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Building Science Fundamentals
Design walls, foundations, and roofs to control water, seal air, insulate, and provide
long lasting structure.
Moisture vapor design
Important principles
Warm air holds more moisture than cold air.
When warm air hits a cold surface, the water drops out as condensation.
In northern climates, the air inside your house is most of the time warmer than the outside air temperature. This suggests putting the vapor barrier on the inside face of the wall.
In southern climates, the air inside your house (thanks to air conditioning) is most of the time cooler than the outside air temperature. This suggests putting the vapor barrier on the outside face of the wall.
Most building materials are permeable to water vapor, but a 6 mil (6 thousandths of an inch) polyethylene sheet can be considered impermeable to water vapor and therefore is used as a "vapor barrier" and if sealed properly an "air barrier".
Walls in practice get wet inside no matter how hard you try to keep them dry inside, so you must have a way for them to dry out again rather than trapping the water inside. This means you only want one vapor barrier and you need to be able to dry from both sides of the vapor barrier.
EPS (Expanded Polystyrene) is moderately retardant to water vapor, but does allow some water vapor through. Some people refer to this as allowing the wall to breath.
Not designing and implementing properly for water vapor is much more likely to make your house uninhabitable and/or fall-down than earthquakes or any other bad thing.
Insulation
How much is enough?
Insulation is measured in R value. Type 1 EPS for example gives you about R-4.17 per inch thickness (in cold climates). (Type 2 EPS is about R-4.5.) Assuming a proper wall design (see below) then the more insulation the better, but insulation costs money so you want the greatest thickness where it's needed most. When deciding where it's needed there are two considerations...
1) What is the temperature difference between the inside house temperature and that particular bit of the outside environment.
2) The fact that heat rises.
Taking the average over the year and assuming you are in the northern part of the USA (or equivalent cold place), the ground below frost level is the warmest, the ground near the surface is a bit colder, the air outside your walls is even colder, and the hot air is mainly trying to get out through your roof. After reading recommendations from the building science guys (see reference below) on the absolute best insulation, here are my recommendations for how much insulation is needed in the different places...
| Ideal Best Insulation | Rec Best Type2 EPS | My Type2 EPS | My EPS R | |
|---|---|---|---|---|
| Under footings | R-10 | 2.22" | 2" | R-9 |
| Under slab | R-20 | 4.44" | 6" | R-27 |
| Edge of footings | R-30 | 6.66" | 9" | R-40.5 |
| Walls | R-40 | 8.88" | 8" | R-36 |
| Roof | R-60 | 13.33" | 12" | R-54 |
"Rec Best Type2 EPS" refers to the recommended thickness of EPS (assuming EPS is the only insulation used). "My Type2 EPS" refers to the thickness of EPS (Expanded Polystyrene) that I am using. The thickness of EPS varies a bit in places on my design so I have tried to use an average value. In the case of the edge of footings, my EPS thickness increases as you go upwards towards the surface. The EPS on my walls and on my roof is on the outside of the structure, which is where it should be for optimum thermal mass performance.
It is ok to vary a bit from the recommendations. In my case the walls and roof are 10% less insulated than I would like, but I made the conscious decision to not make the walls too thick because it was cutting into the floor area too much and reducing the visibility angle through the windows too much. My walls are about 18" thick and making them even thicker seamed not to be a good design choice.
Wall Functions
A wall of a house needs to perform various functions. Sometimes a physical component of a wall might perform multiple functions. What is important is to very specifically know how your wall design will fulfill all of the functions that are required of it. A wall needs to perform the following functions (listed in approximate order of importance, with highest priority at the top)...
Structure
Water liquid control
Air control
Vapor control
Thermal control
Physical abuse resistance
Ultra-violet protection
Beauty viewed from outside
Beauty viewed from inside
Attaching the roof and floors
Attaching doors and windows
Security
Thermal mass
Routing for pipes
Routing for cables
Privacy
Attaching inside lights and pictures
Attaching outside lights
Providing alcoves for ornaments
Let's talk about each of these functions. Given that the list is in approximate order of importance, the answers get more flippant towards the bottom, but even so it is important to keep them in the list so that they serve as a check list for the design of your wall. When evaluating a wall design you should treat the list as a list of questions that need to be answered for your particular wall design.
