5 for a 2x6 wall if you want to use a Class 3 interior vapor retarder.

We could have installed more exterior insulation; from an energy use point of view, more insulation is always better, but there are diminishing returns. Energy modeling showed that going from 2 inches to 4 inches of exterior insulation would save about 250kwh/year — less than $40 worth of electricity and less than the annual output of a single PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. panel. I have designed Passive Houses with R-40 exterior insulation, so I’m not afraid of using too much, but considering the added expense and hassle of installing thicker rigid insulation, we opted to stick with 2 inches. At R-8 for the 2-inch thickness, we meet condensation-control requirements.

Choosing the best exterior insulation

With those decisions made, what should we use for exterior insulation? The most common exterior insulation is rigid foam, either 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. (extruded polystyrene) or polyisocyanurate (PIC). A third option is EPS (expanded polystyrene).

Although my feeling is that foam insulation is probably the best use for oil, in debating what materials to use, I argued strongly against XPS. XPS’s blowing agents are nasty — they stay in the atmosphere, and even though XPS insulation saves energy, in terms of global warming potential, XPS is usually a net loss to the planet, according to research by David White. Additionally, as XPS ages, its R-value decreases, and at useful thicknesses, it’s fairly effective at blocking the ability of the wall to dry to the exterior. In the plus category, it’s readily available, has decent compressive strength, and is easy to work with. Until the industry changes to the more benign blowing agents used for XPS in Europe, though, it’s always my last choice for insulation.

Roxul ComfortBoard IS

Polyiso has fairly benign, pentane-based blowing agents, and it is advertised as having a high R-value per inch, but with its usual foil facing, it is a total vapor barrier. Not only does its R-value decline over time, but it also loses performance as the temperature drops, just when you need it the most.

EPS is vapor semipermeable at thicknesses less than 3 inches, has relatively safe blowing agents, and stable R-value, plus it’s generally less expensive than the other options. It can be crumbly to work with, and it shares with XPS a brominated flame retardant that is not safe to ingest, but if I had to choose a foam, my first choice would be EPS. My second choice would be polyiso.

Fortunately, there is an alternative that is good for the house and good for the planet: rigid mineral wool, in the form of Roxul ComfortBoard IS (the IS is for “insulating sheathing”). It will keep the sheathing warm and dry, and it has low global warming potential and stable R-values. It’s a lot like a wearing a wool coat compared to a rain slicker. It’s a bit of a pain to install compared to the other options, but it’s completely vapor-open. Not only will it allow the occasional raindrop that finds its way past the siding to drain freely, but it will let the wall sheathing dry to the exterior if and when it needs to. Installed with 1x strapping, it makes an excellent ventilated 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. that is open to air movement at both the top and bottom of the wall.

Considering my obsession with managing water and water vapor and my strong desire to build resilient yet energy-efficient homes, I am happy with the ProHOME wall assembly.

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Image Credits:

  1. Fine Homebuilding

Feb 21, 2017 11:03 AM ET

Edited Feb 21, 2017 11:03 AM ET.

membrain and kraft
by Brendan Albano

Isn't kraft paper already a smart vapor retarder? What's the reasoning behind attaching a second smart vapor retarder (the Membrain) to it?

On a related note, why not just use unfaced batts, as both code and building science seem so support their use in this situation? The paint on the drywall would be the required class III vapor retarder.

Thanks for sharing! It's always useful to hear how things shakeout in the field as to quantity of insulation vs payback.

Feb 21, 2017 11:45 AM ET

Product choices
by michael maines

Brendan, some product choices were dictated by sponsors, as long as what they wanted to use did not conflict with good building practices. The idea with the MemBrain is that when properly taped it is also a decent air barrier. Mike Guertin and I both prefer to create the air barrier at the sheathing, in this case taped Zip, particularly as we were going for excellent air tightness (and achieved under 0.6 ACH50).

