I found a really nice and helpful presentation about Solar Home Design and Thermal Mass:
Northern Hemisphere needs the glazing on the South. People should be very careful with glazing ratios over about 12% window to floor area. The paper says 10-20%. Overheating can be a big issue with those kind of ratios even with a lot of thermal mass.
In passive solar design, its common for people to overestimate the importance of Thermal Mass TM. It can be easily argued that Airtightness and Insulation are much more important. The paper mentions that ACH50 is important with the insulation but ACH50 is actually more important than insulation in the home as a system.
TM can help performance but its easy for projects to go over budget and see little cost effective results from using too much. To me, the level that cost effectiveness drops dramatically is when more TM is used than what is needed anyway in the construction of the house; like concrete slabs or that included in the possible wall design.
I'm moving to the Asheville area shortly from Illinois, (grew up in SC) and am planning to build a passive solar log house. I've been planning to put down 4" thick concrete floors with backup radiant heating tubes as thermal mass, not overdoing windows on the S side. I read on this site that low e windows on the south side aren't a good idea... Enough thermal mass? Too much? I plan on having a sun room/porch along part of the south side, possibly with concrete flooring also. It'll be two story so there'll be high windows also. Sound like this will work? Any advice, a way to get in touch with you when I get there (project a few years out), ?... Sounds like you know what you're talking about, especially for the region. I've read and read, but most of the info I get is from either southwestern or northeastern climates where things are so different. Of course the radiant heat would be solar, probably on demand back-up, for the heat and domestic. I could afford consultation for all this stuff but want to do all the actual work myself.
I'm also planning to build my own masonry heater, a job I'm excited about but concerned about codes and stuff. No way I could afford to have it done. I've looked at a few plans, including Thomas Elpel's, my favorite, but am still a bit apprehensive about it. Any advice about local advice?
There is a very timely post on another forum if the moderators allow and it might be nice to start a discussion on this forum with a more permies slant; http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/fixing-leaky-log-home Edit: link may not work but its a horror story of log home weatherization efforts.
I think you are asking the right questions and its great that you are still in the planning stage unlike the person in the other thread who has already built and now trying to fix it. Passive solar is the way to go for any climate that needs heating and has the right site for it. It wont be hard to find that the majority of energy efficient building professionals would agree with going passive solar. Getting those same efficiency experts to agree using actual logs as a good fit for the exterior walls of the home? Not a chance. Iam not saying exterior log walls are the wrong choice for all permaculture projects but it sounds like they are the wrong choice for yours.
Are the looks of a real log home more important than energy costs and chopping a #%$ load of firewood? Have you considered interior and exterior log siding? What about some of the other insulated log home options? A real log wall might get you an Rvalue of 8. Thats not insulation, its thermal mass. To achieve higher than international code performance, you might need R20 under slab, triple pane windows and R100 roof to make up for all the heat you would be losing through your wall conduction. The bigger problem of course is the air-leaks. There are some log homes to achieve acceptable blower door test results but they are very rare and get worse over time as the wood does what wood does; moves.
Love the idea of Masonry fireplaces except for one major issue; Combustion. The more I learn about building performance and Indoor Air Quality IAQ, the more I avoid combustion appliances inside the conditioned space. If you dont mind the increased risk of IAQ problems then the cheap DIY fuel of wood is tough to beat. Masonry fireplaces are notoriously difficult and it usually takes a master mason to pull them off although I know there are kits becoming more available. Have you checked out the RMH here? Personally I would consider trying to create one that had the burn chamber outdoors but it would probably take an extremely efficient building envelope (not logs) or a very small structure to pull off as an acceptable backup heat source. This would have to be after the building department has given you a CO of course. One code approved method of heating with outdoor combustion is the outdoor wood furnace; better for IAQ but terrible for efficiency, but hey if real log walls are in the picture, who cares about inefficiency!?
Low E windows are almost always better than regular windows. Windows on the south should preferably be LowE but also high SHGC. The LowE coating on those windows are done differently.
