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All About Thermal Mass

Interior thermal mass can sometimes help lower energy costs — but in cold climates, it’s won’t help much

Posted on May 3 2013 by Martin Holladay, GBA Advisor

UPDATED on December 4, 2013 with a citation of recent research findings.

What’s the deal with 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. ? Since manufacturers of materials that incorporate concrete often exaggerate the benefits of thermal mass, it’s easy to get cynical and conclude that the buzz around thermal mass is all hype. But in many climates, it’s actually useful to have a lot of thermal mass inside your house. Just keep in mind that thermal mass may not be as beneficial as its boosters pretend.

Thermal mass is a solid or liquid material that can store heat. Most of the objects inside your house can be considered thermal mass, including plaster, furniture, books, and canned tomato soup.

The specific heat capacity of building materials varies. In general, denser building materials have a higher specific heat capacity per unit of volume than less dense materials, which is why concrete, stone, and gypsum wallboard are more likely to be used to provide extra thermal mass than wood.

Three analogies: cistern, frying pan, truck

A building with lots of thermal mass on the interior side of the insulation may have lower energy bills than one without as much thermal mass, for reasons I’ll explain soon. But it’s important to point out that thermal mass can’t heat or cool your house. It’s just plain old concrete. To heat and cool your house, you still need HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment.

Here are some analogies:

  • Let’s imagine that you live in a remote desert town where everyone has to buy drinking water for $10 a gallon. Having a house with lots of thermal mass is like owning a really big cistern. The cistern can store drinking water, but it starts out empty. You still have to pay the water merchant to fill it up.
  • A house with lots of thermal mass behaves like a heavy cast-iron frying pan. When you want to fry an egg, you have to leave the pan on high heat for three minutes before it even begins to feel warm. And once you’re done cooking your egg, you have to remove the food from the pan right away, because the pan will continue to cook the food even when the burner is turned off.
  • Sometimes a house with lots of thermal mass can take advantage of free solar heat. When it does, it’s like a tractor-trailer that always goes 70 mph when it barrels downhill. (The truck is taking advantage of gravity, which is free.) Then, when the truck has to go uphill, it struggles to reach 60 mph. In the same way, homeowners who hope to take advantage of free solar heat have to be willing to be a little cold during the morning and a little hot during the afternoon.

Early high-mass walls were made of adobe and stone

Traditional homes in hot climates often have thick walls made of stone or adobe. If daytime temperatures are often above 80° or 85°F and nighttime temperatures are usually below 65°F, this method of wall construction — one that incorporates a lot of thermal mass — makes sense. At dawn, the wall is cool. When the sun begins to heat the wall during the day, it takes a long time for the heat to penetrate the thick wall. At 1:00 in the afternoon, the interior surface of the wall is still cool, and so is the interior of the house. In the evening, just as the sun is setting, the heat has fully penetrated the thick wall. At night, the warm walls begin to cool, giving off some of their heat to the interior, keeping the occupants warm even if the weather is chilly outdoors. At the same time, the exterior surface of the wall cools, releasing the stored heat to the outdoor air. The next day, the cycle repeats.

As long as the diurnal temperature cycle sticks to this pattern — with the temperature uncomfortably hot during the day and uncomfortably cool at night — these walls work well. In these circumstances, the 12-hour time lag between when the sunlight warms the wall and when the wall gives off heat proves very useful.

However, if the weather doesn’t follow this pattern, the walls stop working well. During the winter, if the weather is uncomfortably cool for 24 hours, the walls never get warm. And during the hottest months of summer, the walls never cool off. During those months, the occupants will probably wish the walls included a little bit of insulation.

Here’s an important thing to remember about these traditional walls from Syria and Arizona: when these walls were commonly built, the builders didn’t have anything else to use for wall construction except mud and stone. They soon learned that there were advantages to making their walls thick, and the houses performed pretty well. But they didn’t have access to insulation, central heating systems, or air conditioners. Had these materials and appliances been available and affordable, they might have chosen to use them.

Thermal mass is most useful in hot climates

Modern homes with high-mass walls or floors usually locate the mass on the interior side of the insulation. A high-mass wall might consist of concrete masonry units (CMUs) insulated on the exterior with a layer of rigid foam. A high-mass floor might consist of a 4-inch-thick concrete slab placed on a continuous horizontal layer of rigid foam.

Researchers have found that hot-climate homes with high-mass exterior walls require less energy for air conditioning than low-mass wood-framed homes with similar levels of wall insulation. According to Alex Wilson, the editor of Environmental Building News, “Nearly all areas with significant cooling loads can benefit from thermal mass in exterior walls.”

There really aren’t any benefits to thermal mass if temperatures stay hot all night long. In an article called “Mass Confusion,” Charles Wardell reported, “Greg Kallio, a professor of mechanical engineering at California State University in Chico who specializes in heat transfer, recently … model[ed] ‘the whole gamut’ of wall systems, from stick-built to SIPs to insulated concrete, using industry standard energy analysis programs like EnergyPlus, as well as his own custom software. His conclusion? ‘The effectiveness of thermal mass is very dependent on diurnal temperature variation. You want nighttime temperatures that get at least 10 degrees cooler than the thermostat set point.’”

There are two reasons why high-mass walls can lower cooling bills:

  • On days when the outdoor temperature ranges above and below the indoor temperature set-point during a a 24-hour period, the direction of heat flow through the wall reverses. During the hours when it is hot outdoors and cool indoors, the heat flows inward. At night, when it is cool outdoors and relatively warm indoors, the heat flows outward. This reversal of the direction of the heat flow reduces the overall heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. during a 24-hour period, because the heat flow reversal permits heat recovery. This heat recovery from the wall's thermal mass reduces the overall need for energy input from the HVAC equipment.
  • A high-mass wall introduces a thermal lag or time delay in the flow of heat from the exterior to the interior. If the effects of strong afternoon sunlight only penetrate to the interior surface of the wall in the middle of the night, the air conditioner is more likely to be operating when outdoor temperatures are cooler. Since air conditioners operate more efficiently at night, when the outdoor temperature is low, than they do during the heat of the day, energy savings can result from the thermal lag introduced by a high-mass wall.

Can thermal mass lower my heating bills?

In the classic thermal mass scenario — a hot-climate house with uninsulated adobe walls — a high-mass wall can provide thermal benefits. But what happens in a cold climate during the winter?

If daytime highs are 50°F or less for months at a time — as they are in colder areas of the U.S. during the winter — thermal mass won’t help much. After all, heat is flowing through your walls in just one direction: from the interior to the exterior. Under these condition of steady-state heat flow, you need insulation more than you need thermal mass.

Studies have shown that thermal mass can provide heating energy savings in only a few areas of the country. “The sunny Southwest, particularly high-elevation areas of Arizona, New Mexico and Colorado, benefit the most from the mass effect for heating,” Alex Wilson has written. “In northern climates, when the temperature during a 24-hour period in winter is always well below the indoor temperature, the mass effect offers almost no benefit, and the mass-enhanced R-value is nearly identical to the steady-state R-value.”

