How much thermal mass in a strawbale house?

I know that straw is mostly insulation. Is the clay/plaster coating applied to the outside and inside of the strawbales thick enough to be a significant degree of earth mass? Earthmass will help to hold heat or cold. Insulation keeps the outside temperature from influencing the inside. Can these two work together to provide a more comfortable environment inside?

Tom Connolly wrote:I know that straw is mostly insulation. Is the clay/plaster coating applied to the outside and inside of the strawbales thick enough to be a significant degree of earth mass? Earthmass will help to hold heat or cold. Insulation keeps the outside temperature from influencing the inside. Can these two work together to provide a more comfortable environment inside?

Plaster would have a lot of surface comparing to it's mass. Because of that heat would get released at a very high rate so the heat inertia would be pretty much nonexistent. Think of a radiator that has its heat exchange rate increased by increasing the surface. Or see how much quicker the same amount of water cools down if it's poured on on a plate vs when it's sitting in a cup.

I think the whole "Insulation vs Mass" argument that came out of mainstream construction with moralities like "build tight ventilate right" have people confused....Build tight toxins in they mechanically ventilated to save your life! They (being ASHRAE 62.2 and other "Building Scientist") argue and debate over the ventilation rate or exchange per hour requirement.....What they fail to get is the more you ventilate(intake and exhaust including all the chemicals in the mechanical device, heat and chem treats of metal, plastics, etc)) the more VOC is pushed around from the intake and exhaust heat exchanger and moisture exchange (HRV/ERV)...

Back before all this non-sense the walls would breath and/or were made of natural materials...meaning the envelope(walls, roof, foundation) provided the air changes per hour not a machine. When drywall was invented they put holes in it, same with siding....The insulation was a heat sink, the render (plaster/stucco) earth that is hygroscopic with a high ability to store and pass vapor through the wall to dry, and equalize pressures on both sides of the wall.....a vapor barrier does not allow and compounds infiltration of vapor pressure into a wall that can not dry well or at all.

"Thermal Mass" needs to be replaced with "Hygroscopic Mass" or a term that also considers vapor pressure and drying it in a heat sink insulation(r-value), not just storing and releasing heat or cold since that is a function of moisture content in a wall too. So it gets complex the way mass and combined insulation works, since the insulation could be assumed as steady state r-value but the mass is both thermally and hygroscopically dynamic.....At the surface it spreads out initially then it penetrates to a depth.

Orlando National Lab did a study on concrete of different insulations-mass configurations (mass-insulation-mass, insulation-mass-insulation (ICF) just insulation and compared....that showed 2" thick concrete held significant heat when in direct contact with air, 4' held more in a lab and field test across the USA. In reality that can change depending on the materials and alot of other factors. Very difficult to quantify.

The interesting part of the study showed a significant increase in energy consumption when ICF or foam is common to the interior conditioned air. 4' inches thick of concrete to the interior showed the least energy consumption......If you put insulation between the two renders (mass-insulation-mass) and thermo-couples on the plaster an stucco you see temperature spikes that differ in a lag time of 6-12 hours....This allows the wall to store heat during the day say from the sun then release it at night when the interior temps drop. When it rained the exterior thermo-couple cooled down. Clay does the same thing with relative humidity....That is why prior to the "sick home syndrome" post WW2 people did not have mold and building related heath issues from vapor barriers/retarders and walls that could not breath heat/moisture.

I did a test this winter high humidity on an exterior rammed earth wall I built @ 50 temperature difference....I got a heat depth of 4 inches after 24 hours for heat using my hand, I think I would get to 6" if I let it store (heat sink) for a week. I need to buy a moisture and temp multi meter if anyone knows of one.

There is a lot more to learn about breathable walls I am reading a book on now

Here is a definition of what I was trying to explain above:

Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.[1] This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid (the absorbate) permeates or is dissolved by a liquid or solid (the absorbent).[2] Adsorption is a surface-based process while absorption involves the whole volume of the material. The term sorption encompasses both processes, while desorption is the reverse of it. Adsorption is a surface phenomenon.

