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Insulation for Passive Solar

Ken Haggard, formerly an architecture professor at California Polytechnic, is an architect and principal in the San Luis Sustainability Group. Since the late sixties, Ken has designed more than 200 solar buildings, from homes to large commercial and institutional buildings—as well as the first permitted straw bale building in California. An active member of the American and International Solar Energy Societies, he received the Passive Pioneer Award from ASES in 1999 and was made a fellow of ASES in 2000. His office and home—in Santa Margarita, California—are passive solar, off grid, and straw bale. With David Bainbridge, Ken wrote Passive Solar Architecture: Heating, Cooling, Ventilation, Daylighting and More Using Natural Flows, published by Chelsea Green in 2011.

Questions and Answers

Q: I'm building a passive solar high thermal mass home and would like it to be earthquake and tornado proof as well. There are all kinds of concrete wall systems out there from ICF's that have the insulation in the wrong place to styrofoam sandwiched between two layers of concrete. The latter sounds like the way to go but I'm concerned about bowing caused by the temperature swings between the interior and exterior concrete. I was wondering if it wouldn't be cheaper and just as effective to have a crew pour a 6 or 8 inch wall and then glue 4 inches of styrofoam on the outside, stucco it and I'm done. Electrical conduit for outside wall outlets can come up from the slab floor. Why go to the expense of all these other systems out there. The only thing I can think might be a problem is how strong and how long the construction glue bond is between the concrete wall and the styrofoam.

A: Attaching styrofoam to the exterior of masonry wall to insulate them is an old tried and true approach to getting insulation and mass in the right place. Usually its 2 inches (R14 or so ) 4 inches is a little more difficult but of course much better insulation. You don't say what climate the house is in. Maybe R-14 will be enough? I wouldn't relay on just construction glue solely for the connection. Tie wires from the masonry through the insulation would be good insurance that you won't have any problem with separation of these wall components in the future.

Q: What materials do you recommend to use for insulating curtains? I would like to see if these can be made locally but I do not know what materials are best. Are there insulation materials you would recommend?

A: (Daniel Chiras) Good question. I would consider using wool to make insulating curtains. Am I correct in assuming there are sheep available for wool? To be most effective, curtains should also reduce air infiltration/exfiltration -- that is, block air flow at night. Be sure the curtains fit tightly in the window opening.

Q: That is a great idea to use wool for making insulating curtains. In the village I am working in they don't have sheep, but they do have yaks and they have great experience with making wool blankets. Is there a specific way you would suggest to make the curtains? Would you suggest two wool layers and another material in between?

A: (Daniel Chiras) You could create an insulated curtain with a layer of fluffy "yak wool" sandwiched between an inner and outer blanket. That would create a nice energy-efficient curtain.

Q: Hello I recently purchased a home that is passive solar, with the south wall composed of 11 windows approximately 81 by 81. While renovating I took the curtains down and they were discarded. Now I'm looking for curtains to replace the old ones. The temperature in Ottawa can go as low as (-40 C). Do you know of a manufacturer or retailer who sells with a R-value of approx 6 or better.

A: (Daniel Chiras) I don't know of any insulated window shades with an R value as high as 6. Various manufacturers sell cellular shades with R values reportedly around 3 to 4. You might consider installing internal shutters made from rigid foam, as described in my book, The Solar House. You'd have to make them or hire someone to make them if carpentry isn't your thing. They're basically made by attaching a piece of environmentally friendly rigid foam such as the green polyiso insulation to a piece of thin plywood. The shutters are hinged and secured to the window frame. They're also covered with fabric to improve their aesthetics. Rigid foam insulations vary in their R value from around 4 per inch (expanded polystyrene) to 8 per inch (that's polyiso). You will need some wall space on the side of the window to accommodate the shutter when the window is opened.

Q: In Sustainable Architecture / keep your cool, you state "The part of the house that is underground needs to be well insulated, or the earth will suck warmth out of the house." I agree. Does this also apply to earth connected mass such as floors, walls? What about earthships? Does one store solar heat in Insulated Mass only ? In other words, 55-58 deg. is great for tempering but won't any added heat (solar or other) get "sucked" into the earth?

A: (Kelly) I do believe that mass intended to help heat a house should be insulated from both the atmosphere and the earth, for the reasons that you state. This view is somewhat controversial though, since I have heard of solar consultants claiming that solar mass floors, for instance, do not need to be insulated. All I know is that I have insulated all of the thermal mass in my house and it works fine!