If you were not going for such high levels of airtightness, Taping the MemBrain-faced batts would be one way to create an interior air barrier. Kraft paper on its own is not a good air barrier. When using fiberglass insulation (which I am not as opposed to as some "greenie weenies") I do like to include a secondary air barrier at the interior, to fully seal the framing cavity, though some say it's not necessary. Drywall usually does the trick, especially when paired with Conservation Technology gaskets, but if you wanted boards or other air-permeable finishes on the interior, taping the MemBrain facing would be one way to seal the cavity.

The continuous exterior insulation is minimal, so there is some danger of the sheathing reaching the dewpoint for short periods. With thicker exterior insulation I would be less inclined to want an interior vapor retarder, but since we are on the cusp, I feel safer with a product that slows vapor transmission, and opens to allow free flow of moisture when relative humidity is high.

Feb 21, 2017 12:31 PM ET

Desperate for approval
by N/A N/A

Guys, Im desperate for some way to confirm that the way im going to install walls and insullation is goint to be ok. Im a young home owner, first time buyer in the US. I had problems with my contractor and now I have to find out how to do things by myself. I live in Washington DC, lots of humidity.
My wall would be (from the interior to the exterior)
1) fiberglass (15″ Kraft – EcoBatts) faced to the interior of the house
2) spray foam closed cell 1″ BASF Spraytite
3) plywood
4) tyveck housewrap
5) roxul comfort board mineral wool 3″
6) 1 inch of air for ventilation
7) hardie planks
is this ok? could I suffer those water and condensation issues inbetween the walls that you talk about sometimes?
best and you cant imagine how much I appreciate your opinion on this

Feb 21, 2017 1:09 PM ET

Re: Product choices
by Brendan Albano

Thanks for the explanation Mike! Being able to tape the MemBrain faced batts as an air barrier to improve the fiberglass performance is interesting, especially for folks who want to use finishes other than drywall. Do you feel like this is easier or cheaper than just using unfaced batts and sheet MemBrain?

Did you end up taping the MemBrain facing on the batts to form a secondary vapor retarder, or did you use it untaped?



A few questions that might help people help you:

- What's your interior finish? Painted gypsum board?
- How thick are your walls? 2x4? 2x6? Something else?
- Why the closed cell spray foam? You probably don't need it, but maybe it's for a specific reason that's not clear from your post.
- What is creating your 1" air gap? 1x4 strapping is commonly used, which creates a 3/4" air gap, which is a perfectly fine size of gap. 1" just stood out as an unusual number.

In general it sounds like you have the right idea. Just gotta sort out the little details.

Feb 21, 2017 1:15 PM ET

Reply to Pablo
by Robert Opaluch

Assuming that you have 2x4 or 2x6 studs 16" on center (15" wide kraft-faced EcoBatts 3.5" or 5.5" thick, not 15" thick), then spray foam against the plywood sheathing...

Any wintertime condensation in the wall would occur at the spray foam on the inside next to the fiberglass, so the plywood sheathing would remain dry. Even with 2x6 walls, the interior insulation would be about R-19 + R-6 = R-25. The exterior ComfortBoard would be R-12. So the ratio is about 2:1 or about 33%/67%. That's more than good enough for DC area winters. The ratio even safer if you have less interior insulation with 2x4 studs. Total wall R-value would be somewhere around R-35 for 2x6 or R-27 for 2x4, after factoring in siding, drywall, air films, and thermal bridging losses through the studs. "Pretty good house" advocates might recommend that level of insulation for DC area. Also good that you have a vapor open continuous insulation layer of mineral wool since your foam layer is likely to be vapor closed except through studs, preventing drying out through the foam layer. Hope you find your owner builder work very satisfying when you have it all completed!

Feb 21, 2017 1:49 PM ET

How difficult was it to stand up those walls?
by John Clark

It makes total sense to attach the exterior mineral wool before the wall is stood up but I'm thinking it makes for one damn heavy wall.