Ive also got to question your radiant floors and hoping I can post my blog on the subject to better explain why. http://www.springtimehomes.com/asheville-builders-blog/?p=196
Lastly, I have concerns about your plans for a sunroom/porch on your south elevation. There are many threads here where I question the wisdom of greenhouses sharing the south elevation with the house. To me, most of these designs create as many problems as they solve. Not saying your planning an attached greenhouse but "sunroom" and "porch" are bad words in my views of what passive solar elevations should be unless they are on the East and West, not blocking or complicating the home's access to sunlight. I suggest sticking to my advice in the GBA article for what I see as cost-effective passive solar design. Now, arent you glad you didnt waste your money on paid consultation? Hope I didnt ruin many dreams today!
You've given me much food for thought, especially the link to your website about radiant flooring. Is this reasoning based solely on the cost of putting in the system?
Also, I'd already thought about the complexity of solar hot water supplying domestic and heating hot water. I believe some have been successful with doing a fairly simple system themselves but I can see how it would easily fail and cause monstrous headaches for years to come.
I'll really have to address all of this further at a later time. I'm usually aboard from 1-2 months and internet access is sparse.
But I'll be in touch and again, really appreciate all of your effort... so much to think about.
I completely understand the allure of a real log home as I use to share it myself. After many years of learning, I find myself in a completely different camp of people. Now, instead of having a warm feeling when viewing log homes I have a sick feeling. Most home performance contractors who have been brought in on log home weatherization projects will probably have the same feelings. When I see a log home now, I think, "those poor homeowners.. they chose looks and aesthetics over energy, maintenance costs and possibly comfort. That thing sure looks expensive and troublesome to keep comfortable and protect from the elements" or something like that..
I also respect your position about hiding the true materials. I can say Ive seen some very convincing insulated log homes, so much so that it fools professionals regularly. I would suggest falling in love with the look of timber framing as this method is much easier to combine with energy efficient construction and is arguably easier to maintain depending on the details. Full timber frames can be pricey but I think timber frame elements and accents can add a lot to the look of a home without adding a lot to the cost.
In defense of real log walls, they can make a lot of sense for homesteading sites with abundant usable logs that are in the tiny range (roughly 300 sqft or under).
Is my blog implying radiant sometimes doesnt make sense because of costs? Yes. Also that they can be more uncomfortable than forced air systems because of the temperature lag times. However, if your project is at a high elevation or a windy site and you think you might be able to avoid cooling with dehumidification then I would consider them more.
Solar thermal radiant floors are generally not a good match for passive solar designs on a budget in my opinion but there are plenty of exceptions out there. One of my future projects on my personal home is to install a thermosiphoning solar thermal radiant floor which Iam hoping will address much of the trouble, cost and complications with theses systems: pumps, controllers and over heating protection. Passive solar design works just fine without all that fuss. Adding solar thermal radiant floors is something for people with money, time and/or enjoy tinkering and maintenance. Also, dont trust any design that uses grid energy for circulation without the measurements to back up the supposed gains.
It's hard for me to grasp, after reading so much from log home owners who talk about how comfortable their home is and how easy they are to heat and cool, even in much harsher climates. But I surely see how difficult it could be to seal them well. I've lived in places that are much, much colder than NC. However I grew up in SC and understand the necessity of dehumidifying also! I now live in Illinois in a stick built house that was insulated to code at the time it was built, 1982, and either the heat or ac has to be going about 360 days a year. When it's 10 deg in the winter, or 98 deg in the summer, the heat pump can barely keep up. In the summer we could fry breakfast on the roof and cook dinner in the attic. In the winter the crawl space could keep our food in deep freeze. It's a very uncomfortable house, even after all the extra sealing I've done, and adding storm windows.
There's lots and lots of things I have to consider. I've done so much reading about log homes, including writings from researchers who've lived in log homes who insist log homes can't be judged on the true R value of logs alone. They attest to the long period of time it takes for the temperature to change in a log house. Most real log homes are built with logs that are at least 10", usually 12" or more. At an R value of 1.4 per inch, that's at least R-14. I'm not building a home with 6" milled "logs". Here's also a bit about R-value/thermal mass of logs... http://energy.gov/energysaver/articles/energy-efficiency-log-homes
I understand most likely your largest concern would be the leaks around the logs, but I don't necessarily want to live in a box that's so tight we'd suffocate if our ventilation system shut down either. Of course there's maintenance to consider to keep leaks out. There's a huge group of people who say they do very little maintenance after the first few years, and when they do it costs very little. You don't have to use chinking made by a corporation that costs $250 a bucket. Have you heard of these guys?: http://www.buildloghomes.org/
After much research and consideration with my wife I went to their seminar, so I already have a few grand into the project, not to mention the bias I now have after all the planning, etc...