Structure
Obviously you need to have walls that will not fall down and will hold up the various floors and the roof. The structure needs to be strong enough to withstand high winds and earthquakes. The best place to locate the structure function is on the inside of the wall so it stays dry when it's raining. It also avoids problems associated with a wet structure that then freezes and expands. Concrete walls are my preferred way of achieving a very strong structure, and I want to get the concrete as close to the inside of the wall as practical in order to thermally couple the huge thermal mass that concrete provides with the warm inside house temperature.
Water liquid control
The water liquid control layer, sometimes known as the rain control layer or water layer, wants to be fairly near the outside of the wall assembly. It can be inside a bit, but personally I think wall design is generally easier if you stop the water from getting into the wall, rather than providing a way to let it drain out somewhere in the middle of the wall. It is actually the case that most houses do not provide the rain control layer on the outside, but rather rely on an internal cavity to provide a drainage plane.
An important aspect of keeping rain out is to provide good flashing with drip edges so that less water runs down the outside of the wall.
Air control
Obviously it's important to the comfort of the humans inside the house not to have lots of drafts, ie air blowing through the walls. Air control, ie providing a sealed enclosure, allows us to carefully control the inside living environment of the house. We can control the inside air temperature and we can filter air coming into the house to make it nice and pure for breathing. There is actually an even more important reason not to let the air blow in. Air carries water vapor and water is bad for the structure of the house. Water causes mold and decay. The amount of water that air can carry depends on the temperature of the air. Warm air holds more water. If the air does get into your wall and hits a colder surface then the air will cool and drop its water in the form of condensation. The bottom line is that if you can keep the air out then you can keep the water out. A 6 mil polyethylene sheet is a very effective air barrier, but only if it is well sealed at all the joins.
Vapor control
Vapor control goes hand in hand with air control and typically it's the same physical layer of the wall were both are taken care of. The best type of vapor barrier is a 6 mil (6 thousandths of an inch) polyethylene sheet as it has a very low permeance (about 0.05 permeance). All materials have a permeance value and there is no material available (with the possible exception of unobtainium ) that has a permeance of zero. A 6 mill polyethylene sheet can though to all intents and purposes be treated as impermeable to water vapor (and air). If you have thick enough polystyrene then it will have sufficiently low permeance and will therefor form a virtual vapor barrier.
It is important to have a vapor barrier as one of the layers in the wall, but equally important is to only have one vapor barrier. If you have two vapor barriers, one each side of the wall, then you will be trapping water in the wall, which is bad as it can cause mold and rot. This bad thing is called a "vapor sandwich". In your wall design you should have just one vapor barrier and on either side it should have materials with a higher permeance so that drying can occur on both sides of the vapor barrier. If you have enough thickness of EPS then this can be sufficient to count as a vapor barrier. It is good to call this a "distributed vapor barrier" and this can often be the best answer as to how to implement a vapor barrier.
Thermal control
You need to prevent the heat in your house (provided by your heating system) escaping to the outside. Nature does not like temperature differences, and nature always wins in the end, but you can slow up the heat flowing to the outside by using insulation. The more insulation you provide, the slower the heat transfer to the outside. Insulation is measured in terms of "R value" and the bigger the R value the better. Typically walls have an R factor in the range of R-10 to R-40. You should strive to get the walls somewhere above R-30, and that is perfectly possible with the building materials available today. There are lots of suitable insulation materials, but I favor expanded polystyrene because it comes in convenient sheets, and more importantly is not harmed by water.
An important question is where in the wall assembly (ie where in the sandwich of different layers) should the insulation be placed. The answer to this question is related to where the thermal mass is placed (see later for info on thermal mass). You want the thermal mass to be on the inside of the wall assembly so that it's at the same temperature as the inside living space. You therefore want the insulation to be on the outside side of the thermal mass, ie on the outside of the wall assembly. Having the insulation on the outside also helps protect the building structure from excessive temperature variations.