According to a document posted on the Oak Ridge National Laboratory (ORNL) web site, “The most favorable climate for application of the massive wall systems is in Phoenix. Relatively worst location for these systems is in Minneapolis (especially for less insulating walls).”

That said, there are some circumstances that call for extra interior thermal mass. The classic example is a passive solar house with lots of south-facing glass.

Passive solar design principles

The more south-facing glass a home has, the warmer the house gets on a sunny day in winter. (Of course, much of the gathered heat escapes through the windows each night, so the south-facing glass is a double-edged sword.) If the home’s roof overhangs are properly designed, and the weather isn’t cloudy, south-facing windows will probably get sun from late September until late March.

If there are only a few small windows on the south side of the home, there’s no reason for the house to overheat. However, if the area of south-facing glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. is large, the house will be flooded with so much solar gain that it risks overheating.

That’s where thermal mass can help. If the sunlight streaming in the south-facing windows strikes a concrete floor or a concrete wall, the concrete will soak up some of the heat, preventing the house from overheating — or at least delaying the event.

Here are some rules of thumb for thermal mass in a passive solar building:

  • The area of the thermal mass should be about three to six times the area of south-facing glazing.
  • The maximum thickness of the thermal mass (usually concrete) should be about 4 inches. Thicker concrete won’t absorb heat quickly enough for the extra thickness to be useful.
  • Dark-colored concrete floors work better than light-colored floors.
  • Concrete floors should be bare — not covered with carpets.

The net result of including plenty of thermal mass in a passive solar building is to reduce the amount of energy needed for space heating. However, it’s important to remember that the reduced energy bills may come with a comfort penalty. Like the tractor-trailer that speeds down hills at 70 mph and struggles up hills at 60 mph, a passive solar home gets the most benefit from “free” solar heat if the indoor air temperature is allowed to climb up to 80°F on sunny afternoons and allowed to drop to 60°F or 65°F in the early morning.

Some homeowners are happy with these indoor temperature swings, while others find them uncomfortable. GBA’s technical director, Peter Yost, reported, “When I have been in well-designed passive solar homes with plenty of thermal mass in Santa Fe, New Mexico (a climate with nearly ideal diurnal swings) during the winter, I’ve concluded that it is best to expand your thermal comfort zone quite a bit in the early mornings until the sun catches up on that mass.”

Finally, it's important to remember that there is a simpler solution to the problem of overheating caused by too much south-facing glass: just reduce the size of your south windows. There is another benefit to this approach: the house won't lose as much heat on winter nights.

Disadvantages of high-mass buildings

In addition to their advantages, high-mass buildings have a few disadvantages:

  • If the building is used as a vacation home or weekend retreat, it will take a lot longer than a low-mass building to heat the building up when you arrive on Friday night.
  • In a high-mass building, nighttime thermostat setbacks won’t save as much energy as in a low-mass building. This effect is small but real.
  • In some climates, high-mass buildings use more energy than low-mass buildings — but only if the insulation is installed on the wrong side of the wall. According to ORNL researcher Jan Kosny, “For buildings located in Minneapolis and Miami that have low R-value massive walls with the insulation material located on the interior side, total building loads can be higher with thermal mass than with the equivalent lightweight wall of the same steady-state R-value.”

Where should the insulation go?

If you want to use thermal mass to help stabilize indoor temperatures, the thermal mass should be inside the insulation, and there shouldn’t be any insulation between the thermal mass and the interior of the home.

Although manufacturers of insulated concrete forms (ICFs) sometimes brag about the benefits of thermal mass, one of the insulation layers in an ICF wall is in the wrong place. ICF walls sandwich the concrete core between two layers of rigid foam. The interior layer of insulation prevents the thermal mass from easily absorbing heat from, or releasing heat to, the building’s interior.

In a study conducted at the Oak Ridge National Laboratory, researcher Jan Kosny validated this common-sense analysis. “The most effective wall assemblies are those in which thermal mass (concrete) remains in good contact with the interior of the building,” Kosny wrote.

Several researchers have investigated whether the high thermal mass in ICF walls provides any reduction in energy use in cold climates. One of these studies was particularly thorough; it was performed in 2007 by Duncan Hill of Enermodal Engineering. Hill looked at the performance of ICF walls in a seven-story multi-unit residential building in Waterloo, Ontario.

I described Hill’s findings in an article (“ICFs Provide No Thermal-Mass Benefit In Canada”) published in the July 2008 issue of Energy Design Update. I reported, “At the time of construction, temperature sensors were placed at nine locations in the building; at each location, four sensors were inserted in the ICF wall. The researchers also installed data loggers to record indoor air temperatures. … While ICF construction creates a wall with very low levels of air leakage — ‘the concrete is a poured-in-place 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.,’ says Hill — the concrete had no thermal-mass benefit in Canada. The researchers wrote, ‘No thermal-mass impact or higher effective insulation value was observed.’ ... The lack of any benefit from the thermal mass in ICF walls was confirmed by computer modeling: ‘Comparing the simulation results of Models 1 and 3 shows that the increased thermal mass offered by the ICF wall construction in this building showed an insignificant improvement when compared to a low-mass wall assembly having the same thermal resistance and infiltration.’ The researchers examined the data carefully for signs of a thermal-mass effect. ‘We couldn’t detect one in the work that we did,’ Hill told EDU. ‘We weren’t able to tease anything from the monitoring data to show anything left over that might be seen as a benefit from thermal mass.’”

Oak Ridge National Laboratory quantifies the effects of thermal mass

In the 1980s and 1990s, researchers at the Oak Ridge National Laboratory (ORNL) undertook several studies designed to better understand the performance of homes with higher than average amounts of thermal mass.

Using data from hot-box experiments and energy modeling, four ORNL researchers — Jeffrey Christian, Jan Kosny, Andre Desjarlais, and Phillip Childs — quantified the advantages (and occasional disadvantages) of high-mass walls in a variety of climates. The team came up with a new metric (or “matrix”) called dynamic benefit for massive systems (DBMS). Kosny wrote, “The thermal mass benefit is a function of the material configuration, building type, and climate conditions, since high-mass walls are of greatest benefit in climates with large diurnal swings in temperature. DBMS values are obtained by comparing the energy performance of a one-story ranch house built with lightweight wood frame walls to the energy performance of the same house built with exterior massive walls. The product of DBMS and steady-state R-value is called an R-value equivalent for massive systems. This R-value equivalent does not have a physical meaning. It should be understood only as an answer to the question, ‘What wall R-value should a house with wood frame walls have to obtain the same space-heating and -cooling loads as a similar house containing massive walls?’”

