Hygric Buffering and Hygric Redistribution

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Hygric Buffering and Hygric Redistribution

Large amounts of hygroscopic building materials — for example, lumber and cellulose — can act like thermal mass for moisture

Posted on Jun 26 2015 by Martin Holladay

Water causes all kinds of problems for buildings. When rain leaks into walls through a poorly flashed window sill, or when the humidity in summer air contacts a cold water pipe and condenses, mold or rot can easily develop.

One possible way to handle localized leaks or intermittent humidity spikes is to build with hygroscopic materials that provide hygric buffering and hygric redistribution. To say the same thing in simpler terms: installing building materials that can absorb and store water may help handle moisture events.

Hygroscopic materials are materials that readily take on moisture. Examples include brick, framing lumber, and cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection.. Green builders who use natural materials sometimes point out that in a house with thick walls made of rammed earth, straw bales, or logs, these hygroscopic materials act as a hygric buffer.

Hygric buffering

A material that acts as a hygric buffer can store (and sometimes redistribute) moisture. Those who sing the praises of hygric buffers compare these materials to thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. . Just as thermal mass (for example, concrete) that is in contact with a home’s indoor air can act as a thermal flywheel, helping to even out spikes and troughs in the indoor temperature, so hygroscopic materials can act as a flywheel for humidity.

In other words, if a house has enough hygroscopic materials in contact with the indoor air, these materials can smooth out episodes of high and low indoor relative humidity. When the air is very humid, the materials can absorb moisture; and when the air is very dry, the materials will release moisture into the indoor air (by evaporation).

Hygric redistribution

In addition to their hygric buffering function, hygroscopic materials sometimes provide useful “hygric redistribution” — in other words, these materials can take on water and move the water away from a wet spot by wicking.

A wall with steel studs and fiberglass batts is incapable of hygric redistribution, because it doesn’t contain any hygroscopic materials. On the other hand, a wall with wood studs and cellulose insulation can perform quite a bit of hygric redistribution.

If water enters a wall with steel studs and fiberglass insulation — for example, due to a leaky window sill — the materials can’t absorb any moisture, so the water puddles on the bottom plate. If water enters a wall with wood studs and cellulose insulation, the materials can absorb and redistribute the moisture. If the leak is minor, the hygroscopic materials can handle the entering water until it can safely evaporate — at least in theory.

How much water is there in your house?

William Rose is a research architect at the Building Research Council at the University of Illinois. In his landmark textbook, Water in Buildings, Rose estimated the quantity of water held in a home’s building materials: “The wood alone in a house may account for over a ton of moisture,” Rose wrote. “Add to that the amount of water stored in other cellulosic material, such as drywall covering, books and cotton fabrics (assume 12% moisture content), and gypsum and other mineral materials, and the water stored in a building begins to appear considerable. … It is obvious that the water stored in the building materials outweighs the water contained in the air by a factor of 100 or more.”

But if your indoor air is air conditioned…

It’s all fine and good to talk about the moisture buffering capabilities of the thick masonry walls of an old villa in Tuscany. Romantics remind us that during Italian summers, the heat of the noonday sun and spikes of high relative humidity can be modulated by thick masonry walls, so that when you come indoors after a morning spent pruning your grapevines, the interior air is less harsh than it might otherwise be.

However, American homes aren’t like Tuscan villas. Most homeowners keep their windows closed, for one thing, and most homes are air conditioned during the summer. When the weather is hot and humid, indoor air is usually dehumidified and cooled by the air conditioner — greatly reducing the need for hygric buffering by hygroscopic building materials.

Studies in Europe

A few years ago, I attended a conference at which a Swedish building scientist presented a paper that quantified the hygric buffering effect of books. If you’re the type of builder who has concluded that hygric buffers are useful, and you’ve lined your walls with floor-to-ceiling bookshelves, you’re in luck. (As a bonus, your books will also act as thermal mass.)

Other published papers on the topic of hygric buffering by European building scientists include a Belgian paper that discusses a proposed metric called the Moisture Buffer Value: “The Moisture Buffer Value (MBV) indicates the amount of moisture uptake or release by a material when it is exposed to repeated daily variations in relative humidity between two given levels. When the moisture uptake from beginning to end of the exposure to high relative humidity is reported per open surface area and per % RH variation, the result is the MBV. The unit for MBV is kg/(m².%RH).”

Presumably, materials with a high MBV are (in some cases) more desirable than materials with a low MBV. The Belgian researchers concluded that MBV is a good indication of a material’s long-term hygric buffering capacity, but is less useful in predicting a material’s buffering capacity over short terms.