Physical abuse resistance
This layer is best provided on the very outside of the wall assembly. It's most fundamental role is to prevent damage to the various other layers in the wall assembly, eg making sure the air/vapor barrier is not punctured, but it also needs to prevent dents in the walls that would look bad. The most obvious examples of abuse that a house wall needs to withstand include debris blown by high winds and kids on bikes.
Ultra-violet protection
Many modern building materials, particularly the various types of plastic materials, are damaged by ultra-violet light such as comes from the sun. Often UV is the only thing that can cause them to break down. For example polyethylene and polystyrene are harmed by UV. You need to stop sunlight from shining on these material or else in 6 months they will experience some degradation. The outer cladding of the wall needs to be immune to UV and then you put the other materials inside the wall.
Beauty viewed from outside
It takes lots of embodied energy to build a house. An environmentally unfriendly thing to do is to build a house that does not last long and therefore you need to keep using that amount of construction energy every 50 years to build another replacement house on the land. It's much more environmentally friendly to build a house that lasts for 500 years. You need it to be a well engineered strong house to last for 500 years, but you also need for the house to look beautiful so that people want to live in it. If it's ugly then someone will tear it down before it's structural life has been fulfilled.
Beauty viewed from inside
Getting the inner beauty right is not as critical as the outer beauty because doing facelifts on the interior are much easier, and will likely happen a few times during the life of the building to keep up with modern tastes. It's worth designing your internal wall structure to be as flexible as possible to allow for this.
Attaching the roof and floors
The walls need to hold up the roof and also the floors. The roof to wall connection is particularly critical as the roof is also part of the external building envelope. The important "wall" layers need to be maintained.
Attaching doors and windows
Windows and doors are also part of the external building envelope so making the connection properly to the walls is critical. The important "wall" layers need to be maintained.
Security
Kind of obvious that walls help stop people stealing your stuff.
Thermal mass
Thermal mass is a concept in building design which describes how the mass of the building provides "inertia" against temperature fluctuations. For example, when outside temperatures are fluctuating throughout the day, a large thermal mass within the insulated portion of a house can serve to flatten out the daily temperature fluctuations, because the thermal mass will absorb heat when the surroundings are hotter than the mass, and give heat back when the surroundings are cooler. This is distinct from a material's insulation value, which reduces a building's thermal conductivity/resistance. Thermal mass is effective in improving building comfort in any place that experiences daily temperature fluctuations (such as between day and night). It is particularly nice in the summer because the cool of the night keeps the house cool during the day and thus avoids the need for air conditioning. It will also help keep you warn at night during the winter. Thermal mass is ideally placed within the building and situated where it still can be exposed to winter sunlight (via windows) but insulated from heat loss. When used well and combined with passive solar design, thermal mass can play an important role in reducing energy use.
Routing for pipes
You want your internal pipes hidden in the walls to hide their ugliness. Also on the outside you want to hide the drainspouts from the gutters.
Routing for cables
Wires also are best hidden in the walls. Wiring needs change over time, so it is particularly good to provide a way to retrofit wires to walls.
Privacy
As long as you don't build the walls totally out of glass you should be fine here from a visual privacy perspective. It is however worth making sure that the walls also provide decent sound-proofing to provide audible privacy.
Attaching inside lights and pictures
You will want to have a solid surface on the inside of the walls that you can put screws into.
Attaching outside lights
On the outside you also need to have something solid to screw into.
Providing alcoves for ornaments
Walls can be more interesting if you provide indents on the inner walls where you can have a narrow shelf for ornaments or assorted artwork. In the case of typical ICF walls it's good to provide alcoves for another reason. Traditional ICF has polystyrene on the inside and outside of the concrete. You don't actually want much insulation on the inner face because you want the thermal mass of the concrete to be coupled to the interior space. Cutting away the polystyrene on the inner face after the concrete has set will achieve the thermal coupling and will give you nice alcoves.
