Envelope Design

The most effective energy conservation strategy is to have a highly insulated envelope, where envelope refers to the part of the house separating heated from unheated space.  The envelope can be divided into three sections: roof, walls and floor (including the slab or whatever separates the floor from the unheated ground).

Although walls may account for only 20-30% of a building's total heat loss, an inordinate amount of energy has gone into wall system design--probably because the wall cavity is traditionally thinner than the floor or ceiling cavity, so more work is needed to get the insulation level up.

Each section of the envelope must be constructed as a similar sandwich of components: a water barrier, and air barrier, a vapor barrier, a thermal barrier, and the structure itself, in pretty much that order, although usually one material will function as multiple barriers, and the thermal barrier and structure is often overlapped.  The water barrier keeps rain, ground water etc out, the air barrier both reduces heat loss to air leakage and helps prevent humid air moving thru the insulated wall cavity and causing mold growth.  The vapor barrier stops vapor movement into the cavity, insulation stops heat movement (both in and out), and the structure, of course holds the roof up.

Structurally, the envelope must not only support its own weight, but the weight of any cladding, as well as any snow load. It also must resist wind and earthquakes, which may result in much extra framing in some regions. Since structure is generally not a good insulator, the design goal is to get the most insulation in while still providing good structural integrity.

Walls Systems

Since walls are where most of the variation in envelope design is, this document will continue the grand tradition of spending an inordinate amount of space on walls.

Traditional 2x4 framing uses more wood than structurally necessary, resulting in both increased framing costs and a lower insulating value.  Advanced framing, which has been around at least since 1980 addresses these issues, but still does not make a truly superinsulated house.  In the meantime, many alternative building systems have emerged, most geared at much higher levels of insulation, each with its own set of advantages and disadvantages.

The following discussions compare some of the common (and some not so common) wall systems, both in environmental terms, level of insulation, condensation potential and construction ease.

Advanced framing.  This is an improved method of "stick framing". Originally called "optimum value engineering", the idea is that converting from 2x4's on 16" centers (16"OC), to 2x6's 24"OC uses approximately the same amount of wood while allowing for approximately 50% more insulation.  Also by changing the way framing is done, areas, like corners and headers that were often left uninsulated because the framing pieces got in the way are now insulated, and various other more careful framing techniques allowed for an overall reduction in framing wood (and cost).

The diagrams at right show some of these ideas: two stud outside corner, a three stud inside corner⁷, and a window section with an insulated header, 24" spacing and cripple studs eliminated.⁸  The insulated header is shown in the cross section on the right (pink area, sandwiched between the two headers.  In this example, the jack studs holding up the header are still used, but the header could be put on hangers instead.

Unfortunately some of these framing techniques impose design constraints and require other changes in a very change resistant industry, and as a result advanced framing is not especially widely used.  Builders who do use it, often don't use every aspect of it.

There are many places advanced framing is discussed in gory detail, and it is complex enough that it is beyond the scope of this site. As a complete solution, advanced framing produces a mediocre R21 wall, and so is appropriate for mild climates or when used as the base for an insulating foam skin.  Even after 30 years, advanced framing is still the state of the art from the production builder standpoint.¹

Summary: One of the easiest systems to adopt, it can be made will all environmentally friendly materials, but has a greater potential for condensation⁹ and a lower R-value than other systems.

Insulating Foam Board skin. There are two main reasons to use insulting foam board either as the sheathing layer or on top of it: first is adds insulation beyond what can fit in a standard 2x4 or 2x6 framing cavity, and second because its not vapor permeable (or at least can be made to be so), it can prevent condensation inside wall cavities.

In the simplest form, an inch of two of foam board is added on top of structural sheathing (or as a replacement if there are small enough shear loads), but the more interesting cases is where two inches or more are added on top of 2x6 framing (the amount depending on climate) creating a wall of at least R27 and up to around R41 (advanced framed with dense insulation - R21 plus four inches of R5 foam - polyiso etc).  This is one wall system that steel framing would be acceptable to use with, but the overall R-value will be less.

The biggest issues with this system is that window and doors have to have special attachment framing, and furring strips must be attached over the foam in order to attach the siding (for an example house, built outside of Boston see this from Building Science Corp's website).  In addition most foams are considered not environmentally preferable, mostly because they contain potentially toxic fire retardants.