Q: I am interested in researching the application of adding an exterior insulation (styrofoam ?) to existing homes with cinder block walls. The goal is to create a thermal load by means of solar radiation. I assume that the walls would be refinished with a stucco coat. I am interesting in refurbishing existing, non energy efficient homes.

A:(Daniel Chiras) I know one person (Matts Myhrman) who actually did this but used straw bales for the "outsulation" as we call it. I think that you are on the right track. This should dramatically enhance the energy efficiency of these buildings.

Q: What do you think of insulation using the wool from sheep? There is a site just for this and sounded interesting...

A: (Daniel Chiras) Wool insulation is a great product. It is one of only a few insulation materials that doesn't lose its insulative properties if it become slightly moist. The R-value of most insulation products plummets by half with only a tiny amount of moisture.

Q: I want to build a stone house. I am building at 9,500 ft, and have an average snow pack of 6-10 feet or more in the winter. The temp rarely gets below 15f. we will be building passive solar heated structure (no need for cooling). Is it unrealistic to not insulate the walls (2 fT thick) in order to keep the beauty of the rock? Will snow-pack be sufficient insulation? What are the best passive solar designs for maximum heat?

A: (Daniel Chiras) As you know, stone is not a good insulator. In fact, it is a very good conductor of heat. Since you want to retain the look of stone, you might want to consider building a double wall--that is, two walls, each one foot thick with a couple inch space between them. This can be filled with insulation to prevent heat loss during the winter and heat gain during the summer.

The best passive solar designs are direct gain systems, in which the south-facing windows allow sunlight to enter the building. Sunlight is absorbed by solid structures and converted to heat. Some is radiated back into the room for daytime heat. The rest is stored in the mass and radiated at night when the room temperature drops below the air temperature. You can read about them in my book The Solar House.

Q: We have a house in southeast PA that was expanded from an old hunting lodge. The sunroom and den (and one foyer) have old flagstone floors that go out under the outside wall and a few inches past, and then drops vertically 3-4 feet with a deck above (the house is build on a slope, and these are enclosed former outside porches). Underneath the flagstone and making up the outside wall is local red sandstone (which tends to break into rectilinear blocks), cemented.

The issue is heat loss. In the winter I've measured 65 degrees in the middle of the den, 55 degrees near the wall or less. We ended up carpeting the den which helps (the sealer on the stone was contaminated with breakdown products from a rubber foam rug pad, and attempts to fix it made it worse, plus my wife and in-laws really wanted carpet). The sunroom (which is fully open to the rest of the house) does act as a good passive-solar collector, and we also have a small modern wood stove in there which heats the stone as well. But there's a LOT of loss to the outside. At the sunroom, outside the window-wall is a concrete sloped cap about 4-6" wide which meets up with the decking, and the wall goes down from there. The deck is supported by full-size steel I-beams anchored into the stone and by concrete block pillars. Outside the den wall (12" concrete block) there's a 1-2" concrete cap where the stone meets the deck. Can anything be done to significantly reduce the heat loss? I don't see any way without major surgery to get rid of the concrete cap area; would insulating the wall below that help enough to be worthwhile?

A: (Kelly) Boy, that sounds like a tough situation to deal with. All of that solid masonry material is obviously leaking energy to the outside, and until you provide some kind of a thermal break, it will continue to do so. The best place for that break (with several inches of insulation rated for burial) would be right at the foundation to the house (on the outside), going vertically down at least to the frost line, but the practicality of this sounds difficult. Having that vertical drop just outside the wall compounds the problem because it exposes even a greater area to the outside temperatures. It sounds like surgery time to me...or just accept the loss.

Q: I'm planning to put a concrete patio directly in front of the house (on the S side) in hopes to reflect some of the sunlight back onto the building (the "two suns effect") .. something more reflective like metal will probably not be culturally accepted here (China).  I had been planning to put a layer of insulation between the foundation and the earth underground, but now I wonder if the concrete patio would absorb enough heat (even in winter which gets down to minus 20 degrees C) that it's better for me not to insulate the S foundation that the concrete patio would butt against.  What do you think?

A: (Kelly) I would advise that you insulate the foundation; overall there would be a net loss of energy from the house without the insulation.

Q: I live on the island of Guam, warm (85degrees) and typhoon area. Most of the homes here are built with 8x8x16 inch concrete block. The house is like an oven during the summer months. I plan to build another house and wish to build a cement block house but wish to install steel studs to erect a wall inside the concrete wall then shoot a foam insulation within the inside wall after running all my electrical and communication lines. Will this method gives me a cooler house to live in? I can't apply the foam insulation on the concrete block on the outside because the u/v will make it crispy and the typhoon winds will blow it away!