My wall design overview diagram
Foundation Functions
It turns out that the air/vapor/thermal building science involved in designing a foundation isn't actually that much different from designing a wall. The foundation is just a continuation of the building envelope. Imagine pushing a wall over from the inside so it now lays on the ground, and that's fairly similar to what you want for a foundation. On the outside, ie the side closest to the wet soil, you want a layer to handle the liquid water, then you want insulation. Then on top of that, ie closer to the inside of the building, you want to have the air barrier and the vapor barrier. And finally on top on the inside you want the structure, ie the concrete slab.
One difference however with a foundation design is that drying cannot happen much to the outside (because it's underground) so you need to make sure it can dry to the inside. Putting a polyethylene sheet over your concrete slab on the inside would be a really bad idea (don't listen to a wood flooring salesman that tells you to do this). If you are going to put something over the concrete slab then either make sure it has high permeability or provide a ventilated air gap.
A foundation needs to perform a fairly similar list of functions as a wall (listed in approximate order of importance with highest priority at the top)...
Structure
Water liquid control
Air control
Vapor control
Thermal control
Radon handling
Structure
Obviously foundations have a structural function in that they need to transfer the weight of the house into the ground. An important part of this is that they need to spread the weight over the widest possible area to reduce point forces on the ground. Footings (the part of the foundation under the walls) need to take the lion's share of the weight, but it's also useful to spread some of the weight by using the concrete slab. Using the concrete slab to take some of the weight, means that you need to pour the concrete slab at the same time as, and as part of, the concrete footings.
As with walls, the structure is best placed on the inside of the house envelope so it is not subjected to water problems.
Water liquid control
There is lots of water in the ground, particularly if you are in a high water table area. Not just is there static water, but also often it is actually flowing through the soil. Static water is bad enough, but flowing water if it comes in contact with a warm surface will carry all the heat away.
The water control layer of the foundation is the outer defense against water getting into your building envelope. The water screen for a foundation is typically a layer of crushed rock under the whole slab area of the house. This lets the water flow away before it even comes into contact with the inner foundation layers. It's also necessary to extend this drainage area in the ground all around the house so that the amount of water flowing under and around the foundations is reduced as much as possible.
There needs to be a well defined path for the water to get out from the layer of crushed rock (the water liquid control layer). This means the water needs to get picked up by a drainage pipe that slopes downwards. If your house is on a slope then it's pretty easy to keep the pipe going down hill until it can safely flow to the outside and away from the house. If you don't have a hill then you will need to have the pipe flow downhill to a sump pump collection point and then pump it to the surface away from the house.
Air control
Contrary to popular belief, there is air under a foundation. It's in amongst the crushed rock, and what's more it's potentially bad air with radon gas in it. You don't want that bad air getting into your building envelope and the air control layer prevents it from getting in. Even if you think there's not much air flow happening underground, it's really a mute point because the function of an air barrier is typically performed by the same physical component as a vapor barrier (a 6 mill polyethylene sheet), so you're going to get an air barrier anyway.
Given that there is not a radon problem in my area I have decided to route the radon vent pipe under the footing rather than have it come through the slab.
Vapor control
Given that there isn't that much air flow underground, this is really about stopping any water from getting in. The function is typically performed by a 6 mil (6 thousandths of an inch thick) polyethylene sheet. The critical thing is that this forms a completely sealed "plastic bag" for the house to sit in. That means that you need to seal up any places where pipes come up through the sheet. When I say sealed, I really do mean completely sealed because water will be coming at the barrier with a fairly high pressure and it will find any holes. Given that the plastic sheeting may not be as wide as your house, you may need to join it, so the join needs to be well taped up to seal it. It's actually not a bad strategy to use two layers of sealed polyethylene sheet with the joins offset so that you will still be waterproof even if the sealing tape degrades. Ideally you should try to buy a huge sheet of 6 mil polyethylene sheet so you don't need any joins.