The biggest advantage for this method over advanced framing, or double wall framing is in reducing the condensing potential inside the wall.  As long as the system is air sealed, it won't matter if a small amount of moisture enters the walls, because it won't condense-- at least as long as enough is put on relative to the amount of cavity insulation so that the inside surface of the foam stays above the condensing temperature.

Summary: More tricky to implement than plain advanced framing, the advantage is much higher R-values and much lower condensation potential, although environmentally, plastic foam is less than ideal. 

Double walls.  This approach is simple to adapt because it means learning very little new: first build a 2x4 framed house (but typically using 24"OC spacing), and then go back and add another non-load bearing wall on the inside.

Of course, its not as straightforward as it sounds: window sills and liners have to be wide, you need to install fire breaks every ten feet (small sheets of plywood nailed across the two studs), and you may have to be careful about how the inner wall is attached to the ceiling to avoid cracking sheetrock.  That said, windows and door mount in the exterior wall just like normal, and as long as the wall cavity is greater than the thickness of two studs, there are no thermal bridging issues.  You can use ungraded reclaimed studs or finger jointed studs on the interior wall because its not load bearing. It would even be possible to frame with steel studs on the interior (although you'll still have an energy penalty.  Its probably still not a great idea on the exterior wall due to condensation worries).

The greatest concern for double wall is accumulated moisture and/or condensation inside the wall cavity.  To avoid this, much greater care must be made in air sealing (since that's the major source of moisture)--in fact fanatically sealed would seem to be a requirement. Ideally the wall should have some way to dry out when the inevitable small amount of moisture gets (for example due to an imperfect vapor barrier).  In cold climates this would mean some kind of vapor permeable sheathing, but since neither plywood nor OSB are especially permeable, any home whose shear loads require full plywood sheathing will be stuck with either very slow drying to the exterior or drying to the inside in the summer (at least in the dry west).²

Summary: In some ways, double wall framing is easier than advanced framing because if you use a few extra studs for attachment, the overall R-value stays pretty high.  Because rigid foam is avoided, it is possible to use all environmentally preferred materials, but the downside is that without the foam, the possibility of condensation in the wall goes up.⁹

I-Joist walls - this is a variation on double stud walls, that is potentially easier to frame, and potentially uses less lumber, yielding a very similar R-value.  It has not been used often, and apparently was developed by the PassivHaus people. Details coming some day..

Post and Beam/Timber Frame - A post and beam structure differs from a standard "stick frame" structure in that rather than using smaller size lumber like 2x4s or 2x6s spaced 16 or 24 inches apart, it uses more sizable lumber like 4x4s and 6x6s or larger spaced 4 feet or more apart.  Structurally, timber framing is a post and beam method, and although traditional timber framing didn't use nails, there is not a significant difference between  the two. The biggest difference in in  implication: in timber framing the large dimensional pieces of lumber are usually left exposed, while a in a post and beam they may or may not be exposed.

Timber frame or post and beam structures are very commonly used in strawbale, cob and light clay buildings.  Timber framing is also combined with SIPs, although this is obviously overkill since building with SIPs generally requires few structural members.   Another method is to surround the exterior with Larsen trusses.

There are various claims out there that timber framing uses less wood than stick framing, but it does not appear that this happens very often in practice.  Likewise, it does not appear that there is any significant difference between post and beam and stick frame construction in terms of wood use.  When the wall in-fill is straw, cob or equivalent, at least the plywood wall sheathing is eliminated--a significant savings in wood.

Although there has been a move away from larger beams (or at least toward engineered wood) in order to reduce pressure to cut old growth forests, this conventional wisdom may be less relevant when applied to FSC certified beams.  By buying larger beams, you're encouraging the woodlot owners to cut some trees on a longer rotation, promoting a forest that is more ecological robust.

Summary: there is a significant learning curve, but nothing any experienced carpenter would have a hard time with.  Since timber frame is usually combined with some other method, it is hard to comment on the benefits of it alone.  Timber frame is often chosen for the look.

Retrofit Double walls - there are various ways to add a second wall on the exterior on an existing one, as with all thick walls, the issues are framing around windows and doors, and how to attach the siding.  The most straightforward way is with Larsen trusses: 2x2s nailed together with chunks of plywood every so often.  One nice thing about Larsen trusses is that you can put the vapor barrier on the inside of the trusses, and since the electric, plumbing etc is all in the old wall, the condensing potential of the new wall is very low.