A: (Daniel Chiras) Insulation should help you stay cooler in the hot sun. You could just blow foam insulation inside the blocks of your wall, though you will get some heat conduction in and out of the house through the solid portions of the block walls. I'd be inclined to insulate inside the walls and also in the interior wall you are building. Also, be sure to insulate the ceiling really well. You might look at building a home with a light colored roof, too. Be sure to ventilate the attic to get ride of heat and provide for good ventilation in the house. You'd do well to shade the structure as well. 

Q: My wife and I are in the design stage of a passive solar home we will build in Oregon next spring. We have been thinking of a Beadwall as the solar glazing, but I was wondering about the comment in the 1970s earth-bermed design, that the beadwall had developed problems and had to be abandoned. Could you tell us more about that?

A: (Paul Shippee) The Beadwall works fine when maintained & operated properly. It is certainly the unsurpassed night insulation for passive solar windows as far as R-value and automation are concerned. But with the advent of thermal curtains of various styles, many people have found those to be effective enough.

Low E glass should not be used in sunny/cold climates on south-facing glass because it restricts more sun coming in than the heat loss it saves at night. Oregon might be another story, depending on how much sun you have in winter.

Beyond that, the beadwall generates static electricity when operating and the beads tend to cling to the glass. The fix has usually been to dose the bead flow with glycerin (or even water has been used). But the glycerin then tends to build up inside the glass after some time.

I am not sure if Zomeworks in Albuquerque is still licensing beadwall or not. It might be worth a call to Steve Baer down there to see what they have to say about it these days.

Q: I am considering installing a beadwall. What were the problems that developed in your house with them?

A: (Paul Shippee) I think you know the benefits of a bead wall, super insulation which makes a passive solar house Really Work. The problems seem to be in the realm of maintenance mostly. Since the styrofoam (polystyrene) beads generate a lot of static electricity when moving to and fro, the question is how to dissipate that static so the beads do not cling to the window panes and other odd attractors in their daily transit.

One solution is to dose the beads periodically with glycerin which does dissipate the charge. But this eventually leaves a film coating on the inside of the glass which no one likes very much. Others have tried plain water injected into the transit piping with some success.

I favor finding or making another kind of insulating bead that does not generate static electricity in transit throughout the piping and window surfaces. If something like this can be found or made, then the beadwall would become universal for passive solar homes. This would take delving into the chemistry industry and finding a solution, then a manufacturer willing to gear up for massive sales, or perhaps something already exists off the shelf?

Or perhaps static electricity could be dissipated within the system by some other means, such as a clever electrical counter charge? In any case the market is potentially huge and I would deploy this system in my designs if it became available static-free. Let me know if you discover anything promising...

C: I live in White Rock, NM. The house (14 Y.O.) is 1680 SF w/attached two car garage. It's a SIP's panel house, vaulted ceilings clerestory windows, stucco finish. R40 (6in) walls and R60 (10in) roof...... pretty incredible stuff. I have 7 - 30"x72" bead ball windows, w/trombe wall in master BR. When we were looking for a house last yr, we looked around for quite a while. Came to see this house several times at different times over the winter months, and every time we would see this house w/realtor (house was vacant) it was always warm in here.... Make a long story short we closed in mid March of last yr. This winter the coolest we've seen it in here is 63-64 degrees. Wife loves it! Cats sleep all day, every day! I love the utility bill! Very efficient wood stove (w/cat converter) has kept this place cozy.

Q: My common sense tells me to install the foil radiant-barrier shiny side facing the sun. Everywhere I research reports dull side towards the sun. Do you have a definitive answer on the correct install for this material?

A: (Kelly) I wouldn't claim my answer to be definitive, but I would say that it depends on whether you want to keep the heat in or out. The shiny side will reflect more radiant heat, so face it in if want to retain the heat, and out if you want to exclude the heat from the space.

Q and A: You mentioned blowing in borate treated cellulose for Victorians but said "be sure to have a vapor barrier." I live in a 1910 heart pine folk Victorian in South Texas were we are primarily concerned about keeping heat and mold out. The exterior walls in this house are open from the crawl-space to the attic and have significant airflow between the two.

(Daniel Chiras)This is not a good situation and can be remedied by blowing in a foam insulation like icynene that provides insulation, keeping the home warm in the winter and cool in the summer, but also blocking the flow of moisture into the wall cavities from the outside.

