Building Science

In our quest to make our houses more energy efficient, some interesting things have happened along the way.  In the good old days, when energy was cheap, we were not concerned about conservation and our houses reflected that.  They were insulated to a minimal level, if at all and there was a large furnace in the basement capable of creating enormous amounts of heat when needed.  One other very important point to make about houses of days gone by, is that they were made from real wood.

Today, there are two important differences from the past.  We are now very much concerned about the cost of energy, and there is not as much real wood used to build our homes.  These two factors have collided in our houses of today, and spawned an increased interest in the field of Building Science.  Building Science is the study of understanding and predicting all the positive and negative things that can happen within a house when we attempt to keep the heat inside and everything else outside in order to provide a healthy place to live.

Barriers


Since we are interested in keeping everything in its place, particularly heat, we need to have barriers.  However it is not just heat that we want to put a barrier around.  We also need a barrier for moisture, air, and the great outdoors.  Hence each house needs a clearly defined, complete and continuous:

barrier

Regardless of which barrier it is, it must be complete and continuous, no breaks allowed.

  • Thermal barrier
  • Moisture (vapour) barrier
  • Air barrier
  • Weather barrier

These barriers are, of course, made from building materials, so they don’t necessarily jump out at you and say, “I’m a barrier”. Any particular building material may be nothing but a barrier (eg polyethylene film), it may be both a barrier and an insulating material (eg rigid insulation), or it may be barrier and a structural member (eg OSB sheathing). Keeping it all straight will make the difference between your house being comfortable, cheap to heat and lasting a long time; or being uncomfortable, expensive to heat, and suffering from decay.

Thermal Barrier


Thermal barrier is another way of saying insulation.  Here we are going to talk about where it should be.  First of all it should be everywhere, forming a complete and continuous thermal barrier on all surfaces of your house.  Continuous is important, because heat has one thing on its mind-to go somewhere colder.  If you have a non-continuous thermal barrier, the heat inside your house knows exactly where to go.  It is like leaving the gate open in a pasture; the fencing becomes irrelevant when the gate is open.  Accelerated heat loss is typically caused by thermal bridging, whereby a structural member provides a path for heat to pass from the inside of your house to the outside, unimpeded by any insulating material.

The most effective way to reduce thermal bridging is to apply a continuous layer of insulation to the outside of the stud cavity.  Since the structural wood studs transmit more heat than the insulation within the cavity, the overall thermal performance of the wall assembly is compromised.  Adding a continuous layer of insulation outside of the cavity greatly reduces heat loss through the studs and enhances the insulation of the cavity, resulting in a positive impact on the thermal performance of the entire wall assembly.

insulation

From the perspective of thermal barriers, it is good to use insulation not only to keep the heat inside the house, but to apply it in such a way to keep your structural elements warm and dry as well.  Revisiting the old days, the structural elements of the houses were kept relatively warm and dry because there was little or no insulation.  There was a lot of heat flow through the walls.  Rather than staying inside the house keeping its occupants warm, the heat made a hasty exit, providing heat and comfort to the studs, cladding, trusses, etc. on the way out.  So today, when we are using insulation to keep ourselves warm, we have to think about where we put it so we do not leave the structural components of our house “out in the cold”, where they become magnets for condensation and dampness.  This is particularly important for components such as Oriented Strand Board (OSB) which is made from thermally processed wood chips, and getting it wet would make it an all-you-can-eat buffet for mold fungi.  OSB is a functional, structural, recycled product; just make sure if it gets wet that it is allowed to dry out.  If we put enough insulation outside of any moisture sensitive structural elements, the temperature will remain above the point where damp air will condense on them.  The rule of thumb for our climatic zone is that you should have at least 20% of your total wall insulation outside your wall cavity.  Therefore if you have R20 in your wall cavity, you would need to have R5 outside of the wall cavity (in the form of external rigid insulation).  

With respect to the insulation used within wall cavities, it is important to make sure that whatever insulation you use, care is taken to completely fill the cavity.  If air pockets or channels are left, then air will circulate around the cavity, lessening the thermal performance of the wall assembly.  Batt insulation is particularly prone to this phenomenon, while blown fibers are better, and spray foams are the best way to combat this.

Some common insulation types and their R-values are shown in the table below.  