SIP - Structural Insulated Panels (sometimes called stress skin panels) are a sandwich of OSB and foam (generally polystyrene) glued together that make an incredibly stiff panel, combining insulation and structure in one package.  They make great roofs (they can span incredibly far), and can be used for walls also.  There are some limitation in the size of window holes that can be put in them, but in general they are quite flexible.

Top: SIP being lowered into place
Lower: SIPs, numbered ready to install

There is generally no option to them made from FSC wood, and the foam cores have all the same environmental issues as standard foam insulation.  Because they are stress skin panels, a cut thru the skin renders the panel nearly useless, and could easily result in catastrophic failure, although there appears to be no cases of this actually happening.

Summary: SIPs have a learning curve, but experienced installers claim significant labor reductions.  The materials are not on the environmentally preferred list, but the walls are generally very tight, have a R-value and avoid condensation issues.

ICF - insulating concrete forms, are foam blocks (typically virgin polystyrene, but at least one product is made from wood waste bonded together with Portland cement, and one is made with recycled polystyrene in a Portland cement matrix) with holes in them that are filled with rebar and concrete to make a solid wall.  Like SIPs, ICFs are both structure and insulation in one package. They have been used for above-grade walls, but make the most sense for below grade walls that will be conditioned (heated/cooled) space.

Since concrete isn't much of an insulator (Ok, its pretty terrible actually), the overall R value of the wall depends only on the block itself, with the concrete acting as a big thermal hole (it does provide thermal mass, although not in inside where its most useful).  Most ICFs don't have that great of an R-value since the hole for the concrete has to be quite big.  Beware of "effective" R-values that take into account the thermal mass of the concrete: there are too many situations where the assumptions behind the effective R value aren't true. If the alternative is a solid concrete wall, the ICF looks highly insulating, but if the alternative is a lightweight highly insulating wall, the ICF looks mediocre, at best.  For below grade the ICF does save concrete. ³

Summary: There is a learning curve to installing these, with some systems being a bit harder than others, but in theory at least, insulated basements can be built for less with ICFs.  For above grade walls, other wall systems generally perform better, and with less environmental concerns.

Straw bale - packs bales of straw left over from agricultural production (typically wheat or rice) and builds them into walls: most typically packed between wooden posts, but sometimes built as load bearing walls (typically in areas with lower shear and snow loads).

The concerns about the strength, durability, issues with bugs and fire are mostly unfounded, although water is a primary concern as moisture will damage straw much faster than wood (due to greater access to air in the bale).  The rule of thumb is that bale houses, have a "good hat, and boots", meaning generous overhangs, and bales well up off the ground.  Bale houses have been built in wet climates (Western Washington), but doing so is more of a challenge.

Although conceptually simple, straw bale buildings are not nearly as easy to build as they are sometimes made out to be, nor do they necessarily save much wood (you save wall sheathing, but otherwise use larger top plate material, and a bunch of other framing material).  Whether the straw is actually a truly a waste product or not, isn't clear, although you'll not likely find organic straw because organic farmers usually plow crop waste back into the soil under the theory it isn't really waste.  No-till farming may turn that question further on it end.⁴  Objective comparisons of the relative environmental impact of strawbale are lacking.

In addition to using a waste product, straw bale walls insulate quite well (at least R30)¹³, and the thick wave walls have an organic feel that is appealing to many and hard to duplicate any other way.   Unfortunately, bale walls achieve their durability by using an amazing amount of cement stucco (mixed with lime, so its breathable) - a product that requires massive amounts of energy to produce. It is also possible to use earth or lime plaster, which are much better environmental choices, but not as strong, and not as durable.

Building strawbale in high shear load areas (like California, or anywhere with significant earthquake issues), adds additional structural complications.⁵

Summary: straw bale has a real learning curve, and if done by a contractor can easily cost more than other wall systems.  It creates a high R-value wall with low infiltration amounts (especially since the outside is usually stucco), and a very lovely building.  Condensation is a huge fear in strawbale, but it often is less a problem than its made out to be--provided, of course, that the builder is careful to avoid problems.

Rammed Earth/Cob/Adobe - these building methods both use some combination of clay, sand and straw as a structural building material.  Rammed earth is just sand and clay, often with some Portland cement for rain durability.  Adobe is the same thing, with the addition of straw, and Cob is just a variation on Adobe.  All of these materials have high thermal mass and low R values: sometimes foam board is added to improve the R-value.

The chief advantage of these methods is that the materials are all natural - the environmental footprint is very small, and for the do-it yourselfer who doesn't have to deal with those pesky code officials, they can be very low cost.  The resulting building has a very organic feel that many people (including this author) like.