Having a 100% sealed "vapor" control layer in your foundation to prevent any water migration will ensure that you do not have a wet basement.
Thermal control
In most areas, the ground temperature is significantly colder than the temperature you are heating your house to. If you don't want the heat from your house to be lost to the ground then you will want to have insulation under your concrete foundations. Most people just put the insulation under the concrete slab, but ideally from a thermal perspective, it should be under the entire concrete foundations including the footings. You want to have a thermal break (ie insulation) between all the concrete of the foundation and all the soil that the house sits on and in. The argument for polystyrene under footings is explored in more detail here .
Radon handling
It is good practice to provide ventilation for the layer of crushed rock that's providing the water control layer, but there is another reason why this is necessary. The center of the earth is slightly radioactive and a small amount of radioactive gas permeates up through the ground. It's worst in areas that have lots of granite, but many areas have radon gas to some extent. Radon gas is lighter than air so it can accumulate in roof spaces. Roof ventilation is a good technique for avoiding harmful gas build up, but even better is to catch the gas and vent it away before it even gets into the building envelope. The same layer of crushed rock that collects the water can also (and will also) collect the radon gas coming up from the earth's core. The water will collect and flow along the bottom inch of the crushed rock layer and the radon gas will collect and flow along the top inch of the crushed rock layer, trapped by the polyethylene sheet above. If you provide a vent pipe from the top of the crushed rock layer then you can route the gas in a pipe up through the roof of your house and safely away. It's another roof vent just like the sewer stack needs a roof vent. The annoying thing about providing a radon vent is that it ideally needs to go upwards out of the capture umbrella formed by the footings and the slab. That means it needs to go up through the polyethylene sheet, so it is a perforation through the polyethylene sheet that needs to be well sealed so water and gas cannot get through.
Given that there is not a radon problem in my area, I have decided to route the radon vent pipe under the footing rather than have it come through the slab. This means that if I ever did have a radon problem (very unlikely in my area) then I would need a fan to suck out the radon as it is lighter than air.
The radon vent pipe also provides a way to route a high pressure water hose down into the under slab drainage system to clean it out.
Termite control
Termites are mainly a problem with a wooden house. Termites don't eat concrete or polystyrene. They can dig through polystyrene, but they will only do this if they smell wood on the other side of the polystyrene and want to get to the wood. Even so, it's best to make sure they cannot even get to the polystyrene insulation sheet. Termites cannot eat through a 6 mill polyethylene sheet, so if you put a polyethylene sheet under the polystyrene then that should do the trick. Note that this is a different sheet of polyethylene than the air/vapor barrier sheet. The termite sheet does not need to be waterproof so leaky joins are fine.
If you are in a high termite area then the other thing you can do is use poison on the soil under the house. I have a few concerns about the health effects associated with using termite poison (so I don't use it), but at least the various layers of polyethylene sheet will stop the poison from getting into the building envelope.
Having this extra layer of polyethylene sheet violates the rule about never having more than one vapor barrier as that will prevent drying. In this case however we will only be trapping a sheet of polystyrene insulation between the polyethylene sheets and we can ensure it is dry before it is sandwiched (do this construction step on a dry day). If any water is accidentally trapped it won't actually be a problem because polystyrene is not harmed by water.
Tree root control
Tree roots are best kept away from your house. You don't really want them growing up through the crushed rock water drainage area. You can help dissuade them from doing this using a sheet of geo-fabric. You don't want this layer to be a waterproof layer because you want water to be able to get from the soil to the layer of crushed rock so it can drain away.
My foundation design overview diagram
Roof Functions
A roof needs to perform the following functions (listed in approximate order of importance with highest priority at the top)...
Structure
Water liquid control
Air control
Vapor control
Thermal control
Radon gas venting
Structure
A roof only needs to hold its own weight, so it does not need to be made of concrete, but concrete is best as it lasts indefinitely. In places like Italy, Spain, and Croatia it is the norm to use concrete roofs rather than wood. A roof needs to handle plenty of wind force, so it needs a fairly strong. It is nicest to use a cathedral type ceiling in the attic as that makes the attic more useable.