This new metric was both useful and easy to misinterpret. Ever since ORNL researchers started writing about the concept of “an R-value equivalent,” advertising copywriters have been having a field day with the idea.

Is there such a thing as “effective R-value”?

While R-value is defined by federal statute, the term “effective R-value” has no agreed-upon definition — other than the narrow one created by ORNL researchers when they defined the “dynamic benefit for massive systems.” As noted by consulting engineer Maribeth Bradfield in her article, “The Effectiveness of Effective R-value,” “A quick online search for effective or equivalent R-values reveals a wide range of results. Depending on the industry and the building assembly being marketed (or marketed against), effective R-values can mean: the combination of standard R-value and air leakage (or lack thereof); R-value of insulation adjusted to account for 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. ; R-value plus thermal mass effects; R-value plus thermal mass plus air infiltration; and probably other combinations as well.”

Alex Wilson has echoed Bradfield’s observation. Wilson wrote, “All sorts of claims are being made about mass-enhanced R-value (usually called ‘effective R-value’) with little standardization.” Because the term is all but meaningless, “effective R-value” claims in advertising brochures should be ignored.

“The mass effect is real,” Alex Wilson wrote. “High-mass walls really can significantly outperform low-mass walls of comparable steady-state R-value — i.e., they can achieve a higher ‘mass-enhanced R-value.’ But (and this is an important ‘but’) this mass-enhanced R-value is only significant when the outdoor temperatures cycle above and below indoor temperatures within a 24-hour period.”

Facts rarely deter those aiming to profit from exaggeration. For example, consider claims made for Rastra walls. (Rastra is a manufacturer of ICFs made from recycled polystyrene. According to tests performed at Oak Ridge National Laboratories, a 10-inch-thick Rastra wall has an R-value of R-8.2 or less.) A Rastra brochure boasts, “A typical ‘advertised’ R-value for new wood frame construction ranges from R-13 to R-19. A Rastra wall provides a much higher Effective R-value of up to R-46.” Notice that the copywriter capitalized “Effective,” to make the adjective sound more official.

Or consider a claim made by Massachusetts Building Products of Warren, Massachusetts. The company boasts that the “actual R-value” of its concrete-filled iForm is R-24, while the “effective R-value” of the wall is R-32+. While that might be true in Phoenix, it isn’t true in New England. The company notes its area of distribution proudly on its website: “Serving Massachusetts, Connecticut and Vermont.” It would be hard to choose three states in the lower 48 where thermal mass is less useful.

What the building code says

The 2009 International Residential Code includes a definition for “mass walls.” (All walls have mass, so the phrase “mass walls” is awkward. For that matter, all walls have thermal mass, too. But people who write code books are immune to logic.)

According to section N1102.2.4 of the 2009 IRC, “Mass walls, for the purposes of this chapter, shall be considered above-grade walls of concrete block, concrete, insulated concrete form (ICF), masonry cavity, brick (other than brick veneer), earth (adobe, compressed earth block, rammed earth) and solid timber/logs.”

If you choose to build a wall out of one of these materials, the code allows you to include less insulation than would be required for a wood-framed wall — even in colder climate zones. For example, wood-framed walls in climate zone 5 need to be insulated to R-20, but concrete block walls can get away with R-13 (as long as at least half of the insulation is on the exterior of the wall).

It’s hard to know whether the code provision that allows “mass walls” to have less insulation than wood-framed walls is logical or not. A modeling study by Cheryl Saldanha and Joseph Piñon (“Influence of Building Design on Energy Benefit of Thermal Mass Compared to Prescriptive U-Factors,” 2013) shows that a three-story office building in climate zone 2 or climate zone 5 with high-mass walls will use less energy than a comparable building with lightweight walls, even when the high-mass walls have a lower R-value than the lightweight walls. The study supports the conclusion that prescriptive tables in the code that allow lower R-values for high-mass are logical.

In many cases, however, the improved performance of concrete block walls or ICF walls may be due to factors other than the wall’s thermal mass:

  • Most walls that are built of concrete or adobe have lower rates of air leakage than most wood-framed walls. (This wouldn't be true, of course, if air-sealing requirements in the building code were consistently enforced.)
  • Most concrete walls are insulated with continuous insulation (for example, rigid foam) rather than ribbons of insulation interrupted by thermal breaks (otherwise known as studs).

Here's my advice: if you want an R-20 wall, build a wall that includes (at a minimum) R-20 insulation. (Of course, continuous insulation performs better than insulation installed between studs.) It doesn't make any sense to build your wall with only R-13 insulation and hope that “R-13 will perform just as well as R-20 because the wall includes lots of thermal mass.”

One thing is for sure: the code provision that allows log homes to get a break when it comes to minimum R-value requirements is nuts, since most log homes are leaky.

Does your new home need interior concrete?

Should you include some concrete on the interior side of your home’s insulation to increase your home’s thermal mass? Maybe — but only if you are planning to include lots of south facing glass or your house is located in a hot climate.

If the thermal mass is part of a passive solar design strategy, the best type of thermal mass is probably a concrete floor on the south side of your house.

If you live in a hot climate, and want the thermal mass to help lower your air conditioning bill, your thermal mass should be located in your exterior walls. Remember, though: your walls still need plenty of insulation.

Even though interior concrete sometimes has thermal benefits, it also has drawbacks — including its high cost. In most climates, you can get the same benefit that concrete might provide by simply installing more insulation — and in most cases, the added insulation will cost less than the concrete.

Remember: the better insulated your house, the less thermal mass matters.

Martin Holladay’s previous blog: “Choosing HVAC Equipment for an Energy-Efficient Home.”

Click here to follow Martin Holladay on Twitter.


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  1. Martin Holladay
1.
Fri, 05/03/2013 - 12:54

Excellent article.
by Gavin Farrell

Helpful? 0

Thanks for clearly illustrating the importance of a 24-hour temperature cycle when considering thermal mass. It makes alot of sense!


2.
Fri, 05/03/2013 - 13:56

Clarification of IRC prescriptive interior vs. exterior R
by Dana Dorsett

Helpful? 0

Martin writes:

" For example, wood-framed walls in climate zone 5 need to be insulated to R-20, but concrete block walls can get away with R-13 (as long as “more than half the insulation is on the interior” of the wall). "

Under the IRC there are TWO prescriptive R values, and the higher value applies when more than half the R is on the interior, thus by default the lower value applies when half or more is on the exterior. For zone 5 mass walls are specified as R13/R17, so the R13 only applies if half or more of the insulation is on the EXTERIOR, and R17 is required if more than half is on the interior, which is the converse of Martin's interpretation. (And yes, parsing all of that CAN be confusing!)

See:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

Read note "i" carefully.

Also note, that for framed walls the prescriptive values for zone 5 is R20 (cavity-only) or R13 cavity + R5 continuous insulation.