Clay plaster is a "sorption" process containing both a moisture and thermal function.

The manufacturing industry uses "r-value" which is a steady state measurement of the resistance to heat flux through a material in one dimension....some materials such as fiberglass, straw, etc, will see less resistance when wet or a knock down factor of say 2-4 r, while other materials like clay will not. Clay has several dimensions r-value alone can not measure because it is dynamic mass.

Some say r-value applies to all materials. I agree and disagree. R-value can be misleading. For example, some including myself may think clay slip @ r=1.7/inch provides less insulation than straw @ 3. CS will have more mass effect which is measured dynamically and difficult to put a value to. Since we do not understand mass some may think render over straw is a safer approach than slip. ORNL put a Dynamic Mass Value to concrete, no-one has to clay plaster or slip I know of. It would be helpful but not completely revealing since the whole house dynamics is complex.

In theory, clay based renders or skins sandwiched between a straw core will provide a breathable wall....The wall dynamics combined in a home should reduce hot and cold spots, regulate humidity, and yes provide a comfortable home. That is what homeowners have reported. Care has to be taken to avoid over heating or cold temperatures of the mass system, it's orientation to the sun, hvac size, glazing, etc. Some climates with large daily temperature swings do best, other that stay above the internal desired daily temperature or below it most of the day will not do as well. High relative humidity that surpasses the mass ability to suspend, hold, pass, and equalize dynamically will not do as well unless there is enough mass to manage it. It gets tricky quantifying thickness, 2-6" of render is what I have seen max with stucco-straw-plaster. 12" of slip clay or limecrete, rammed earth, etc....unless in a extremely hot or cold climate then a thermal brake core like straw has may be needed for clay-slip, rammed earth, limecrete casting's. If the clay content is low the wall will not perform as well, that is why the video Tom posted on slip shows a 40% min as tested by a lab.....The denser the wall like 3000+ psi concrete the less hygroscopic and ability to store heat...Portland cement is not very hygroscopic but is has a lot of compression strength. Same with lime and MGO compared to clay. If adding lime, mgo, pozzolans to render it may reduce the ability to store, but increase compression strength for a straw load bearing wall for example where the render takes more load.

That is my level of understanding at this point, right, wrong, or indifferent I personally probably like most am a little afraid of mass since I don't have a lot of experience with it nor measured data in my climate or design tools(hvac load software, material data, etc) like insulation has in r-value out there. The modeling we do have like WUFI do not have natural materials in the library and have been proven unreliable and inaccurate with insulation, nor do these models understand mass. The best info are local builds and from homeowers. If the home over heats or is cold often like some I have read there are limits on what can be done about it, not that comfortable. I'll probably start with clay based solar passive wall (trombe type sorta) in my first build with an well insulated envelope see how it goes.....Alot has to do with windows too, solar heat gain, e-coating, overhangs, shutters, trees, etc....

Terry - excellent research!

I couldn't agree more, water has a large latent heat of vaporization. Which means that it takes up a lot of energy(heat) when it changes phase from water to vapor and gives that heat back when it changes back. So hygroscopic mass makes sense to me.

Vapor goes in the direction of heat flow through a wall. So right now(morning) my adobe walls are colder than the air in the home, so moisture carrying heat is driving through the wall assembly. The trapped heat and vapor will accumulate throughout the day as sunlight warms the walls and interior air and then radiate back in the night as interior temps drop below wall temps. I can say that this works best in every season but right now. When it is really cold, the 24" thick adobe walls only perform as well as a modern home, but in every other season, the modern home is seriously lacking in thermal performance and especially in interior comfort.