Characteristics of Common Insulation Materials*
Insulation R/inch Appearance Advantages/Disadvantages
Batt Type
Fiberglass 3.0 - 3.7 Sold in plastic wrapped bales.  Batts are like fibrous blankets, 48 in long and wide enough to fit snugly between wall studs Readily available
Mineral wool 2.8 – 3.7 More fire resistant, sound deadening
Loose Fill
Fiberglass 3.0 – 3.7 A very light fibrous fill, usually pink or yellow Can be affected by air movement in attics
Mineral fibre 2.8 – 3.7 A very light fibrous fill, usually brown
Cellulose fibre 3.0 – 3.7 Fine grey particles, denser than glass/mineral Not as much air movement in attics
Board Stock (rigid)
Expanded polystyrene 3.6 – 4.4 White board of small (0.3” dia) foam beads pressed together Typically HCs used in production.  Must be covered.
Extruded polystyrene 4.5 – 5.0 Commonly blue or pink foam board Works well in wet conditions.  HFCs used in prod’n.  Must be covered.
Polyisocyanurate 5.6 – 6.7 Foil-faced rigid foam, usually yellow in colour Works well in wet conditions.  HFCs used in prod’n.  Must be covered.
Spray Applied
Wet-spray Cellulose 3.0 – 3.7 Fine particles held together by a binder Moisture content must be monitored
High density blown Fiberglass 3.8 – 4.2 Fibers held together by a binder Fills the cavity better than batt fiberglass
Open cell polyurethane 3.6 Soft compressible spray foam that expands into the cavity Must be covered with a vapour barrier
Closed cell polyurethane 5.5 – 6.0 Rigid spray foam that expands into the cavity Can act as air & vapour barrier.  HFCs used in prod’n, must be covered

*Based on CMHC Fact Sheet “Insulating Your House”

Moisture (Vapour) Barrier


The moisture (vapour) barrier has a very simple role, to prevent moisture from diffusing into the building envelope.  Moisture has one thing on its mind-it wants to go somewhere drier.  The act of moisture moving from a wet area to a dry area through a medium is called diffusion.  The only thing that will stand in its way is a moisture barrier.  Moisture is constantly generated by human activity, and it is generally borne by the air.  If the moist air comes into contact with a dry surface of a material (medium), the air says, “I am getting off here now!”  If the material (medium) can absorb a certain amount of the moisture when the air is moist then there is no problem, provided the air dries out later on.  When that time comes, and the moisture that had been absorbed by the material (medium) sees that the air is now drier than where it is now, says, “I’ve changed my mind, I want back in the air”.  Obviously there is a problem if the air is continually moist, as it will diffuse through the entire water absorbing materials which that it contacts, to the point of saturation.  

Obviously a moisture barrier has to be impermeable to moisture; the most common material used is 6 mil polyethylene.  The best place for the moisture barrier is on the warm side (inside) of your house, right underneath the drywall.  The drywall can absorb and release a certain amount of moisture as long as the humidity of the inside air fluctuates in a normal fashion.  If the humidity is constantly high within the house, the drywall will become saturated.  The drywall has paper on its surface which mold fungi like to eat if it is wet enough. This raises the issue of indoor air quality, and the need to exhaust moist air from our houses in an energy efficient manner.

Air Barrier


An effective air barrier is important for two main reasons:  heat loss and moisture control.  This is where there starts to be some confusion between air barriers and moisture barriers.  We have dealt with the moisture barrier in the previous section.  Moving air will move heat (convection), and air moving from the inside of your house to the outside will take heat with it.  Moving air is caused by a pressure difference between where the air is, and where it wants to go.  In a house scenario, the pressure differences between the inside of the house and the outside are caused by blowing winds.  Even though there is insulation standing in the way, if the insulation is air permeable, the air, and the heat, will go right through it, a phenomenon called air washing.  Some insulations are less air permeable than others.  Fiberglass and mineral fibre batts are the most air permeable, and require the usage of an air barrier material to prevent air from passing through them.  Closed cell spray foam, on the other hand, is not permeable to air and does not necessarily need an additional air barrier material.