I've also considered the short distance materials would have to be shipped, and eliminating materials that are manufactured by nasty corporations with most likely harmful industrial processes. I've been looking into plaster for interior walls that I can make with local materials. I'd need concrete like any other home, insulation in the roof and floor, but would be doing away with huge amounts of materials that would have to be shipped.
I think my idea is that I can live very comfortably in a house that's not airtight, and can sleep soundly if I'm using sustainable energy to heat/cool it.
About radiant floors... I figured we'd need radiant floors to keep our feet from freezing since we want tile floors with some type of concrete underneath for thermal mass. Even with a good passive solar design, on days when the sun is not shining wouldn't our feet freeze on top of all that concrete and tile? Obviously I plan to have lots of insulation under those floors, but wouldn't they still be chilly no matter how much warm air was blasting about?
Thanks to you I'm learning so much more about geothermal heat pumps. It certainly seems like a better way to go considering such need for A/C. Aren't these systems compatible with radiant heating? I understand the added cost, but I plan to build my own house with my own two hands, hiring out very little. I'm sure I can lay the tubing before concrete is poured.
Iam sure there are some high performance log homes out there but I wouldnt accept anecdotal comments from those living in log homes. People that have chosen their paths and spent their life's savings are not likely to see things from a balanced perspective. I would suggest interviewing home performance contractors who have experience in evaluating log home performance. They would be a valuable third party perspective. Surely there is a home performance contractor out there who lives in or built their own log home but that would be a rare bird I suspect.
R 1.4 per inch does not seem like an accurate R value for most wood. I think the average is closer to R1 per inch and 1.4 would be the highest possible number with the driest, most favorable species. I would disagree that most log walls are 10-12" and up. I think 6-8" is average and those are nominal values, actual measurements are usually less than advertised just as with nominal lumber sizing.
I was very surprised to find the 1.4 number and other such favorable views from the DOE site you linked to. I think its good how it suggests using passive solar design and a "whole house system approach" to building log homes. However, I dont think it does a good enough job of pointing out the biggest weakness in most log walls (air leakage) and tends to downplay the other major weakness (pathetic R value). Yes there are loopholes, exceptions and Thermal-Mass weighted calculations to make log homes (and other systems) appear more favorable but for the most part a passive solar home designed with a "whole house system approach" will have much leakier walls if they are made out of logs compared to other forms of tighter and more insulated construction. Air leaks and thermal leaks will be most log homes weakest links.
For thermal mass weighted R value performance calculations, the 2012 IECC allows thermal mass walls to get by with considerably less insulation. Your site sounds more like zone 5 than 4 which would need R13 minimum (zone 4 is R eight). Stick framed would need R20 or R13 cavity + R5 continuous. In my experience with energy modeling, the code is far too lax for mass walls. Most energy modeling is effected very little by tweaking the TM numbers and there is a pretty big disconnect between performance modeling and code minimums in the Insulation VS Thermal Mass comparisons. The performance advantage of having more mass, than what typical construction includes anyway, has very questionable advantages compared to good insulation especially when you start replacing wall insulation with TM. I suspect the reason for this is that there are many systems out there that would be illegal to build without this code crutch. People dont like being told what they cant build enough as it is. The most common research I like to reference is that done by Oak ridge national labs. http://web.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/index.html
The R1 versus R1.4 becomes significant as does what climate zone minimums you are shooting for. Zone 5 minimum R13/1=13" thick wall. R13/1.4= 9.3" thick wall. I suspect splitting hairs over climate zones and R values is probably meaningless compared to the air infiltration as measured by the blower door test. This is where log home living has the most to prove in my book. They are just so hard to make airtight and keep airtight. Sure you can live comfortably in a home thats not airtight but you will probably use more energy to do so. Passive solar energy disappears quickly through air leaks, sustainable energy is not cheap, and wood energy is usually not healthy IMO.