At a typical 25% framing fraction the whole-wall R of the sheathing + studwall for R20 cavity fill comes in around R14 as does R13 2x4 construction with R5 of continuous insulation (unbroken by studs) on the exterior.

That means that a mass wall with more than half the R on the interior is required to have a substantially HIGHER whole wall value than a stick built wall (R17 rather than R14), but if at least half the insulation is on the exterior it can be modestly lower-R (R13 rather than R14.) There actually IS method to the madness in the IRC prescriptions, since with higher interior R on a mass wall uses more energy in a zone 5 climate, as noted.

Curiously, that's at variance with TABLE N1102.1.3, using the whole-wall U-factor method, where U0.057 (R17 whole-wall) would be required for low mass construction, but U0.82 (R12 whole-wall) with half or more of the insulation on the exterior, or U0.065 (R15) for mass walls with more R on the interior. The methods are not totally in sync.

But in the case of ICF, most current offerings start at R20 (R16 if you really hunt), and none of the of the lower-R ICFs have more than half of the insulation on the exterior- they would all meet code min for zone 5 using either method.


3.
Fri, 05/03/2013 - 14:05

Response to Dana Dorsett
by Martin Holladay, GBA Advisor

Helpful? 0

Dana,
It looks like I was correcting my editing mistake at about the same time that you were composing your comment. I recently corrected the bad sentence -- you'll see that it now reads, "For example, wood-framed walls in climate zone 5 need to be insulated to R-20, but concrete block walls can get away with R-13 (as long as at least half of the insulation is on the exterior of the wall)."

Thanks for your comments, and for your sharp eye.


4.
Fri, 05/03/2013 - 14:12

The other common exaggeration...
by Dana Dorsett

Helpful? 1

ICF companies are also prone to mentioning (without qualifying) the 40F center temp R values of EPS rather than the 75F number. This is because Type II EPS runs about R4.5/inch when it's average temp is 40F, about 7% higher than the R4.2/inch when it's average temp is 75F. Were it being sold as insulation rather than a wall system, they would be required to label at the 75F number. What's never stated in the ICF literature is that in order for the foam to average 40F in an 50/50 ICF, the temp of concrete would have to average 40F. In a house that's maintained at 70F it means the outdoor temp would then have to average +10F (!)

While that's a mid-winter average one might expect in US climate zone 7 or maybe the cold edge of zone 6, nowhere is that the outdoor average temp across an entire heating season in ANY lower 48 states, or for most of the population Canada & AK. Using the 40F R-values for EPS without clearly qualifying what that means is another sleight of hand to add another R1-2 of implied performance (that isn't really there) to an R20-ish ICF. They seem to get away with it because they're selling a wall system, not insulation, but it's pretty sleazy marketing, IMHO.


6.
Fri, 05/03/2013 - 20:34

Excellent post
by Dan Kolbert

Helpful? 0

It's one of those subjects people tend to get all gooey about - the perpetual motion machine of building science. Thanks for demystifying it.


7.
Sat, 05/04/2013 - 06:35

Windows and Thermal Mass
by Dennis Dipswitch

Helpful? 0

Every time I read about thermal mass and south facing windows,I always wonder what the net effect is.Doesn't seem to work too well during the night time hours.And it would seem that you get an added counter productive effect during the seasons that you do not want any added heat.

Martin,you give a quick mention of this in the article,but is there ever a circumstance,that the south facing glass/thermal mass floor and wall scenario is ever a net positive in energy usage?Or is the best possible scenario a situation where the view is to the south and a lot of glass would be included there anyway,so might as well capture some solar gain?


8.
Sat, 05/04/2013 - 07:01

Response to Dennis Dipswitch
by Martin Holladay, GBA Advisor

Helpful? 0

Dennis,
Q. "Is there ever a circumstance,that the south facing glass/thermal mass floor and wall scenario is ever a net positive in energy usage?"

A. Yes, of course. If you choose the right glazing, a window can gain more energy than it loses on an annual basis. (This is what you want in Minnesota, but it probably isn't what you want in Miami.)

The trick is to make sure that you have the right type of glazing, the right amount of glazing, and the right roof overhang -- so that the heat gain occurs at the times of the year when you want the heat. If the house has solar heat gain in July, it's usually unwanted.

These elements of passive solar design can be tweaked with a good energy modeling program like PHPP. For more information, see Windows That Perform Better Than Walls.


9.
Sun, 05/05/2013 - 22:06

ICF in the humid south
by Curt Kinder

Helpful? 0

Lots of good stuff in the original article and follow-on comments. Something I haven't seen mentioned that I would like to run up the flagpole for reaction / analysis:

ICF seems confer an extra benefit in the hot humid south. We here endure about 4 continuous months, call it 5/15 - 9/15, where the overnight lows are around 70-75, and that low is dry bulb, wet bulb, and dew point; in other words, nearly 100% RH obtains from midnight until dawn.

With outdoor temperature and indoor setpoint about matched, AC runs little during those hours, because, by midnight AC has removed most of the solar heat gained during afternoon by a typical low mass frame house as well as the internal load from the evening's cooking, bathing, media. and lighting activities.

From about midnight onward, indoor RH rises owing to infiltration and respiration by sleeping occupants, some of whom may react to the humidity by reducing AC setpoint, causing the house to transition from hot and muggy to cold and clammy; still uncomfortable, but with the added zest of mold risk in some building assemblies.

Typical AC may cycle only once or twice after midnight, and barely long enough to make a dent in RH, since the thermostat tends to be satisfied just as the coil gets cold enough to dehumidify.

ICF seems to help with all that, in two ways:

One - In the manner of "traditional" thermal mass, it shaves the peaks from torrid afternoons during summer and frigid nights in winter, allowing HVAC equipment to be a bit smaller, costing less up front and operating more efficiently owing to longer on cycles.

Two - specific to humidity - I suspect, but haven't the data to prove, that the concrete "meat" between the foam "bread" within ICF stays at at temperature of 80-90 all summer, west walls at the higher end of that range, and north walls at the lower end.

Running some numbers, suppose an ICF home (such as mine) has 5000 SF of wall with average 85*F concrete in contact with R10 worth of foam and drywall (ICF mfgs might argue the number is closer to R12, but there are no end of small bridges and penetrations, so I'm going with R10 for the example...maybe R8 is closer to the mark...)

With cooling setpoint of 75, that translates to 5000 Btu / hr sensible moving through my walls, 24/7, all summer. That, in turn, results in about a minimum 20% duty cycle for my AC, 24/7. That effect, coupled with the traditional thermal mass of the home, causes the AC to run for 10-20 minutes 3-4 times during the midnight - dawn near-100% RH period, resulting in superior humidity control and comfort compared with typical frame homes.

Right-sized two stage AC coupled with reasonable infiltration (~3 ACH 50, tight, nowhere near PH standard) results in RH being held to 45-50% throughout cooling season, which in turn allows drybulb setpoint to rise a bit above typical 73-75 to about 77-79, reducing HVAC cost even more and reducing building assembly mold risk as well.