Now back to moisture.  In the moisture barrier section we discussed the role that the moisture barrier plays in preventing moisture from diffusing into your building envelope.  Moisture diffusion is a kitty cat compared to the moisture damage that can be done by air movement through your building assembly.  As mentioned above, air moving through your building assembly will carry heat with it.  Air moving through your building assembly is also carrying something else, moisture.  Moisture in the air in your house is a necessary part in keeping our mucous membranes, and ourselves, healthy.  Some bad things happen if the moisture content in our indoor air is too low or too high; again, the role of your ventilation system is to keep the moisture levels at healthy levels.  Within your house, the moisture will remain as a vapour in the air, it will not condense to a liquid on any of your indoor surfaces because of good energy efficient building practices.  You have not provided any cold spots, due to thermal bridging, or other discontinuities in your thermal barrier for condensation to occur.  

When it is cold outside and warm inside, it makes sense that the temperature of your building assembly varies from being warm on its inside surface to being cold on its outside surface.  In other words, there is a temperature gradient across your building assembly.  As the air carrying its moisture moves through the building assembly, the moisture remains as a vapour as long as the temperature that it experiences is above the dew point.  The water vapour will remain in the air as long as it does not experience a temperature below its dew point.  Should the temperature within the building assembly fall below the dew point, some of the moisture will condense as a liquid.  As a consequence, whatever material is present in your building assembly at that point will get wet.  Back in the Thermal Barrier section we talked about having 20% of your insulation as outboard insulation.  This will prevent condensation within the wall cavity (on the fiberglass batt) or on the OSB sheathing.  Of course, if the material in the cavity is not susceptible to moisture, such as closed cell spray foam, that concern is somewhat diminished.

We have highlighted two major problems caused by air movement through the building assembly heat loss (air washing) and moisture (condensation).  Air washing is best controlled by having the air barrier on the outside, because that is where the effect of the wind is the greatest.  The moisture control component of air movement is best controlled by having the air barrier on the inside, because that is where the warm, moist air is.  The best approach then is to have two air barriers, one on the outside and one on the inside.

A typical choice for an outside air barrier is house wrap.  House wrap is not permeable to air or moisture in the form of water, but is permeable to moisture in the form of vapour.  Hence it is not a moisture (vapour) barrier.  Again, the house wrap air barrier has to be complete and continuous, making a successful mating with the air barrier of the ceiling, basement and windows.  The joints of the house wrap are typically sealed with tape.  Compared to the inside of the house, there are relatively few penetrations that must be dealt with, penetrations present opportunity for air movement through them.

When the external air barrier (house wrap) cannot be relied on to completely stop air movement, the 6 mil polyethylene installed behind the drywall is generally considered part of the air barrier system.  It would be better if, through care and good building practices to have an externally applied air barrier (house wrap and/or sheathing -insulative or not) be the sum total of the air barrier, and relieving the need for the 6 mil polyethylene at all.  If the movement of air can be completely controlled by the external air barrier, then the issue of the movement of air through the building assembly is eliminated.  In a case such as this, having a 6 mil polyethylene moisture barrier on the inside is not practical.  The moisture barrier needs only to prevent moisture diffusion, which generally, short of a flood, does not require a significant barrier.  Some paints are formulated to form a moisture barrier when applied to the interior of drywall.

Weather Barrier


Like most everything else in Atlantic Canada, your house needs to be protected from our weather.  We live in a Maritime climate with weather that changes rapidly, and as far as our houses go, the worst weather is driving rain.  Roofing, siding and our concrete basements form the first line of defense against the weather.  Design elements such as sufficient eave overhang width to keep the rain and the sun off of your house, eavestroughing, and a minimal amount of roof pitch changes will help prevent rain from flowing over your building enclosure. 

Although siding and brick do repel weather, a certain amount of rain will get in behind them.  Therefore a drainage plane is created behind the external siding layer.  A complete and continuous weather barrier is required on the other side of the drainage plane to ensure no free moisture enters the building assembly.  The width of the gap needs some thought, it needs to be wide enough so that water will drain through, and not get hung up due to capillary action.  It also has to be wide enough to allow drying to occur behind it.  Typical widths of ¾” are used, primarily because this is the width of commonly available strapping.  The strapping is applied vertically, allowing vertical drainage and horizontal application of the siding material.

The complete and continuous weather barrier is oftentimes house wrap, the same material that forms the outside air barrier.  Again it must be tied into the windows and care must be taken that it is lapped properly.