Barry: The reason wood is not good thermal mass is because its specific heat(C) is high. It takes twice the amount of heat to raise woods temperature one degree Celsius than concrete, for example. You’d need a lot deltaT to transfer or add or release heat or cold.
Here is the basic steady state heat transfer formula for materials: Q = CMΔT. The dynamics of the room can knock Q up or down.
Note mass, or thickness, density, in grams.
Brian: It’s hard to determine how much heat transfer will occur in log homes given the many parameters and variables. Thermal couples don’t speculate, one could run a temperature and pressure bench test of a given log, species, thickness, cross sectional area, sealant, etc…sustaining the sealant is key. If I wanted logs inside I’d go with timber or post-n-beam with an envelope wrapped in hay-bales, plaster and stucco to get some mass effect, since clay and bales are abundant in KS zone 4 where I am. I have to agree hiding lumber in walls is a sin, and look at all the issues with it and stick building. The only ones benefiting from are “building scientist” and theory how to bandied them, and of course giving my restoration company business. If you don’t want moisture in the wall bond skins to a core, such as SIP or SCIP. But some adhesive bond lines will fail depending on degree of thermal and pressure cycling.
Mass effect is difficult to quantify by code and relate to R-value. I agree with them being low until they are better understood, tested, and quantified. ORNL tried with DBMS and their ‘mass calculator’ but, that is still limited data. I do not see basic HVAC load software as having the ability to model this complex interaction of fluid, thermal, and aerodynamics to determine flux loads accurately. We struggle using sophisticated software and lab test data (hot box testing). To me, this is entirely different than steady state R-value. If you isolate the ΔT with Mass-Insulation-Mass or “CIC” as ORNL proved to be the most effective, steady state R-value goes away, and isolated thermal cycling between the inner mass and outer mass in lag times (preferable 6-12 hour cycles) becomes more significant. It becomes a complex matter of how thick is the isolation core, inner and outer mass, have to be. ORNL showed that 4” of concrete inner mass performed better than 2”, with the same 4” foam core or isolator but what is the COP? I offer again, a good model that can handle material properties and complex air flow as a start, then testing to quantify the DBMS as ORNL did.
I’d have to agree, basically it is a system that integrates to itself. The more mass inside the building connected to each other by dynamic thermal coupling the better. Having a single sun facing wall, and the rest of the floor and walls with a much lower specific heat (wood, drywall, paint, carpet, etc.) would show a lower DBMS value. The loading and unloading of walls can come from the sun or HVAC, therefore my understanding of ORNL shows a benefit is many climate zones, other than where outdoor daily temp swings are not far from the internal set point. I ran their mass calculator and see a benefit where I am at in Zone 4, compared to a stick build.
I agree a tight building is ideal if it is toxin free. Sealing toxins in makes no sense. ASHREA. 62.2 ventilation rates makes no reference to toxin in a building and is misleading. You need an IAQ meter to see where you are once the building is complete, then determine a rate based on ASHREA 62.2. Even then, if occupants bring in toxins it be a health hazard in tight buildings and a liability to builders of tight homes. That is why we have a clause in our contract for IAQ.
I’m leaning towards a tight envelope perhaps SIRE (Structural Insulated Rammed Earth) with a single zone air source mini-split location that distributes air to bedrooms doors closed and mass evenly that will require send and exhaust ducts to from passive solar facing bedrooms to colder rooms. The way I see it is, the solar walls that heat and cool need the most mass. The HVAC has to push conditioned from it to rest of the mass I hope to boost my ventilation system to do. If there is little to no airflow the bedrooms may see too large a ΔT to the single source mini-split air handler for cooling/heating if it is located in a hall or dining room. The loading can happen by HVAC or natural air or open fenestration plan too, with supplement mini-split. The non-solar walls may perhaps be of less mass if another method proves to be more cost effective (e.g.: than SIRE) since those walls are not a primary storage source. I think the idea of load distribution of walls and lag time is key, but difficult to quantify or design to up front. The biggest bang is in phase change, but I am not sure how to tap into that resource yet. Since grid electric is only $.10 KWH where I am PV is not a good ROI.
As far as cold floors, I have not looked at rammed earth floor much yet but, from what I gather as Brian points out the right thermal mass should stay at indoor conditioned air temps, if you can counter stack effect by sending heated air to the floors. Earths specific heat (more packed, denser the better) is a little higher than concrete but, some clays such as kaolinites are good for this application. I’m thinking it would not be a good idea to isolate-insulate a RE floor from ground source energy and let the indoor air condition the floor(hot HVAC air to cold floor and visa-versa) with the natural ground source temperatures? The clays plasticity and ability to expand, handle moisture fluctuations, if designed right makes it a better choice than concrete especially with Portland cement. Just add a fungi resistant such as borate. I looked for borate only place I find it is online and it did not look cheap. I’m thinking no vapor or air barrier RE
posted 6 years ago
Interesting I was able to post all that today on a different computer. The error I was getting said it did not like abbreviations. When I tried one paragraph at a time, then edit to add another, it worked.....when it got to a point I got the error again I went to another post to get past it. Seemed it was related to # of words but some on that thread have more than what got through. Weird, perhaps all the internet traffic on mother's day..my signal was weak
I know that many years ago out at CodeRanch, there were some people that were posting with the texting shorthand and some of the developers did not like it. So they implemented something that would catch the shorthand and reject the submission. It sounds like you accidentally tripped that. It would be good to know what tripped it so that we might be able to refine the algorithm.