Thoughts, anyone?


10.
Mon, 05/06/2013 - 04:26

Thermal mass of lightweight 'insulating' concrete products?
by Andy Parkinson

Helpful? 0

Am I correct in my understanding that lightweight concrete and CMU products (such as the AAC illustrated) are less effective thermal mass than regular concrete or CMU, due to their lower density? So should these products be called 'high mass' or not? More 'medium mass' maybe?


11.
Mon, 05/06/2013 - 06:51

Response to Curt Kinder
by Martin Holladay, GBA Advisor

Helpful? 0

Curt,
Your theory is intriguing. You may be right, but it would take some research to verify it -- for example, a research project involving an ICF home with embedded sensors.

It's certainly true that a high-mass wall tends to shift some of the air conditioning load from the late afternoon to nighttime hours.

I think it's ironic that you are proposing a somewhat counterintuitive justification for the use of ICFs in a hot climate -- namely, "You'll have hot concrete in your walls, all summer long. In fact, the concrete will be so hot that it will make your air conditioner kick on in the middle of the night!"

In Houston, researchers trying to come up with a solution to high indoor humidity problems had good success by simply installing a $250 stand-alone dehumidifier somewhere in the house.


12.
Mon, 05/06/2013 - 07:09

Edited Mon, 05/06/2013 - 07:13.

Response to Andy Parkinson
by Martin Holladay, GBA Advisor

Helpful? 0

Andy,
In general, you're right. Denser concrete products have a higher specific heat capacity per unit of volume than less dense concrete products.

Two observations:

1. For any given application, there is an optimum level of thermal mass, beyond which any additional mass provides no energy performance benefit. There's no need to pay for concrete that doesn't do anything. Modeling the optimum level of thermal mass for any given application gets complicated, but often rules of thumb and experience give results that work just as well as modeling.

2. The exception to the "denser is better" rule of thermal mass concerns phase-change materials.

Phase-change materials used in construction are usually some type of paraffin or wax. (Small spheres of these paraffins or waxes can be incorporated into gypsum wallboard or cellulose insulation.) These materials have the ability to absorb and then release heat by changing phase from solid to liquid and back. The materials are chosen because they have a melting point that is close to the temperature setpoint of the typical heating or cooling systems.

During this phase change from solid to liquid, a material absorbs heat from the surrounding environment. Conversely, in cooling from liquid to solid, heat is given off to the environment. The energy that can be stored and released during the change of state occurs over a very narrow range of temperature. During the change in physical state, the material itself remains at nearly constant temperature until the phase change is complete. (Remember 8th grade science class, when you put a thermometer in a pot of water, and then brought the pot to a boil?)

Phase-change materials only provide benefits for indoor environments where occupants are willing to allow indoor temperatures to range above and below the thermostat set point.

Phase-change materials behave like thermal mass, but store more heat per unit of volume or per unit of weight than concrete or water. So these materials are an exception to the "denser is better" rule.

For more information on phase-change materials, see Storing Heat in Walls with Phase-Change Materials.


13.
Mon, 05/06/2013 - 15:49

Response to Martin...
by Dana Dorsett

Helpful? 0

"In Houston, researchers trying to come up with a solution to high indoor humidity problems had good success by simply installing a $250 stand-alone dehumidifier somewhere in the house."

The definition of "...good success..." get's a bit fuzzy when you're converting a latent load directly to a sensible load, inside the thermal & pressure boundary of the house, in a climate where you'd rather be dumpng that latent heat outdoors rather than raising the indoor temp with it (along with the heat dissipated in the motor & compressor, which are all indoors.)

I like the Daikin Quaternity mini-split approach of having independently settable relative humidity and temperature setpoints, and proprietary valving on the interior coil which allows it to hit it's marks on humidity even without sensible cooling. (I'm told there are some ground source heat pumps with similar functionality, but have yet to look into them in any detail. ) In this case the latent heat and compressor-motor heat are all sent outdoors where they belong, not adding to the sensible cooling load.


14.
Mon, 05/06/2013 - 16:03

Edited Mon, 05/06/2013 - 16:05.

Response to Dana Dorsett
by Martin Holladay, GBA Advisor

Helpful? 0

Dana,
It's all fine and good to like the Daikin Quaternity, which I don't doubt works well. But the Daikin Quaternity is hardly an affordable solution for an existing house with high indoor humidity.

It takes energy to remove moisture from the air, and there is no way around that fact. Running a stand-alone dehumidifier takes energy, and of course it dumps a little extra heat indoors. Most central air conditioners are oversized, however, and easily handle the extra load from this small appliance. Moreover, if the central AC runs for a few extra minutes each cycle, that helps to lower indoor RH as well.

No solution is perfect. I don't doubt that a Daikin Quaternity is more efficient than my suggested solution. But for $250, the stand-alone dehumidifier approach is hard to beat.


15.
Mon, 05/06/2013 - 16:56

Edited Mon, 05/06/2013 - 16:56.

I'm familiar with the approach....
by Dana Dorsett

Helpful? 0

Since I have effectively no sensible load most of the summer, even with the ever-tightening house the place still needs dehumidification (especially in the basement where there's an as-yet uninsulated slab), and the $250 standalone dehumidifier does the trick (to the tune of a measured >300kwh/year- it's a significant plug load.) It's contribution to the almost non-existent sensible load isn't something that needs to be actively pumped out of the house- the basement rarely breaks 72F even with the dehumidifer and other plug loads.

In TX the both the latent & sensible loads are much higher than near me in central MA, with no "passive cooling" slab to soak up the additional sensible load. I get that the central AC can handle it, but was pointing out that it's less than ideal. (To be sure on new construction you'd like to be able to do something different.), but it's certainly true that from a retrofit point of view a best-in-class mini-split (or GSHP) isn't going to be in the cards. In TX there's a pretty good case for applying the $250 cost of the dehumidifier toward a heat pump water heater, pumping the latent and compressor-motor heat into the hot water tank where it does some real good, though it's not exactly under dehumidistat control. Every house will be different...


16.
Tue, 05/07/2013 - 00:47

Edited Tue, 05/07/2013 - 00:50.

Mass effect offers almost no benefit in cold climates?
by Ted Kidd

Helpful? 0

Very interesting topic, thanks for posting this information!

Is load linear to temperature? Or does it look like a mpg curve (as speed increases mpg goes down), having sort of geometric relationship?

Mass seems to have this flywheel or momentum effect. Doesn't thermal mass provide a buffer to worst case load? If load is not linear, then wouldn't the buffering effect of mass create lower load experienced by buffering temperature spikes?

If the temperature ranges between 10f and 30f, would a high mass structure experience a load of 20f?

Also, can you change design temperature for high mass buildings and install smaller equipment, providing further benefit of the type Curt suggests?