If you insulate underneath your slab as you should, then the slab temperature will be about the same as the indoor air temperature without radiant tubing. That might feel cool to some but slippers and socks are cheaper than a 5000-10000$ extra mechanical system to keep your feet warm which you shouldnt really feel anyway with a proper building envelope. But with walls as leaky and poorly insulated as logs, you probably would have to crank your radiant system high enough to feel the warmth by touch to make up for all the heat escaping through the cracks and thermal bridging of the logs. Tubing in concrete is easy. Its the appliance, plumbing parts/labor and complication that makes radiant an expensive way to heat. Tubing in wood framed floors is difficult and expensive.
As for ground source heat pumps (geothermal), you can get a special compressor (water to water) to handle radiant needs but you will also need a more typical compressor (water to air) to handle your cooling. I only like geothermal systems on very large, inefficient or very high load projects. For small or modest sized homes, especially high performance and/or passive solar, they have a questionable payback and would prefer traditional air source and mini-splits.
You're probably right that the R value of wood is more likely to be somewhere between 1 and 1.4 than a solid 1.4 considering all the variables. I suppose when a log home is first built and the wood is not so dry it would be worse and over a few years would get to its attainable R value.
We have to remember there are many different types of log homes. What I consider to be a log home is a home made of real logs, craftsmanship, love, diligence, and lots of attention to detail. Kit "log homes" are made from milled lumber they call logs, dumped on a site and put together by a contractor with probably no attention to detail when it comes to any type of sealing process. There's no scribing or custom cutting to ensure a good fit. All the thought is put into how the design looks in a pamphlet, and salesmanship. Putting it together is really an after thought. They're made with an assembly line mentality just like most stick built homes, except there's not the same minimum code standards that hold stick frame contractors to a certain standard no matter how cheap and shoddy their building is.
You're right that these kit log homes are often made with lumber that's no more than 6"-8" thick. I've seen a few of these and they are no doubt horrible things. A friend of mine rented a "log home" near Waynesville that was built from a kit and it was one of the worst examples of workmanship I've ever seen. There were such gaps there were bats in their house. There were corner notches that I could see through and my friends were not only run out by the bats but also the heating/cooling bills! There were huge windows, and the eaves, 18'-22' from ground level, were no more than a foot wide. The home couldn't have been more than ten years old and stone veneer was falling off several areas of the home, including the outside of the horribly inefficient fireplace. This to me serves as an example of how little care or thought was put into the home when built. Whoever had it built thought they were getting something completely different than the horrible thing they got!
I know there are manufacturers of kit homes that have efficiency in mind, and have processes like sandwiching insulation between D shaped lumber. To me these shouldn't be called log homes. They're homes with wood veneer, not log homes. Anyway, having said what I mentioned above, I can understand your concern and detest of what is being built out there under the name of log homes. It's horrible and many people are being taken for a ride in more ways than one. There needs to be some distinction, and possibly a whole new set of standards for them.
You mentioned codes, and I agree that requiring more and more stringent codes is not the way to go. I understand the need for government codes for safety reasons, (though fire retardant could be giving us cancer) but it's going too far to force us to build our homes using one and only building technique that favors the construction industry and banks. I believe I'm right when I say corporations lobby for codes in order to sell their product, not because of their concern for the environment or energy usage. I have a real problem with corporations having more and more say about what I can and cannot do as a human being. They already have more control over our food supply than... you understand. I kind of feel it's corporations who got us into this mess in the first place and can't be trusted to get us out of it. Of course I'll need the help of them for solar technology, among other things, but I don't want them... You get the idea. I'm really hoping that, along with our food supply, people will take things into their own hands. More and more people will see the advantages of and the need for change, and will do so regardless of residential housing codes.
Well, you can probably see by now that I don't plan to change the type of house I'm going to build, but hopefully you see also that I'll do so responsibly, with the utmost concern of attaining a structure with as little dependence as possible on fossil fuels. It may not be quite as sealed as a plastic bubble, but I hope to make up for that downfall in other ways. My goal, along with my concern for the environment, is to end up with a home that will cost very, very little to sustain. My wife and I presently make very comfortable salaries, but both of us travel frequently for work and I'd very much like to get rid of the need to earn what we do. I want to put whatever we need to put into the home now so that we can do what we want to do and not what we have to do in the future.
