I've done a lot of research lately in sustainable building, zero-energy building, and budget building techniques. I've looked mostly at earthbag, straw bale, and earthships.
While it sounded so excited to have a zero energy home that produces its own water, electricity, and food.. from what i understand the earthship design doesn't work well in a lot of climates. In many parts of the country its too hot and humid to keep itself cool with the thermal mass principle. The night temperatures in a lot of places don't drop enough for the thermal mass to re-cool and get ready for another day of absorbing heat. And since the earth itself is not a good insulator and absorbs the heat, running A/C becomes sort of a challenge when you are fighting to cool all of the thermal mass that stores the heat from the day and then losing all of the cool to the environment by poor insulation.
Am I correct about this whole thermal mass thing? Is it only valuable in the desert and cold climates? Or is it still a viable option to aid in the heating and cooling of a home in places like the Carolinas?
I'm really just trying to figure out the absolute best building technique for my area that will be cheap and also be super efficient or even self sufficient. I'm assuming that insulation is much more important than thermal mass in my area where its probably impossible to really get away without using A/C to maintain a comfortable environment. So instead of using my home like a heat sink (thermal mass), its more important to keep the internal temperature sealed in and contained. Is there anything that combines insulation properties AND thermal mass properties to allow efficient A/C cooling and thermal mass heating in winter?
There seems to be so little info on this topic when it comes to earthen structures. Nobody really seems to address the issue with warm, humid climates.
Am I missing something? Do any of you have any experience building earthen buildings in these sort of climates?
Joined: Nov 11, 2010
Location: Burlington, NC - Woodland, Clay - Zone 7
I live in North Carolina so I am in similar conditions. What you have very valuable under your feet is clay soil. I am working on utilizing clay soil in the form of stabilized compressed earth blocks. I do not plan on using much insulation but instead relying more on thermal mass to absorb temperature fluctuations. I am also working on a design that utilizes a solar chimney to draw fresh air from the outside which will travel through a subterranean air vent causing the air to chill in summer and heat in winter. I can also arrange the solar chimney in a fashion to allow me to cool my home using a rocket stove to accelerate air intake while I heat water and generate biochar plus woodgas.
Those who hammer their swords into plows will plow for those who don't!
I remember them building an Earthship in East Texas. You're right that earth is not a good insulator, but that's why they surround the dirt with insulation. Use cooling tubes buried in the north side earth to bring in cool air. Vents on top of south to let out.
Joined: Nov 02, 2011
Location: Asheville NC
I think youve got it Tyler. Thermal mass TM is not going to keep one comfortable year round in our climate (and most North American climates) compared to Air sealing and Continuous Insulation. Sure, include as much TM as whats economically feasible but personally, I think that ends with concrete slabs and below grade foundation walls.
There are a lot of compelling reasons to use earthen construction. In our climate however the high labor costs and difficulty with code and airsealing/insulation make it a tough sell. I would love to see examples of earthen systems that are cheap and quick to build airtight and continuously insulated. In my opinion, they just cant match SIPS, Pre-cast concrete panels or advanced/double wall framing with exterior insulated sheathing.
The SC climate is a little more forgiving as far as recommended insulation levels. I would go with ICC, BSC or Energy Star minimum recommend R values, shoot for a Blower Door Test result of ACH 50 < 2.5 and plan on an efficient heat pump (PTAC, GSHP, Mini-split, or variable speed ASHP). I would also explore passive solar design which can provide 40-90% of your heating needs for free.
While I can't speak of first hand knowledge of Thermal Mass working in the Carolinas, I can comfortably relate how well it works in Florida.
With relatives that live down there I've visited at various times of the year and can assure you that the mass does remain relatively cool, at least in a filled CMU house. Cool enough that it's heaven to lie on a tiled floor and a fan can handle most of the necessary air movement.
The greatest problem in my opinion is air control. You simply can't condition the air quickly enough when you open the door, and let's not even consider the ASHRAE air change requirements if you aren't using an ERV.
The earth tube solution can be, depending on where you live, a workable solution to the problem, allowing you to draw in noticeably cooler air. Unfortunately it's a bit of a problem when the water table is a foot deep.