17.
Tue, 05/07/2013 - 04:53

Edited Tue, 05/07/2013 - 04:55.

Response to Ted Kidd
by Martin Holladay, GBA Advisor

Helpful? 1

Ted,
Q. "Is load linear to temperature?"

A. Good question! As far as I know, it is. But one has to consider temperature like a physicist, and think in terms of degrees Kelvin, with absolute zero as one's starting point.

So when the outdoor temperature is 40°F, and you want to heat your house to 70°F, you have to remember that the outdoor temperature is 278°K, and you are raising the temperature to 294°K.

When it's cold outside -- let's say that the temperature is -5°F, or 253°K -- the outdoor temperature is 91% as warm as it was when the temperature outdoors was 40°F (278°K).

Q. "Doesn't thermal mass provide a buffer to worst case load?"

A. Yes, and this benefit occurs with all houses -- not just houses that include concrete. Every house includes partitions and drywall, and most include furniture and books and kitchen cabinets and appliances. Because of this thermal mass, you never really need HVAC equipment to handle the worst-case load. In theory, Manual J knows this.

However, Manual J includes a fudge factor, so it is extremely rare for anyone to actually specify HVAC equipment that is comes close to the actual load required on the worst-case day. For one thing, you can't even buy a furnace small enough to match the Manual J load on a well insulated house. So it's really hard to take any advantage of the load-shaving mechanism you describe.

Where I live, the temperature occasionally gets down to -40°F. But that happens rarely. Many engineers might use a design temperature of -30°F for this part of Vermont. On the rare morning when it is -40°F, the thermal mass of the house prevents occupants from being cold. With any luck, the temperature will warm up to -30°F before long.

With lots of thermal mass, you might think that you could use a design load of -25°F. And you probably could. However, there will be an occasional day when the thermometer starts out at -40°F, and warms up to -18°F at noon, and then drops down to -28°F by dinnertime. If you have many days like that, you might be getting chilly after a while.

But really, since equipment is always oversized, it's very hard to save money with this trick. The concrete is expensive, and there aren't any cheap, small furnaces to save you any money.


18.
Tue, 05/07/2013 - 07:21

Edited Tue, 05/07/2013 - 07:24.

Good building science rarely lends itself to sound bites...
by Curt Kinder

Helpful? 0

...But I guess I asked for it.

ICF causes some highly welcome late night HVAC operation during hot weather. I suppose that could be viewed as a negative, except that the extra hour or so operation after dark is more than offset by the ton or so shaved off the design load and also by the substantial delay and reduction of operating time during sunny afternoons.

I'm familiar with the Houston study. I agree that portable dehus can solve certain problems at certain times but I cannot advocate their continuous operation unless all other humidity control strategies and techniques have been exhausted or ruled out: We advocate a ten point defense-in-depth approach to controlling humidity (WITHOUT separate active dehumidification appliances), to wit:

• Minimize enclosure air infiltration
• Properly size HVAC system to minimize short cycling - endeavor to install the smallest feasible system.
• Ensure ductwork is configured to provide individual room design air flows - stave off thermostat wars
• Ensure ductwork is within conditioned space or at least minimize duct leaks to / from unconditioned space
• Avoid single stage HVAC systems that short cycle during part load conditions
• Include controls that modulate system airflow (CFM / ton) in response to humidity
• Manage point sources of humidity with ventilation - encourage use of bath vent fans by installing very quiet models controlled by timers and / or motion. Ensure range hood is properly sized, selected, positioned, and ducted so that it both works well and is reasonably likely to be actually used.
• Discourage use of continuous fan in cooling mode.
• Discourage use of natural gas or propane for cooking.
• Evict panting dogs, sweaty children, and thirsty houseplants to the extent possible during summer months.

Specifying a dehumidifier before or in lieu of implementing the ten steps above amounts to an expensive, irresponsible band-aid.

In my experience portable and central dehus tend to have flaky, inaccurate controls, consume inordinate amounts of energy, add substantial sensible load, and confer additional system complexity. All that and they are loud and unreliable. No worthy building scientist would specify one before attacking the roots of a humidity problem as outlined above.

Note to Dana - my HPWH just finished a continuous run from 7:30 until 10 PM recovering from the evening's multiple showers and along the way made the basement zone the driest in the house.

I'm not totally opposed to dehus...we just used one to drive RH down from 70% to 40% in a space being drywalled during 3 days / 8 inches of rain. 40% RH greatly reduced time between mudding cycles, allowing drywall crew to stay productive during the storm. As I wrote earlier, dehus do have their uses...


19.
Tue, 05/07/2013 - 08:19

Response to Curt Kinder
by Martin Holladay, GBA Advisor

Helpful? 0

Curt,
I can't argue with any of your suggested measures -- they all make sense. You're right, of course, than any suggested solution to an indoor humidity problem must take a whole-building approach.


20.
Tue, 05/07/2013 - 10:51

Linearity of loads
by Dana Dorsett

Helpful? 0

Heating loads are only approximately linear with temperature differences between interior & exterior temps, with MANY secondary & third-order aspects to screw it up. The U-factors of low-E windows is VERY non-linear, with the radiated portion of the loss being a function of the difference of the fourth powers of the absolute (relative to absolute zero) of the temperatures on each side of the low-E surface. The U-factor of insulation materials is also non-linear with temperature & delta-T. Mid to high density fiber insulations increase in R with lower outdoor temps. Foam insulations also have highly non-linear R with temperature & delta-T- polystyrene goods increase in R with falling average temp, falls with rising temp, and it's by more than 10% across the annual temperature ranges seen in most of the US. Polyiso falls somewhat with both rising OR falling temp around a middle sweet-spot. Then there are the not strictly air temperature aspects like wind-washing, wind driven air infiltration, passive solar gains, etc.

But despite all of these effects, linear approximations based on temperature alone are still pretty good across typical temperature ranges one would expect, though not better than 10% accuracy at any particular set of indoor & outdoor temperature conditions across the spectrum of other possible second & third order effects. It's definitely NOT a geometric or exponential fit, more of a linear fit to the average, but the noise coefficients are large.

For example, in a house with a lot of south facing glass the heat load at 30F outdoors, 70F indoors may be near zero or even NEGATIVE at noon on a sunny calm day when there's a reflective snow cover boosting the solar gain, but quite substantial a 2AM during a 40mph blizzard. The temperatures are the same, the heat load isn't.

Cooling loads are even less linear with temp, with solar gain through glazing being a sometimes dominating secondary effect, followed by outdoor air humidity & air infiltration & ventilation rates.

Curt: Heat pump water heaters are the best dehumidifers EVER, eh? In a steamy gulf-coast climate you just can't beat the double-duty aspect!