Papercrete - combines waste paper, sand and Portland cement, resulting in a structure that is similar to Adobe, but unlike Adobe blocks which contain mostly clay and sand, Papercrete usually has a very large amount of paper.  The basic issue is that you need a reasonable amount of sand & cement to be able to carry the compressive loads, but by doing so you lower the insulation amount.  Virtually all the Papercrete buildings are load bearing (ie not post and beam, and built in areas where building permits are not required.

Papercrete Blocks

Papercrete wall. Completed building in background

The main advantage is the use of waste paper as a building material, and the potential to achieve highly insulating walls (although it would probably be better to use some kind of post and beam structure).  As a building technique this is one of the fringe movements that hasn't gotten much attention, even in the green building community.⁶

Final thoughts: Each of these building techniques has its own user groups and cheerleaders, and each group will claim their building technique is better than everyone else's.  Needless to say, they're all wrong, because each has its own unique set of tradeoffs. In considering each of the techniques, you need to not only evaluate their relative merit, but to figure out whether you can find the materials and expertise to build in that style.  If you live in an areas with building code, you may also find the building department not especially open to alternative construction techniques they are not familiar with.  Although not insurmountable (patience and charm help), it can add a significant burden to the construction process.

Roof/Ceilings

Unlike walls, roof/ceilings framing is usually quite thick for structural reasons, so its relatively easy to put a lot of insulation in them.  In truss roof systems, the attic is unused space, so as much insulation as need be can be put there, but because its often blown in and left at a fluffy, low density, the R-value is less, often much less than it appears.  There are three general options (with many variations): stick frame (rafters), trusses or SIPs.  Each is a variation on a wall system.

Stick Frame: this could be done with dimensional lumber (usually 2x10 or 2x12), or with I-joists.  The two main variations are (1) insulation in the floor (2) insulation directly under the roof.  Since trusses are generally a much cheaper system, the real use of stick frame roofs is when there will be occupied space under the roof.

No matter which method is used, it is important that there is enough room for the full depth (or at least nearly so) of roof/ceiling insulation all the way across the wall system.

Roof insulation is generally vented, which is to allow the insulation to dry out and prevent condensation.  In fact most building codes require it.  If insulation is to be blown into the roof rafters in a vented roof, a 2" or so air gap needs to be created first, adding to the cost of insulation.

For an unvented roof, the key is avoiding condensation, and so all the techniques for walls apply here.  A thick enough layer of foam board could be added on top of the roof sheathing (usually with a second layer of sheathing to attach roofing to), or a continuous air barrier could be installed along with a limited amount of foam board.  Getting code officials to accept an unvented roof can be difficult (even though they accept a similar condensing problem in walls without a blink).

Trusses:  The two main advantages to trusses are low cost and that they use much less lumber than stick framing.  The main disadvantage is that the space is generally not useable (the caveat  is that if the roof pitch is steep enough, attic trusses make some of the space available.

When using trusses, in order to get the maximum insulation, use raised heel or oversized trusses.

SIPs: These make a very solid, highly insulating roof, capable of spanning large distances, and so make an alternative to stick framed roofs for occupied attic space.  They have little moisture retention ability, so they should be sealed with paint.  The joints can open up a little over the course of the year due to moisture changes, or so may have condensation potential if the SIPs are not protected from moisture entry.

Floors

Floors, especially slab on grade, have traditionally been under insulated on the theory that earth is a more moderate temperature than air, and that earth is a decent insulator.  Wood frame floors were also often not insulated at all.  While it will generally true that heat loss toward the ground will be less than thru other parts of the envelope, the loss can still be substantial.

Crawlspace/Basement: this is the easiest to build, as since floor joists are often 12" wide, high levels of insulation are quite easy. In dry climates, an unheated crawlspace or basement is no problem, but in humid climates, summer air entering this space will likely cause condensation.  Not venting it will lower the problem, but likely not eliminate it. Needless to say there are thousands of homes with musty basements due to excessive humidity all around the east coast.

Heated (conditioned) basement:  if the basement space is to be used¹⁰, the underground walls system needs the same attention to detail as the above ground one.  The major choice here is whether to insulate on the exterior of the foundation and including it as thermal mass, or on the interior.  The most common choice seems to be the interior, as it allows more options for insulation.

Thick wall on a insulated slab
(dark pink area)

Slabs should be installed on visqueen (or equivalent vapor barrier) on top of coarse gravel (no fines).  The gravel acts as a capillary break, keeping water out, and the plastic keep most of that limited moisture from migrating thru the slab.