Feb 21, 2017 3:54 PM ET

Response to Pablo (Comment #3)
by Martin Holladay

Like Brendan Albano, I don't think you need the spray foam layer. If you want to cut down on air leakage, just tape the seams of the plywood sheathing with a high-quality tape.

-- Martin Holladay

Feb 21, 2017 4:13 PM ET

Edited Feb 21, 2017 4:13 PM ET.

by N/A N/A

Thanks so much Brendan and Martin! I really appreciate it!

Feb 22, 2017 10:26 AM ET

Response to Brendan
by michael maines

My opinion is that if a functional air barrier at the interior is needed, it would be easier with MemBrain or another membrane in sheet form, or with the airtight drywall approach, but that's based on my experience and others may find taping the MemBrain-faced batts an effective approach. They taped the batts on the ProHOME. This is a good overview of the insulation used on the project: (There is a short ad before the video plays.)

Feb 22, 2017 3:46 PM ET

Edited Feb 22, 2017 3:47 PM ET.

Table R702.7.1,not R402.1.1 for dew point control! (CAREFUL!)
by Dana Dorsett

"The 2012 IRC has a handy way to make sure the sheathing stays above the dew point most of the year: table R402.1.1, which shows the minimum R-value of cavity and exterior insulation. For Rhode Island (Zone 5), it’s 13 + 5. "

Not so!

Table R402.1.1 Chapter 4 is only about thermal performance, not dew point![RE].html

Dew point control R-values for the wall sheathing with only Class III interior side vapor retarders are found in Table R702.7.1 :

As it happens R13 + R5 c.i. happens to work for both thermal minimums and dew point control for 2x4 framing in Zone 5.

In zones 6-8 relying on Table R402.1.1 would fall short (sometimes woefully so) with the R20+ R5 c.i. option, even though R13+ R10c.i would still work (if only marginally in zones 7 & 8)

See Martin's prior blog pointing this out:

Feb 22, 2017 4:05 PM ET

Good catch, Dana
by michael maines

And what I usually do because I usually design for zone 6, where the proportional numbers do not correspond. I'm not sure what I was thinking but will blame it on late night blog writing.

Feb 22, 2017 11:06 PM ET

response to John (post #6)
by Timothy Godshall

I was also concerned about lifting a 50' long 2x6 wall with 2" of mineral wool on the exterior, but this set of wall lifts my friend made did the trick. Video here:

Here's the company whose idea my friend adapted:

Just another general comment regarding using mineral wool over sheathing: To anyone considering this technique, I would advise that you do a bit of testing to see how far out you should build your window boxes before you frame up your entire wall. I didn't factor in how spongy some of the mineral wool is, especially at the edges, so my window boxes are generally sticking out 1/8" to 1/4" beyond my vertical furring.

Feb 23, 2017 10:11 AM ET

Wall jacks
by michael maines

Mike Guertin, the builder (and owner) of the project, uses QualCraft wall jacks to raise the walls. They operate similarly to a pump jack. Mike blogged about it here:

Mar 1, 2017 8:27 AM ET

Edited Mar 1, 2017 8:33 AM ET.

might pick a material designed for air and vapor control
by Ken Levenson

If one is looking for a durable airtight layer inboard with robust vapor control, INTELLO Plus delivers tested and consistently strong results for both vapor control AND airtightness, as part of a complete system of smart high-performance assemblies. Disclaimer is that we sell the stuff - more info here: Cheers.

Mar 1, 2017 9:08 AM ET

Smart vapor retarders
by Martin Holladay

MemBrain and Intello Plus are two different brands of smart vapor retarder. They are similar but not identical. Both brands can act as an air barrier if properly detailed; both brands have variable vapor permeance.

Ken Levensen is one of the founders of 475 High-Performance Building Products, and as he noted, he is promoting one of the products he sells. GBA occasionally allows manufacturers and distributors to promote their products in our Comments section, but in general, we discourage the practice.

-- Martin Holladay

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