There remains a problem for designers, though: even if you know the MBV of a certain material, it’s hard to know whether that knowledge is useful.

Log walls act as hygric buffers

Another relevant study was performed by five German researchers: Hartwig Künzel, A. Holm, K. Sedlbauer, F. Antretter, and M. Ellinger. The title of their 2004 study was “Moisture buffering effects of interior linings made from wood or wood based products.”

The researchers set out to compare the hygric buffering effect of ordinary plaster with five other types of wall coverings. The researchers wrote, “To determine the moisture buffering capacities of various internal linings [finish materials], the test room … was covered with untreated linings consisting of spruce panels made from tongue-and-groove boards, acoustic elements [panels], cellulose fiber insulation units [panels], solid round logs made of Scandanavian pine, and wood fiberboard sheets. By comparing the indoor air humidity amplitudes resultant during a moisture production peak in the lined test room and in the plastered reference room, the moisture buffering capacities of the various surfacing materials under test could be quantified.”

The winners? Unpainted logs and unpainted fiberboard. “While the log walls had the best long-term buffering capacity, the best short-term moisture buffering effect was achieved by wood fiberboard.”

Although interesting, these research findings aren’t very relevant to the needs of interior decorators. Unpainted fiberboard makes a lousy material for interior walls, since it is soft and hard to clean. And very few homeowners are willing to put up with the well-known air leakage problems associated with log walls.

Straw-bale walls with natural plasters probably have excellent hygric buffering capabilities, but the German researchers didn’t test this type of wall.

More on moisture redistribution

For another take on moisture redistribution, it's worth pointing out that even non-hygrosopic materials (including hydrophobic materials like mineral wool insulation) can sometimes help redistribute moisture. For example, when mineral wool insulation is installed on the exterior side of 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 mineral wool allows damp sheathing to dry by evaporation to the exterior.

This type of evaporation isn't the same phenomenon as wicking, but it points to a characteristic that distinguishes mineral wool from rigid foam.

I recently telephoned building scientist Joseph Lstiburek and asked him a few questions about hygric redistribution. "If you install rock wool on the outside of the wall, the rock wool ends up sucking the moisture from the OSB," Lstiburek told me. "The water goes out in vapor form and condenses on the fibers of the rock wool. The liquid sits in the spaces. So a material doesn’t have to be hygroscopic to be a hygric buffer — it just has to have spaces."

Max Sherman’s analysis

A dozen years ago, at the 2003 Westford Symposium on Building Science in Massachusetts, Max Sherman, a senior scientist at Lawrence Berkeley National Laboratory, gave a jocular presentation on mold problems. He explained that the brouhaha over mold had given rise to three responses.

The first response was “the Terry Brennan approach,” which can be summed up, “Get used to it” or “Mold happens.” According the Sherman, Brennan devotees, having grasped the Zen of mold, accept its reality and adapt. As a result, they advocate the use of moisture-tolerant materials like concrete and ceramic tile.

Sherman dubbed the second response “the ‘Joe Lstiburek’ or ‘Tough Guy’ approach,” which he summed up with the phrase, “Real men suck it up and spit it out.” Sherman reminded the audience (which included Brennan and Lstiburek) that Joe sometimes praises the virtues of wall assemblies with hygric buffers. As long as a wall includes materials able to absorb moisture, and as long as the moisture level never exceeds the buffer’s upper limits, such a wall should be resistant to failure.

The third approach, Sherman continued, is summed up with the phrase “Squeeze it till it cries.” Adherents of this path believe in keeping mold at bay through dehumidification. If a house is equipped with a dehumidifier that keeps the indoor relative humidity below 50%, mold is unlikely to pay a visit.

Of course, most building science experts, including Brennan and Lstiburek, recommend a combination of at least two, and often all three, of the strategies described in Sherman’s light-hearted presentation. Brennan, a consultant and president of Camroden Associates in Westmoreland, New York, is also known as the originator of the Wile E. Coyote analogy. Brennan notes that “the difference between six inches on this side of the cliff and six inches the other way is really very large. I try to tell people that we need to take steps away from the cliff, not towards the cliff.”

That’s why it might be considered wise, even in a home with hygric buffers and low indoor humidity, to consider the advantages of moisture-tolerant materials. Sherman calls a three-pronged response to mold “the approach formerly known as redundancy.”

So how do we use this knowledge?

While it’s easy to describe examples of hygric buffering or hygric redistribution, it’s much harder for builders to get a sense of what this buffering means for building design.