What Makes a Good Envelope?

A building's envelope job is to keep the outside out and the inside in. This doesn't mean it never lets the outside in, only that the outside is let in only when you want it. That's the whole idea.  Until recently (say the last 100 years, but probably less), buildings did a terrible job of this.  More recently we've been using large quantities of energy to compensate for poor envelope design.

Beside holding the building up, the envelope does the following:

1. Most important, of course is that the envelope keep water out so the whole system doesn't fail.
2. It must keep air from moving thru it, because air brings moisture with it, which ultimately will also cause the building to fail.
3. It must keep vapor movement to a minimum, but more importantly keep it from condensing inside somewhere.
4. Fourth, it must be a thermal barrier.

The last three requirements are tied together, since we know that it is possible to use a small amount of insulation while allowing quite a bit of moisture and a lot of air flow, and have the building survive—it just uses a lot of energy.

What's a good insulation amount?  Of course it depends on climate, and on the specifics of the building (ie residential versus large commercial, single family versus multifamily).  Here are four ideas on how to decide:

• Put in enough, so that the mean radiant surface temperature of the envelope is reasonably close to room temperature 99% of the time.
• Put in enough so that the predicted energy use is 80% less than what houses currently use (see the 2030 challenge). This is the predicted amount to curb global warming.
• Put in enough so that the house can use no any fossil fuels.
• Put in as much as you can afford.

Another idea, from the PassivHaus program, is to insulate enough so that normal ventilation levels can supply all the heat necessary on the coldest day (presumably also that it can supply all the cool necessary on the hottest day).  This generally comes out to somewhere in the vicinity of 3-7Btu/SF at the coldest temperature, but that depends on what the coldest temperature is.¹¹

Does building shape matter? Of course, it does to some degree, because, for example the surface of a one story 20x100 ranch house is nearly 50% greater than the surface area of a two story 25x40 house.  In practice, however, the difference is often negligible because the heat loss thru windows and infiltration are relatively larger¹². This isn't just because its hard to reduce heat loss via windows and infiltration, but because its also so much easier to reduce heat loss thru the floor and roof.

The best way to figure out what matters is to figure out the relative contribution of each component of the building, and then focus mostly on those components that contribute the most.

Effect of building type and size & climate:

Looking at multifamily (ie townhome type, with shared walls), the effect of walls is further reduced because the shared walls lose no heat (assuming of course, that all units are heated equally).  In multi-story buildings (units on separate floors), heat loss thru the roof is further curtailed because only the topmost unit has roof heat loss.

As a general rule, as the building gets larger, the energy for ventilation gets larger relative to the energy loss thru the envelope, because the volume grows faster than the area of the skin.

The following tables compare four different buildings, each of which the individual housing units are all 2000SF.  Each building is well insulated and decently tight.  The last line shows how the heat loss is distributed, in each. This is a simplified calculation, and in particular an assumption is made that floor is over a crawlspace, and so the loss is 40% of what it would otherwise be.  Someday this will be a calculator where you can change the values yourself to compensate for assumptions you think are wrong.  Also there is currently no provision to add HRV ventilation.

Currently, all losses are in Btu/Hr/°F, since we're only looking at relative contributions.

Building 1: Single family, Ranch style

Building  2: Single family, City Lot Two Story

Building 3: Eight 25x40 Town Homes, Two Stories Each (loss is per home)

Building 4: Sixteen 40x50 Condos, 4 per floor, 4 floors (loss is per home)

..also compare 2500 HDD with 7500 HDD for yearly loss, but note 2500HDD often has 1000CDD also.

What's a good tightness amount?

Of course, it depends.  From the energy perspective, tighter is virtually always better: less energy is used supplying ventilation air than would be lost due to infiltration.  But then there is the issue of how tight can you cost-effectively get—for which of course it also depends: on the wall system used, on how complex a shape the building is, etc. And it also depends on the relationship of house size to number of occupants: a big house with fewer occupants needs less ventilation air, and so making it tighter saves energy at no penalty.

Most green builders can easily reach 3ACH50, many can get to 2ACH50, a few can get down to 1ACH50 (especially if the client is willing to keep a simple design and be willing to use plastic foam), and only a very few can get below that.  Needless to say, getting low is a matter of have worked out a system and being willing to do the extra work.