From the attic I can look down the walls and see the beams supporting the floor joists. Some interior wall boards are not tongue and groove nor are they tight - there is a quarter inch gap between the boards. So we have lots of direct airflow to the outside. There is no vapor barrier or house wrap. This is an affordable sub-100K house so anything elaborately expensive like taking off the outside siding is not an option. I plan on re-roofing and finishing out the attic with a few rooms in the future. When I do this I'll heavily insulate the ceiling to keep out the Texas heat.

You should also give strong consideration to installing a radiant barrier. And, you want to be sure the insulation you install in the ceiling stays dry.

I assume this insulation will stop the current exterior wall airflow.

Most insulation materials (e.g., cellulose and fiberglass) do very little to slow air flow.

So I am interested in insulating these walls but want to make sure I'm not going to create a mold problem or other nightmare. I don't see any signs that suggest I have water running down the walls anywhere. The narrow opening at the top would not allow me to drop batt insulation down the walls.

I wouldn't do this...try blowing in a liquid foam insulation.

I'm thinking I'll stuff the bottoms of the walls with cotton insulation and blow in borate treated cellulose on top. The bottoms of the walls are hard to get hands to and are irregularly shaped so I can't imagine cutting plywood or stapling wire mesh to cover these areas. Does all of this this sound like a good plan?

You really need to prevent moisture from entering the wall cavities (stud cavities). Dropping in batt insulation and blowing in cellulose isn't going to work in my opinion. You will be creating conditions perfect for mold and moisture buildup.

It was my understanding that blowing in foam insulation requires tearing out either the interior or exterior walls. Is that not the case? And would the exterior wood siding be OK with the foam behind it?

I'd have to see the home to answer your question but for most homes, foam can be blown in through holes in either the interior wall (usually drywall) or through the siding. It sounded as if you had a pretty good access to the wall cavities from above and, if that is so, you should consider snaking a tube down into the wall cavity from above. You may want to contact a local insulation installer to get their opinion after seeing the project.

Q: I am a ninth grade student and recently conducted a science fair project where I compared the effect of different types of insulation on heat transfer. Basically I placed a digital thermometer probe in a can in a pot and then surrounded it with insulation. I then placed this in a 300 degree oven and monitored the temperature in the can over a 20 minute period. In the end, sand was a far better insulator than the professional home insulation material. Why would that be? I thought it was related to the density of the sand but my teacher says that's way off. I would appreciate your insight. BTW - car coolant also worked very well.

A: (Kelly) This is a very interesting experiment. I'm not a scientist, so I can't give you a definitive answer. I suspect that the reason that you got this result is because of the way that you set up the experiment. A 300 degree oven is very HOT, hotter than any house insulation would likely experience in its lifetime! The standard insulation became over-powered with such intense heat for that period of time, whereas the the sand and car coolant performed differently.

Sand is generally considered a thermal mass material, rather than an insulator, but in this instance its mass acted to slow down the transfer of heat so that by the end of the experiment it was still cooler inside. If the duration of your experiment went on longer, then I suspect that the sand would not have worked so well. As for the coolant, this is a chemical that is designed to keep engines cool without boiling, but it too would eventually fail at that temperature.

So, I think that you are right, that the sand worked the way it did because of its density!

Q: I am living in the northern part of India where temperatures range from 0 degrees C in winter and 45 degree C in summers. Is there one type of insulation that would work for this temperature range? Please specify for for interior, exterior walls and roof.

A: (Daniel Chiras) All types of insulation will work in your area, my friend. What kinds are available?

Q: I've been researching insulation and have come to the conclusion that they all have significant drawbacks. Strawbale appears to be the most acceptable but the local experience is nil and I'm leery about burying straw below grade. I'm impressed with the R-value of closed-cell polyurethane foam. The product is not mentioned in your book or your review published in " Mother Earth News - Guide to Home Energy Savings". I would appreciate your thoughts regarding this product, its R-value claims, thermal drift, industry experience, off-gassing of flame retardant, claim of no need for vapor barrier.

A: (Daniel Chiras) You are absolutely right about not applying straw bale as an insulation material underground. The only times it's been done, at least that I'm aware of, turned out to be disastrous. I published an article on insulation through Mother Earth News several years ago, which you might look up to learn more about rigid foam insulation, although it is slightly out of date now with recent changes. Your best best for foundation insulation is extruded polystyrene XPS (it's also called pink board or bluebeard in the US). This form of insulation will prevent moisture from being absorbed, but I would sill apply a sealant to protect the foundation because water has a way of getting past all barriers. It will for instance very likely leak between the foam boards.