The deep soil temperature here in SC is in the low 60's F (its 61F here in Greenwood), so if you can build your house in a manner (bermed, earth sheltered, earthtubes,etc.) to take advantage of this, you can keep cool doing the summer without A/C and require only minimal heat during the winter. The main problem to be addressed is dehumidification in the summer when that humid 90F summer air enters the house and gets cooled down into the 70's F and becomes even more humid. Can an earthtube system be designed to collect and drain off the condensate without any mold/mildew problems?
I have also been trying to find information on building techniques in hot humid climates, as I am located in southeast Tennessee. I am no resource on the subject, but I have put a lot of research into different techniques. You may look into Rammed Earth or SIRE wall (Stabilized Insulated Rammed Earth), the latter one not looking so easy to do by yourself. This seems a very labor intensive process though, maybe even more so than cob. It seems to be used in many different climate zones, even in South Carolina. The Stateburg Church of the Holy Cross was built in 1852 in Stateburg, South Carolina near Sumter. Also the Borough House Plantation is a home built across from the church, constructed of rammed earth in 1820. I don't know how autonomous it would be as far as heating and cooling, but it's worth looking into. Good luck!
Tyler I live in South Carolina as well, near Greenville, and I feel your pain brother! From the research I have done I think cobb is still a viable building technique in our area, and I plan on building a cobb home near Landrum S.C.
In The Hand Sculpted House they say that for a climate like ours you need to surround the home with fast growing deciduous trees to provide constant shade during the hottest times of the year, Tulip Poplar is what I will be using, which can grow as much as 6 ft per season. If you look at many old homesteads in our area you will see that is exactly what people did back before air conditioning. I think the combination of cobbs breathability and constant summer shade will keep it plenty cool in the summer months, and when the leaves fall I can still reap the benefits of a passive solar design to help heat in the winter.
Keep me updated on how you decide to build, maybe we could swap ideas/problems we have ran into. I hope to start on my cobb home next spring.
I also live in NC, and as a mechanical engineer, I have insight into HVAC issues. Thermal mass is great anywhere, all the time. If you have enough you can run your air conditioning during the night. This is good for two reasons. During the night, you will be rejecting heat from your house at at lower temperature, so the pressure in the heat pump's condenser will be lower, and the compressor's motor will use less power. Night is also off peak, so if you have time of day rates kiloWatt hours are cheaper.
I would have a timer set, so that the heat pump would start at such a time so the target temperature would be reached about dawn. That way the outside temp would be as low as possible.
Sufficient thermal mass would make it unnecessary to damp a wood stove at night to keep it burning. You could run it wide open until you reached a target temperature, and then just shut it down.
The problem with solar chimneys, earth tubes, and the rest of the unpowered ways to keep cool, is they do not deal with humidity. Today, vapor compression heat pumps are the only technique available to homeowners to dehumidify. All dehumidifiers available commercially are actually heat pumps.
If you want to air condition as responsibly as possible, look into a ground source(often called geothermal) heat pump, and some photovoltaics to help power it.
I want to build a home with CEB load bearing walls, and non bearing straw bale walls for insulation. Between the thermal mass of the block, and the insulation of the bales it would take days to get cold in the winter.
Joined: Nov 02, 2011
Location: Asheville NC
Great post William, I hope you can become a regular contributor. Absolutely agree that nothing beats a heat pump for staying comfortable in hot and humid climates. Also, time-of-use billing is a great way to lower energy bills and people shouldnt forget about using this same strategy with a water heater timer.
I have to disagree with the ground source heat pump GSHP comment though. I think they can make a lot of sense but mainly for very large or largely in-efficient homes. PV powered absolutely, but I think most building scientists and high performance home builders are favoring mini-split heat pumps instead of GSHP in climates as far north as new england. It really depends on the project but I think a small to modest sized home will see a better financial and environmental return by focusing on the building envelope to the point that the extra resources spent on adding the GSHP is a waste. Take the extra money you would spend on GSHP and put it into air-sealing, insulation, windows or PV. The life of the GSHP mechanicals will be a fraction of the life of the building envelope. Also the advertised Coefficient of Performance COPs of GSHP dont seem to be living up to their real world measured performance while the measured COPs of most mini-splits are surpassing their advertised numbers.
I also think that how many days the house would take to get cold in the winter would depend FAR more on the blower door test results, R values and how continuous the "R" was installed. I think people waste way too many resources on thermal mass.