21.
Tue, 05/07/2013 - 22:47

Yes times two down here
by Curt Kinder

Helpful? 0

Windows nearly always dominate cooling load down here, unless a home is hopelessly leaky. Window cooling loads are about on par with penalty caused by ductwork in unconditioned attics.

While I routinely specify / retrofit heat pump water heaters into unconditioned garages, saving clients approximately $100 per person per year, the absolute best place for an HPWH down here is somewhere within the conditioned enclosure, where the home can make use of the free cooling and drying occasioned by water heating.

My own HPWH routinely maintains our basement 5*F cooler and 10% RH dryer than the rest of the home during shoulder seasons while it totes the whole water heating load (~4 hours operation / ~3 kwh / day) We are in the process of finishing the basement for a teenaged child, but I routinely threaten to move down there out of envy for conditions best suited for a displaced New Englander...


22.
Wed, 05/08/2013 - 16:22

50% improvement is significant
by Derek Roff

Helpful? 0

This article says several times things along the lines of "Studies have shown that thermal mass can provide heating energy savings in only a few areas of the country." However, both the linked ORNL and "Mass Confusion" articles say that in the worst climate modeled, Minneapolis, high thermal mass will give a roughly 50% improvement in the Dynamic Thermal Performance, with an R-17 wall. 50% improvement is a lot, particularly since this is the worst example. The improvements are even better in Denver, Miami, Washington, and Atlanta, peaking with Phoenix, as mentioned. The effect is far better than negligible in all locations, which represent a variety of climates across the country. These figures appear to be averages for the whole year.

The advantage of the high mass wall is greater in all locations for an R-17 wall than for an R-9 wall. Figures for low R-value walls (R-1.6 to 2.3) wall show a negative effect for high thermal mass in all locations except Phoenix. This might imply that advantages of high thermal mass could be even more significant, with levels of insulation higher than R-17, as many of us would prefer. However, the numbers for the R-13 wall are similar, and often slightly better than those for the R-17 wall.

I would appreciate your comments on each of these issues.


23.
Wed, 05/08/2013 - 17:09

Edited Wed, 05/08/2013 - 17:13.

Response to Derek Roff
by Martin Holladay, GBA Advisor

Helpful? 1

Derek,
Here is a summary of the conclusions of the ORNL research: "Potential whole building energy savings, available when lightweight walls are replaced by massive walls of the same R-value, were calculated for 143 m2 (1540 ft2 ) one-story ranch houses located in Minneapolis, Minnesota and Bakersfield, California. For high R-value walls, up to 8% of the whole building energy could be saved in Minneapolis and 18% in Bakersfield when wood-framed walls were replaced by massive wall systems. Thermal mass layers must be in good contact with the interior of the building in these walls."

You're correct that 8% energy savings in Minneapolis are not insignificant. My main unanswered question concerns the assumptions made for the control house -- namely, the "one-story ranch house with wood-framed walls." What rate of air leakage was assumed for this house? (For that matter, what rate of air leakage was assumed for the house with high-mass walls?)

In other words, if this one-story ranch house wasn't built very well, the results may partly reflect the fact that most one-story ranch homes with wood-framed walls are poorly built.

My second question is: what would be the most cost-effective way to lower the energy bills of the wood-framed Minneapolis house by 8%? Here's my guess: a little bit of air sealing would save 8% of the heating energy -- for a much lower cost than building concrete walls.

When I have the time, I'll contact Jeff Christian or Jan Kosny at ORNL and ask about the assumptions made for the lightly built ranch house.


24.
Thu, 05/09/2013 - 11:10

Edited Thu, 05/09/2013 - 11:13.

how about longer thermal cycles?
by Dustin Harris

Helpful? 0

RMI claimed that their HQ, build with huge thermal mass, could even out seasonal fluctuations. Same claim was often made for other underground/earth-sheltered homes. This seems intuitively reasonable, even though I have not run the numbers...

At the very least, larger thermal mass (like a large battery bank) would allow one to coast over extended extremely cold or hot periods.


25.
Thu, 05/09/2013 - 11:31

Response to Dustin Harris
by Martin Holladay, GBA Advisor

Helpful? 0

Dustin,
Count me a skeptic when I hear that someone claims that their house has so much thermal mass that it evens out seasonal fluctuations.

Of course, an underground house can take advantage of the fact that the soil deep underground has a temperature equal to the annual average air temperature. But if you are in a northern climate, and your house is surrounded with dirt at 48°F, you'll still need to heat your house all winter. (However, you will probably be able to get away without using air conditioning, as long as you minimize your windows -- which, if your house is truly underground, shouldn't be that difficult.)

One thing is for sure: if you are worried about power outages, fuel shortages, or natural disasters, then lots of thermal mass on the inside of your home will help keep your pipes from freezing as you wait for the linemen to repair the electrical lines. But it's possible to achieve the same result with lots of insulation and attention to airtightness.

There is no doubt that thick concrete inside your home has some advantages. For the average homeowner, however, the question is simple: are the advantages worth the high cost of the concrete?


26.
Thu, 05/09/2013 - 13:39

Seasonal thermal storage
by Derek Roff

Helpful? 0

Responding to Dustin's comment, I share Martin's skepticism. I have run the numbers on several different thermal mass systems that claim seasonal heat storage, and my calculations have never come close to matching the claims. In most cases, this is a good thing, from a comfort perspective, in the given designs. If they could really store significant heat energy from late summer to mid-winter, in their passive thermal mass systems coupled to the living space, then that living space would be overheated through October and November, and probably December, as well. I don't see how you could do seasonal thermal storage without an active system, and I haven't seen any of those that look cost effective.


27.
Fri, 05/10/2013 - 13:16

Comfort too
by Jim Baerg

Helpful? 0

I really lucked out. I bought a 100 year old house 7 years ago. Storey and a half with double wythe brick walls on the main floor, so no stud cavity and 2.3-3" air gap between the bricks. It's always been cold in the winter, wonderfully cool in the summer (8400 HDD, good diurnal swings, but we can get temps up to 100F in the summer). Last fall, I blew the wall cavity with dense pack FG. Immediate noise reduction and comfort. We turned off the furnace a month ago, and are increasingly leaving the doors and windows open. Come summer, we'll regulate the doors and windows and stay cool.
Can't beat it.
Here's a question. Can you recommend any software that has good building input capability (with thermal mass), uses hourly weather data and does hourly output, so that I can mess with daily and weekly temperture swings? Annual energy summaries are useful, but I'm often concerned about worst case scenarios in my design work. Thaks.


28.
Fri, 05/10/2013 - 13:49

Response to Jim Baerg
by Martin Holladay, GBA Advisor

Helpful? 0

Jim,
I'm not really sure which energy modeling program will meet your needs; perhaps another GBA reader will make a suggestion.

In the meantime, here is a link to an article that provides an overview of energy modeling programs: Energy Modeling Software.


29.
Sat, 05/11/2013 - 15:24

Edited Sat, 05/11/2013 - 15:26.