Thanks Bill....

http://hopeforarchitecture.com/blog/

Scroll down about 2/3 and read and watch the Thermal Mass video Clemson University Brick Research Center did.....If you look at the thermo-couple graph of the interior and exterior wall you see the spikes in temp are in a 6 hour lag. This could reduce energy cost in some locations that have a surcharge for peak usage. The sine wave shows that the wall is absorbing heat and releasing it. The inner wall is independent of the outer wall and influenced by the HVAC that stays close to the hvac set point of 70F. The HVAC never lets the wall get below 65 and can control the mass in any climate zone. The outer is not bridging in the hot humid climate of Atlanta, GA or the spikes would be in the same time frame.Studded walls would bridge. Straw walls with clay render would perform like the graph and what I was trying to explain above. These guys are doing some hot box test with some bricks that are not allowed by American code, ceramic, more aerated, used in Europe, and a double wykes (rows) with an air or insulation gap or core like strawbales and clay. I would have like to see a moisture test, but you can see how the outer wall cooled down when it rained.

The conclusion here since the HVAC is influencing the inner wall is the same as ORNL that mass can work in any climate zone. The difference is free solar passive which will show different benefits in different climate zones. The materials used should match the climate zone. Having two wykes like bales do gives you more control to design to the inside set point separate from the outside conditions.

Structurally, you want uniform plaster and stucco mixes if load bearing.

I love this:

Less than 200 years ago, sawmill technology entered the North American scene, and with it, along with our vast forest resources, came the advent of light framing. Before we understood the full implications of this quick and ready, stripped down form of construction — before we had the mindset or faculty to grasp the many gross shortcomings, it became familiar and normal, and part of our social and economic fabric.

Only after stick framing was a defining part of our building culture did we begin to realize flaws. Not all at once, and this is important to note, because it explains how we incrementally backed ourselves into the current proverbial corner. One by one, we began to address issues as our ability to understand them matured; in each instance adding ‘resources— energy— man power’ to shore up what is, essentially, shack technology… and each time, backing up; adding uniquely designed products, highly manufactured composites and even language like ‘renewable’ to soothe the collective green conscience; all the while, backing up. Each instance has been marked by a discovery of some new hole in convention needing to be filled until we now actually put more into the practice of disposable building than we would building authentically and for the ages.

From what I've read on the topic, about two cm depth of the wall is involved in a 24 hr heat cycle.
If that is the case a heavy plaster should make a difference indeed.

Common practice around here when building a straw bale house is 4-5 cm thick clay plaster on the inside.
Should make a difference.
Sure hope so anyway, otherwise we're doing a whole lot of work for nothing...

/Peter

Thermal Mass vs. Insulation.

From what I understand on this topic, thermal mass only works well in climates with large daily temperature changes. Think of high altitude deserts (hot during the day cold at night). The idea is the mass absorbed heat during the day and releases it at night. In places that are cold for several months, and then hot for a few months, good insulation is much more practical.

Thomas, not entirely accurate many US and German climates have proven that mass can perform well in just about any climate. Refer to NREL and ORNL in the US, mass calculator that shows utility bills will lower in many US climates. The software is based on many built and tested homes, and 1000's of simulation models. It performs extremely well in wet climates if designed right due to latent heat of evaporation. The best designers understand it's dynamic properties and since static r-values are easier for most builders to understand most will stick with it. It take a long thread to explain....look at some of my threads like "Breathable Walls" and Healthy Homes. We here in the US build with more insulation as opposed to Germany whom has set the benchmark for passive net zero mass homes that breath and handle vapor. We in the US recently decided not to use their standards to design passive homes since we do not understand it and how it applies to the MANY layers we design to and air cavity flows primarily, now we have a standard called US_PHPP as opposed to PHPP from Germany. The divorce between the two happen mid last year, it is quite entertaining to say the least. A better more accurate term is "Hygrothermal Mass" used by the better German design software like WUFI they have focused on for decades as opposed to most including the USA....It takes a knowledgeable person to run proper inputs and materials like ASHRAE 160 and understand the outputs. There are alot of human factors such as comfort and health many have not understood nor factored into the equations yet since it is difficult to place values on. In time I believe natural mass will dominate the US but that is going to take a different building science & knowledge base that we lost decades ago due to mass home productions. There is alot of politics and $ obstacles involved in focusing in on these less expensive better natural renewable and sustainable designs.