We have our heart set on the log home, so if I need to put in a larger solar system we'll do so. You've helped me greatly with your advice about radiant heating, heat pumps, etc. It seems you still believe radiant heat in my situation is just not worth the extra cost, especially considering there would be added cost to supplying it with a heat pump also. Maybe the way to go is just a simple air sourced heat pump powered by solar? I've thought about ducting and believe I could do so efficiently even in our log house. Is there a large difference in cost/efficiency of ducted heat pumps and mini-splits? What about the difference between ground sourced and air sourced heat pumps? Would the difference mostly be what it costs to get someone to dig it for you, or do the materials make the largest difference? I really like the idea of having geothermal, but am confused of just what is considered expensive. I've done some research into the actual units and see there can be a large price difference among brands. Would getting the high-priced super efficient air sourced pump be comparable to a lesser quality ground sourced? I've tried to learn as much as I can through reading online but still seem so ignorant when it comes to these things.
Your advice has been an enormous help to me in the planning of this project. I thought it was all said and done, supposedly simple, with solar powered radiant heating. So I'm glad you steered me away from that complicated system. I'm still pretty set on thermal solar domestic hot water though. Are these systems manageable without an enormous amount of maintenance?
Brian Knight wrote:
In passive solar design, its common for people to overestimate the importance of Thermal Mass TM. It can be easily argued that Airtightness and Insulation are much more important. The paper mentions that ACH50 is important with the insulation but ACH50 is actually more important than insulation in the home as a system.
Too many permaculture pioneers are desert rats, who oversell the thermal mass concept, not taking into account that there are folks in places like Minnesota, where focusing on thermal mass without insulation means VERY cold winters!
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 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
I have 2 questions: (1) is there any way to insulate the concrete floor so that I don't loose heat in the winter? Or is the loss negligible?
(2) What is the best color and type of tile to use over the concrete floor to maximize the solar heat.
With my existing concrete floor, I plan on adding 2" of polystyrene and pouring more concrete on top but Iam also considering an experimental thermosiphoning radiant floor. The energy paybacks are doubtful with no radiant but there are comfort and condensation concerns of no insulation too.
For floor coloring, the darker the better. It will probably not matter with all the heat you will be losing through all those sunroom windows though. Try to scale back focusing on higher quality (better Ufactor) on the north and south.
Brian Knight wrote:Cynthia, your plans are eerily familiar to my planned carport addition. Be very careful with your amount of west-facing glass. Use as little as possible, locate it higher on the wall and protect it with as big an overhang as possible. Sunrooms are crappy performers for the most part. What's your main intent? Plants or living space?
Both plants and living space. I'm looking for a place to entertain guests.
Because of local laws, I can't extend the footprint to the west. The house was built before the law requiring 12 feet between the structure (house) and the property line. Mine is only 11 feet (or 10 feet) if I recall correctly. Another house is another 11 feet from the property line to the west. Thus, the footprint can't be extended to the west with a overhang nor extensions on the concrete slab. I can hang insulating curtains to keep the heat from escaping at night and make the windows smaller. But it would be dark in there without at least two windows based on a similar house that I visited recently with very similar parameters (facing the same direction and the same style of house, less than 2 miles from my house). My parents also have a similar house with the same parameters. They have 3 foot windows on the south side and the west side but theirs is only about 15 feet long while the rest of the space is taken up with a utility room. The part that would be different (besides the size) is the use I hope to make of the concrete slab and the brick wall on the east side as thermal masses.
The only place I might be able to do something is on the south side where the concrete slab is above the ground. The north side connects to the driveway.
Does it matter what kind of material the tiles are? That is porcelain or not, glazed or not?
Without pictures or drawings its tough to make further suggestions for improvement. I would add that sometimes overhangs dont count for setback requirements so look into that as you want deep ones on the west. Also, you can add more south or north windows than the other projects you are comparing to make up for less west.