Just like with the wall and foundation designs you want the concrete structure on the inside and the insulation on the outside. Think of a roof as being a wall that is tilted to eg 45 or 30 degrees.
Note that the roof overhangs must not be made from concrete because that would form a radiator fin that would lose heat from the overall concrete of the house. Roof overhangs must be considered as triangular bolt on extras and should be make from eg plastic lumber.
Water liquid control
Keeping the rain out is accomplished by the waterproof roof membrane. In terms of permeance this similar to a polyethylene sheet, but for convenience it is good to use a self adhesive product such as Ice & Water Shield.
Air control
The continuous concrete of the roof structure provides air control, as does the EPS insulation and the liquid control layer.
Vapor control
This is provided by the water control layer and the EPS insulation.
Thermal control
As with the walls and the foundation, you want to have the insulation on the outside of the structure.
Given that heat rises, it's worth having lots of insulation in the roof. You want something like R50.
Radon gas venting
This is just a pipe from the foundations coming out through the roof. Obviously you want the opening through the roof well sealed so that water does not get in. Given that in my area there is no radon, I have decided to route the radon vent pipe under the footing rather than have it come through the slab to the roof.
Avoiding ice dams
When there is lots of snow on the roof it acts as another layer of insulation. The temperature gradient is linear across the complete set of insulation on the roof so the snow next to the roof cladding is warmed by the heat from the house attic (if the attic is conditioned space). If there is lots of snow and not enough roof insulation under the roof outer cladding then the snow next to the roof cladding can be above freezing and will melt. The water will run down to the roof eves and freeze to ice as it comes in contact with outside air. The frozen water will then migrate up the roof under the snow layer. This ice and icicles can then slide off the roof and hurt someone or lift up the roof cladding.
Ice dams can be avoided by keeping the outer roof cladding cold, ie below freezing. This can be done by ventilating the underside with outside air.
It is ok to not have an air gap under the roof outer layer (eg the metal roof) if you are not in a high snow area (snow loads under about 50 pounds per square foot) and you have good roof insulation (better than R50). I am not in a high snow area so I chose not to have a ventilated air gap. Also the use of a steep roof pitch and slippery metal roof cladding all helps avoid snow buildup. It avoids the complication of implementing a ventilated air gap.
My roof design diagram
Putting it all together to make a building envelope
Material properties
How permeability is measured is a bit variable and involves some estimating and even guesswork, but here are the figures that I use. Also below are the R values.
6 mil Polyethylene sheet
0.05 perms.
R0.
Expanded Polystyrene (EPS)
3 perms per inch. (So 3" thick EPS is 1 perm.)
R4 per inch. (The actual R value varies with temperature between about 3.85 and 4.17.)
Polymer-modified Stucco with sealant
2 perms for 3/4" thick.
R0.1.
Porcelain tiles
1 perm.
R0.1.
Gypsum Drywall
35 perms for 5/8".
R0.1 for 5/8".
Latex paint
5 perms.
R0.
No 15 Asphalt saturated felt
6 perms.
R0.
House wrap (Tyvek)
35 perms.
R0.
Concrete
R0.5 for 8" thick.
References and further reading
Dr Joseph Lstiburek and Dr John Straube at the Building Science Corporation
There are many different ways to build the walls for a house. Whatever design and materials you choose, it is important to draw a diagram to show the vapor barrier and how the wall on either side of the vapor barrier will dry out after it becomes wet. A foremost authority on proper water vapor design is Dr Joseph Lstiburek at the Building Science Corporation and he has published lots of useful papers such as can be found here. He has also drawn up vapor diagrams for lots of different wall designs (although not for ICF). You can see some of his diagrams and read his good words here.
A good paper on rain screening and drainage planes by John Straube who is the other expert at the Building Science Corporation can be found here . It describes 3 strategies for keeping rain from penetrating walls.
The building science guys' recommendations on insulation thickness can be found at http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/how-much-insulation-needed .