I think we would both agree mass can be good to moderate internal temperatures. However mass as an outer wall (versus straw insulation) will conduct the heat away. Thermodynamics works well in lab settings because it is based on observed lab values. If it can realistically be applied to a house it would be amazing but I would still be skeptical......then again I am skeptical of the entire theory because is does not account for most of the mass and energy in the universe.

Study heat of vaporization external mass does not conduct heat "away" it is renewed internal and hidden with phase changes. Mass performs as well or better as an exterior stucco to regulate it's boundary condition in some cases better than internal plasters. If you know how to isolate the internal and external boundaries from one another it performs extremely well as insulation of a high r-value and dynamic mass. Run the free ORNL WUFI model for your design there are limited natural materials in it to start to give one an idea of how mass performs. Heat transfer, fluxes, and specific heats of materials and boundary conditions are just parts of the equations, moisture and airflows or vapor pressures, along with many other interactive dynamic material properties are some others. Thermodynamics, chemistry, physics, and more specially "Computational Fluid Flow Dynamics (CFD)" or in other words "Hygrothermal Mass" applies. You have to load your climate files too which comes from daily field observations in your climates zone, they can be found on the DOE websites. Another myth is that these models and climate files are derived in a lab under controlled conditions which is far from the truth. I have been a part of many lab and/or continuous field test to develop material properties. If you want a good mass design pro's with applicable experience have to be involved just like any other industry (aircraft, auto, powersports, etc). For the most part CFD software's like WUFI have built, tested, and continue to back calibrate the software for years. We in USA are just now getting more accurate but, Germany has been doing it for over three decades and there is much more proof of mass performing very well in history depending on the designs and people conducting the designs. There have been cases that WUFI and history have been inaccurate but, far less than most that try and make sense out of something that is VERY complex without the proper education, experience, and design tools.

Terry, this is what I mean by Thermodynamics "mass does not conduct heat "away" it is renewed internal and hidden with phase change" & "moisture and airflows or vapor pressures, along with many other interactive dynamic material properties". These theories/calculations have less do with mass or insulation than maintaining a closed system.

You mentioned human factors as well: A dry air tight home conserves heat better, but feels colder to people because the air does not conduct heat very well. A damp air tight house grows mold. In order to avoid these problems you exchange air or water with the outside so it is no longer a closed system.

I am sure the people at ORNL are very smart, but they are doing grant based research and not so much applied or independent research.

Thomas, I'll restate the above hopefully less confusing this time. ORNL whom has conducted decades of applied building science here in the USA, or has built many test homes in many US climate zones, in conjunction with BSC whom also has also conducted MANY applied building science projects, is now hosting a free version of German based WUFI for US climate zones that is more applicable to common USA building technologies. Previous to that, ORNL had developed a very accurate "Mass Calculator' that has proven successful and very accurate in comparing cast-in-place concrete and ICF construction to stick frame insulated construction utility bills. Once again, these calculators ARE NOT THEORY! Anyone that has read the reports and ran the software and knows what they are talking about knows this.

WUFI and ORNL offers new software again based on WUFI over three decades of German building and testing homes, no theory, BUT, WUFI is more "Mass" based due to Germany's codes, not highly insulated such as that found in the USA. To obtain accurate software the pro version of WUFI is needed and it cost a couple thousand. Air tight passive homes whether constructed with mass or insulation can be certified to a new US "Passive house" standard that now involves two paths, using a US "Passive" -WUFI version with limited materials or a pro version, or, per Manual J. The software is also used to determine ventilation rates for tightly constructed homes per ASHRAE 62.4. DOE also offers very accurate modeling software for US climates that is not founded in theory.

It is pointless to go back and forth with all the variables that can occur in a building without having a complete understanding of these design tools and organizations so I will leave you with that. Good luck!

This page does a good job explaining where and when you may want to use thermal mass (without the modern mysticism)

Thermal Mass

Keep in mind when they say cool or cold climates they may mean something a little different in Australia than where you live.