Q: I've been researching insulation and have come to the conclusion that they all have significant drawbacks. Straw bale appears to be the most acceptable but the local experience is nil and I'm leery about burying straw below grade. I'm impressed with the R-value of closed-cell polyurethane foam. The product is not mentioned in your book or your review published in " Mother Earth News - Guide to Home Energy Savings". I would appreciate your thoughts regarding this product, its R-value claims, thermal drift, industry experience, off-gassing of flame retardant, claim of no need for vapor barrier...

A: (Daniel Chiras) You are absolutely right about not applying straw bale as an insulation material underground. The only times it's been done, at least that I'm aware of, turned out to be disastrous. I published an article on insulation through Mother Earth News several years ago, which you might look up to learn more about rigid foam insulation, although it is slightly out of date now with recent changes. Your best best for foundation insulation is extruded polystyrene XPS (it's also called pink board or bluebeard in the US). This form of insulation will prevent moisture from being absorbed, but I would still apply a sealant to protect the foundation because water has a way of getting past all barriers. It will for instance very likely leak between the foam boards. The closed cell foam insulation products appear to live up to their promise. They're hard to beat when it comes to R-value per inch.

Q: We're building a passive solar home with a tuck-under garage. Should we spend the extra money for R-17 insulated garage doors, since we're going to be opening those doors twice a day and losing all our saved heat (or cool air)? The garage walls are well insulated, and it is in a daylight basement. Over the garage is the living room and kitchen area, with some flooring of concrete slab and some standard wood joist flooring, both insulated from the garage.

A: (Kelly) Even though you will be opening those doors with some regularity, you will not lose all your stored heat/coolness. The concrete floor of the garage will store much of this, so keeping the space insulated will help moderate the temperatures in there, as well as the surrounding spaces. So I suggest insulating the door as well. It will be better for you (and other belonging stored in there).

Q: I am in the late planning stages of a passive solar home in Calgary, AB, Canada, and have been trying to find a temperature controlled automatic blinds system to regulate the temperature (especially on warm, sunny, fall days) inside the home. Can you recommend something? Would a home automation system to run blinds, temperature and lighting be advantageous?

A: (Daniel Chiras) I have not seen any temperature regulated shades, but have seen a couple systems controlled by light sensors mounted outdoors so the shades open in the morning and close at night. Theses systems were designed and installed by the homeowner in one case, who was an engineer, in one home and by the builder in another. I have never seen nor have I read about commercially available systems. I think it would be easy enough to design and engineer your own system, if you are an engineer or technically inclined.

Comment: I am a Window Quilt™ dealer since early 80's and they have a proven R-value near 5. They also stop 63% of infiltration. On large window walls, the ICW was available commercially in the 80's, at R-10. I replace and repair those shades found often boosting performance of Trombe walls and large window walls. Shade max size is 20 feet square. 503-449-0527

Q: We are in the process of purchasing a large old church with concrete block walls that faces south on top of a windy hill in northern Colorado and plan on capturing as much passive solar heat as we can. Will straw bales slow or inhibit the heat from the mass (concrete blocks) from getting into the south living spaces? How do we insulate with straw bales on the inside of a 8" block concrete wall? And will the weight of the bales require any more support below grade? Losing space is not an issue since it is so big (6000+ ft).

A: (Kelly) I would say that if you want to insulate the structure with straw bales, they should go on the outside of the walls, so that the concrete blocks can act as thermal mass on the inside. You would need to provide an adequate foundation for those straw bales that raises them well above grade, and you would need to protect them with an extension to the roof eaves.

Q: You really must research your data to be posting factual information on your site where Green Building enthusiasts will be taking your word for it. Read this article please. Maybe you want to rethink the "building with container homes" blog including the insulating paint theory now that it has played out in court with an actual dealer of the paint and the demise of his own home. http://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-paint-salesman-tripped-his-own-product

A: (Kelly) I read that article some time ago and have done my own experiments with a different brand of insulating paint that you can read at http://www.greenhomebuilding.com/articles/inuslpaiintex.html. I just modified my article about container homes to alert people that this paint might not work all that well. I think that that builder/distributor was foolish to think that it would insulate a house that size without any thermal mass on the inside in the first place. I have actually used some similar paint on some metal window frames and it does seem to have a positive effect. I think a lot of it depends on the specific application.


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