Thermal mass - not really for homes?
by Roger Anthony

Helpful? 0

The thing with thermal mass is, it can only emit heat when the room air temperature drops below that of the emitter.

When you use concrete as a thermal store, the store has to be completely insulated and separate from its surroundings, otherwise the input heat disappears into the far reaches of the structure and is so diluted as to be irrecoverable.

Concrete mass is not really suitable for homes, it is mainly used in glass office building where the concrete absorbs heat and delays the moment when the air conditioning comes on, this is usually about six hours.

The system works best where you have regular hot days and cold nights. Where the mass can be cooled overnight by opening all the windows making it ready for the next hot day.

When you have several hot days and nights in a row, the mass doesn't cool and the system breaks down.

There is no point in having dense walls or floors that are not warmed by the sun, they merely add
cost and contribute nothing.

If you decide to have mass you need floors that can be warmed in summers direct downward sun and perhaps, walls that are positioned to be warmed by the sun in spring and autumn. Keep in mind that the days are shorter now, the sun is low and it does not have much heat in the morning, and your windows will bounce a lot of the available heat as the suns angle sharpens.

My sun lounge has floor to ceiling windows facing south and smaller windows facing east and west, this is because we like to see the sun rise and sun set, not because the sun adds much heat in the morning and evening. Morning sun lifts the temperature from 22C to sometimes 25C. Daytime sun can rise 22c to 40C or more - far too hot.

We then retire to our north facing lounge where the temperature is a more comfortable 22-25C.

With concrete floor and brick walls the residual heat from the sun in the sun lounge and other rooms often keeps the heating off for the whole of our home until after midnight.


30.
Sun, 05/12/2013 - 09:33

Another aspect of thermal mass difficult to assign value
by Curt Kinder

Helpful? 1

When commissioning HVAC in a high mass home, I typically adjust the system for less frequent, longer cycles. Within the Honeywell thermostats we often use is a parameter, CPH cycles per hour, that governs this. It defaults to 3, but I'll set it down to 2 or even 1, taking advantage of the lower rate of temperature change characteristic of a high mass house.

Longer less-frequent operating cycles by the HVAC system confers 3 benefits:

1) System spends more time in steady state operation where it operates closest to rated efficiency

2) Improved dehumidification - full dehu only begins when indoor coil drops to steady state temperature, and that takes many minutes, especially with higher SEER systems whose coils are larger.

3) Fewer starts / stops - presumably less wear and tear on system.


31.
Sun, 05/19/2013 - 11:17

We're all cavemen
by Gerard Celentano

Helpful? 0

Nice article. I like that it distinguishes the two properties. I think in the years to come, the building "science" industry may start talking about the thermal time constant, which is a metric that helps describe how quickly a house heats up and cools down.

Drawing analogies from electronics, the time constant is influenced by both thermal mass and R-value. Although the time constant is the product of the two, it's relationship to the temperature of house is described by more complicated equations, which is beyond the scope of this thread. However, understand that as you increase either R or the thermal mass, you increase the amount of time the house will take to respond to a temperature difference.

This is why people often confuse the two concepts and talk about an "effective" R value of thermal mass. However, they're different physical properties and every material has both. Most of us have a good feel for R value, which influences the speed of heat transfer. Thermal mass acts like a battery to store and release heat. It is more than concrete walls and earth married structures. Everything has mass, and it affects the thermal response (ie comfort) of the structure. The mass of an asphalt roof, for example, will keep your attic uncomfortably hot for many hours after the sun sets. To complicate things, a material's R value influences it's "effective" thermal mass and vice-verse. The fact that building codes meld the two together shows just how primitive the building industry is in their understanding of these principles.

The challenge we have is that as we super-insulate a house, we drastically lengthen the time constant and our historical understanding of how a house should behave may no longer be accurate. For example passive gain becomes a fleeting commodity, and allowing your house to cool down during the overnight hours may not be practical.

This illustrates that heating and cooling a house is a dynamic process. The demands constantly change with varying outside and inside temperatures, wind and solar load, etc. Modeling them with static conditions and linear equations can only be so accurate. More complicated models based on differential equations are used in other industries because they more accurately account for this. In the years to come, the building "science" industry will start to explore these concepts.


32.
Sun, 07/14/2013 - 11:14

Good article
by Jon R

Helpful? 0

I'd like to see more data on the effect on nighttime thermostat setback. Up to 15% used to be quoted as the savings for a low mass home, but with better insulation, this figure needs to be revised downwards. Is it still viable and does its benefit outweigh interior mass? I should add that using % is misleading - btus saved would be much better.


33.
Sun, 11/17/2013 - 18:01

Desert SouthWest
by Peter L

Helpful? 0

As mentioned here, the desert SW is the perfect place for mass walls like ICF. Hence one of the reasons why I am building with ICF. We get vast diurnal swings out here at 4,800 feet elevation (Northern AZ). During winter It can be 65F by 4PM and then by 6AM it will be 25F. During summer it can peak at 95F at 7PM and then by 5AM it will be 55F.

My house design will be geared towards Passive Solar/Passive House with exposed concrete slabs on the south exposures to benefit from the thermal mass. It is a Zone 4 climate. The summer sun is brutal but 30" roof overhangs and thermal curtains will help curtail the summer sun. I was also going to incorporate "throw rugs" on the exposed slabs on the south elevation. Any direct sunlight from the lower windows will not hit and warm the slab since the rugs will prevent the slab from warming and keep it cool during summer months. Then when fall - spring comes, one can simply remove the throw rug and let the sun do its thing and warm-up the slab.

I also don't mind the vast indoor temperature swings. Warming up to 80F during the day and then down to 70F by morning is not a big deal to me. I would rather wear shorts and then put on a sweater than pay high utility bills to the energy monopoly known as APS.

The Native Americans understood thermal mass and the adobe dwellings lets them survive the brutal summers and cold winter nights. I think ICF gets a bad "rap" but some of that is deserved because they made some outrageous claims in the beginning. ICF offers a lot of advantages that wood structures don't and vice versa. Out here builders don't know how to seal a wood frame home. They build them "leaky" so they can breathe. At least with ICF the walls are air tight and as long as you pay attention to the windows, doors, and roof transition areas, an ICF home can hit PH Standards of <0.60 ACH more easily than a wood frame home. A lot more to detail in a wood framed home than stacking and pouring a 6" monolithic concrete ICF wall assembly. Builders would think you are crazy if you asked them to tape the OSB seams. Shoot, they don't even put OSB on the exterior walls, it's mostly open framing (2x4 @ 16" o.c) with no OSB sheathing out here.

ICF/thermal mass works but is very location dependent and design dependent. If you get both right, you can have a home that performs better than a wood framed home would. Getting the wood frame home to perform as well as the ICF home in that scenario would require more money and out-of-state expertise that would break the bank.


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