For houses in the 2000-3000SF size, the building can be made at least as tight 2.5ACH50 and mostly not need mechanical ventilation (provided all the healthy house guidelines are following in both construction and operation).   Below 2ACH50, mechanical ventilation starts to become a necessity.  Except in mild climates, being leakier than 3ACH50 will increasingly result in excessive ventilation during the worst weather.

Resources

Building with Vision: optimizing and finding alternative to wood, Watershed Media (2001).

Buildings of Earth and Straw,  Bruce King covers all the structural issues of these alternative buildings.

Wikipedia article on strawbale at: http://en.wikibooks.org/wiki/Straw_Bale_Construction 

SIP manufactures association at www.sips.org

Insulating Concrete forms association at www.forms.org

Wikipedia article on rammed earth http://en.wikipedia.org/wiki/Rammed_earth 

Notes

1: My evidence is mostly anecdotal and quite limited.  For a thorough view of production builders and energy conservation, look at the Building America program, for which there is much info on www.buildingscience.com

2: the real answer is that I have yet to see exactly the best way to build a double wall (at least as long as you need full plywood exterior) and feel very safe that moisture problems will never occur.  Luckily the entire western US has long periods of warm dry weather that will help dry any moisture out.

3: which make me think that most foundations are overkill, at least in terms of the amount of concrete used (or alternatively ICFs make foundations that are lousy).  The issue presumably is not the 3000PSI typical compressive strength, but spreading the load out appropriately over not very strong soil.  It just makes me think there must be a way to build non-insulated foundations with much less concrete, which will probably never happen unless concrete gets very expensive.  Then again maybe they'll come up with a way to make a yard of concrete with much less than the current average of 1.5millon Btus of energy.

4: Farming appears to be in the middle of a similar upheaval as building is, and like green building, farming is applying a combination of older farming methods with newer ones.  No-till farming is even rarer than organic farming, and unless I happen to run across a knowledgeable resource, the question will remain unanswered. In the meantime, as long as 99% of our grain is produced with chemical fertilizers, the straw will be continued to be a waste product, and hence straw bale buildings will be a good option in some regions.

5: There are two structural issues with strawbale: (1) whether you can carry compressive (eg roof) loads on the bales (2) whether you can carry shear (wind, earthquake) in the stucco.  The problem in both cases is that the bales are gushy, and hence the load really ends up in the stucco skin.  While it is appealing to use the bales as much as possible, its appears that when the load gets tough, engineers turn to methods other than using the bales+stucco.

In wetter climates, there would appear to be an issue with shear load cracking following by rainstorm causing much water damage because there is no tar paper behind the stucco.  But then its not clear how standard building hold up in this earthquake+rainstorm sequence.

6: I spend the night in a Papercrete B&B in Marathon, Tx and found the building seemed suitably strong, yet I wouldn't want to be in a hurricane or earthquake in one until I've seen how they perform under the circumstances.  Someday I hope to make a post and beam, infill papercrete shed and see how it performs.

7: there must be a better way to do the inside corner, but I don't know what it is.

8: the theory is that windows should be either 2, 4 or 6 feet wide, and lined up on a two foot increment, but in practice this is often hard, even for strawbale builders who have much more motivation not to  chop up those 4 foot bales. Still, the strawbale experience indicates that you can still do a lot of aligning, and use a lot of 2-0 windows, even though you inevitably end up with some 2-6, 3-0 and/or 5-0 window units, not to mention the standard 3-0 door size.

9: condensation only really a problem with air-filled insulation: using non permeable spray foam would solve the problem--but even closed cell polyurethane is somewhat permeable (in addition, spray foam sticks so hard to the studs, that it makes them nearly impossible to reuse).

10: multiple place recommend conditioned crawlspaces or equivalent basements, and while this solves the musty basement problem, it seems like the energy performance of doing this couldn't possibly be better than putting the house on a well insulated slab-- but then I've never seen any such calculations, and what intuitively seems wrong isn't always wrong.

11: there is significant controversy surrounding the sensibility of the PassivHaus requirements, especially in very cold climates where insulation levels need to be extremely high to meet the standard. My personal choice would be to go for zero fossil fuel use, but being pragmatic, I'm attempting to stay neutral.

12: earlier versions of this website over emphasized the effect of shape.

13: the Oak Ridge lab test that rated the bales at R30 also gave much lower values for standard 2x4 and 2x6 wall assemblies.  The bale community still tends to think of bales as being closer to the originally stated R50. What the truth is just isn't clear.