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Fundamentals - Wall Functions

Star Listing all the things that walls needs to provide.

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)...

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.


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 better 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 another reason not to let the air get through the wall.  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.

Importantly, you also want to avoid air moving from the house inside to the outside.  If it does then it will carry warmth outside and thus your heating bill will be higher.  Arguably this is the most important reason to have a sealed house.

A 6 mil polyethylene sheet is a very effective air barrier, but only if it is well sealed at all the joins.  Many walls rely on the inside drywall to form a tight air seal.  Concrete walls (with all tie rod holes filled with foam filler) are my preferred solution.

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.  A good 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 therefore form a virtual vapor barrier.  8" thick EPS (as I use on my walls) is about 0.25 perms.  I judge this to be sufficient to avoid needing a separate vapor barrier membrane in an above ground wall.

It is important to have a vapor barrier 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, is not too expensive, and importantly is not harmed by water.

A big mistake in most wall implementations is to use batt insulation between wall studs rather than using continuous sheets of insulation without any gaps.  Air can get through at the edges of the batt and this dramatically reduces the R value you get in practice.

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.

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 materials 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.

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.  Where there is no abuse issue, use Type 2 (15psi) EPS on the walls (a 6" inner layer and a 2" outer layer), but if you have a have a wall area that is likely to have to withstand abuse then you can use Type 14 (40psi) EPS for the 2" outer layer in that wall area.

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 over 500 years.  You need it to be a well engineered strong house to last for at least 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.  Personally I design my interiors using classical architecture rules.

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 without any discontinuities.

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 without any discontinuities.


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 warm 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 drain spouts from the gutters.  I hide the drainpipes in the outside EPS and on the inside I use wainscoting (and ceiling channels) to hide plumbing.

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.  I use wainscoting (and ceiling channels) to hide wires.


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 reasonably 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 relatively 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.  Using wainscoting and cornices is another useful way to make a wall more visually interesting.


My wall design overview diagram

Wall design overview diagram