Most building scientists agree that some wall assemblies that seem risky are made safer by hygric redistribution. The classic example is a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. insulated with dense-packed cellulose. In a cold climate, moisture tends to accumulate on the cold side of this type of wall during the winter months, leading to damp wall sheathing. Yet when these walls are disassembled and inspected, the wall sheathing is almost always in good shape. The lack of mold or rot is often attributed to two factors: hygric redistribution by the cellulose (which pulls moisture from the sheathing and redistributes it toward the center of the wall) and outward drying in April and May.

That said, it’s not as if builders can cut corners with water-management details, and glibly announce, “We’re fine — the cellulose provides hygric redistribution, and that will keep us out of trouble.” So hygric redistribution may be one of those phenomena which is interesting and worth studying, but is so hard to model that it is useless as a design principle.

Moreover, there is no easy way to compare the advantages and disadvantages of blown-in fiberglass insulation with the advantages and disadvantages of cellulose insulation. Water is more likely to drain quickly through fiberglass than cellulose, and that’s good — right? And damp fiberglass dries more quickly than cellulose — also good, right?

But on the other hand, cellulose provides both a hygric buffer and hygric redistribution — characteristics that are absent from fiberglass insulation — so the cellulose must be preferable, right?

Assessing the effects of these pluses and minuses is difficult even for building scientists. To weigh all the relevant factors, field observations may be just as valuable as, or more valuable than, hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. modeling.

In our recent phone conversation, Lstiburek noted that cellulose insulation can't perform miracles. “A mass wall built out of several wythes of brick is a spectacular example of hygric distribution,” Lsitubrek told me. “The rainwater penetrates, is wicked away, stored, and redistributed in a material that is not water-sensitive. But hygric redistribution doesn’t work that way with cellulose. Yes, the cellulose wicks and it stores moisture, but the moisture all ends up on one side of the wall because of the thermal gradient. The hygric redistribution buys you something, but it doesn’t buy you enough to keep you out of trouble. It you have a window that leaks, cellulose will not save you compared to fiberglass insulation.”

Weighing the value of hygroscopic materials

These days, coming up with a successful wall or roof assembly requires designers to pay attention to local energy codes, building scientists, technical journals, and their own understanding of what has been proven to work in the field. Integrating these various sources of information is somewhat of a juggling act.

When it comes to weighing all of these factors, hygric buffering considerations usually don’t claim very much of a designer's attention — and that's probably as it should be. From my perspective, here's where hygric distribution and hygric buffering fit into wall and ceiling design considerations:

  • Hygroscopic materials have multiple advantages, including acoustic advantages and hygric buffering capabilities that can even out some ups and downs in indoor relative humidity.
  • The best walls are designed to stay dry. This goal can be accomplished by including good exterior flashing; by including a 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. gap between the siding and the water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material.; and if possible by including a layer of insulation on the exterior side of the wall sheathing.
  • Wall and roof assemblies that include hygroscopic materials are sometimes better able to handle minor water-entry events or moisture accumulation than assemblies without any hygroscopic materials, but designers shouldn't rely on this fact to keep them out of trouble.

Martin Holladay’s previous blog: “How To Buy a Ductless Minisplit.”

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  1. Fraunhofer Institut Bauphysik

Jun 26, 2015 1:25 PM ET

All very well and good, but
by Hobbit _

All very well and good, but what's the R-value per inch of
damp cellulose??


Jun 26, 2015 1:53 PM ET

Edited Jun 30, 2015 11:17 AM ET.

Response to Hobbit
by Martin Holladay

If your cellulose ever gets so wet that the R-value per inch of the cellulose is significantly degraded, your wall assembly or roof assembly has failed.

At that point, you have much bigger problems on your hands than a concern over R-value. You have a major flashing failure, roof leak, or plumbing leak -- or an assembly with an annual drying potential that is less than what is required to handle the assembly's annual moisture accumulation. And that's something you don't want.

Jun 26, 2015 10:31 PM ET

Book him, Dano
by Dan Kolbert

I love the idea of our bulging book collections being our flywheels. If only my grandparents were alive - my grandmother spent the last 25 years of their marriage trying to get my grandfather to part with his collection of books in archaic German type.

It reminds me of Paul Eldrenkamp's proposal for the deep attic retrofit - shredding the contents of a client's attic in situ for loose fill insulation. I'm sure teddy bears, prom dresses and papier mache dragons all have excellent MBV's.

Jun 28, 2015 11:09 PM ET


Martin- Thanks for another informative and well written article.

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