Avelar Home Inspection Inc.

What is Building Science?

What is Building Science?

First, I would like to say that this is a high-level overview of what Building Science. The amount of content and knowledge regarding Building Science is vast and we have been studying it for thousands of years. It is the collection of knowledge that focuses on analyzing and controlling physical phenomena affecting buildings

The study includes:

  • Material Sciences
  • Thermodynamics
  • Hydrodynamics
  • Fluidics
  • Architecture
  • Engineering
  • Construction Sciences

It is the study of structure construction, safety, maintenance and more recently – energy efficiency. We break down the house into systems, components, and items:

  • Chimneys
  • Insulation
  • Ventilation
  • Roofing
  • Drainage
  • Windows / Doors
  • Exterior / Interior
  • Structure
  • Electrical
  • Plumbing
  • Heating
  • Air Conditioning
  • Foundation

Building Science studies these major factors:

  • Heat
    • Resisting heat, and how heat affects the building.
  • Air
    • How air and wind affect the exterior.
    • How to understand air flow and ventilation of the interior.
  • Moisture
    • How well the exterior resists water and moisture.
    • Controlling the interior environment.

Since the dawn of humankind, we have tried to make living easier. By building something to live in. To protect us from the outside environment and to have an inside environment that is more comfortable. Buildings are environmental separators, creating a building envelope to regulate the interior environment. Besides providing shelter, they also provide space for community gathers, places to increase various knowledge, and to commemorate our past.

What factors effect this environment we've created?

These are things that our out of our control, like:

  • Wind
  • Rain
  • Water

What can we control?

Inside the home we can effectively control the following:

  • Air flow
  • Heating
  • Cooling
  • Lighting

The building envelope is the combination of systems that provide the separation between the inside and the outside.

  • Roof
  • Walls (Including the exterior and interior wall coverings)
  • Insulation
  • Foundations
  • Windows and Doors

Remember those three major environmental factors?

  • Heat
  • Air
  • Moisture

These three things are also what is most responsible for the deterioration of materials. These factors add energy to the building, but this energy works against our efforts to control the interior. The effects are also exponential and synergistic in nature. That means that the effects grow quickly, and they love to interact with each other.

Here are some examples:

  • When asphalt shingles heat up, their degradation increases.
  • They dry out and become more brittle.
  • They have granule loss.

When your exterior wall coverings heat up:

  • Paint/Primer chalking and peeling.
  • Increases differential movement in masonry.
  • Degradation of vinyl siding.
  • Split block sealer failure.

When we add moisture to that mix, we get increased speeds of degradation and increases biological organism growth (mold, mildew, and wood rot for example). 

Water and moisture are the most harmful damaging factor to buildings. Buildings retain water and moisture, at differing rates, depending on the materials involved. The building envelope needs a drying period, which is usually done in the winter, not the summer.

How do we maintain the temperature inside the building?

It is maintained by two factors:

The Heating, Ventilation and Air Conditioning (HVAC) system and how well the building keeps the inside and the outside separated. These two factors work in concert to control the environment inside a building.

The air flow, inside and outside, and the moisture levels (humidity and rain) effecting the heating of the building.

Let us talk about heat loss?

Air leakage is the most common source of heat loss. However, we lose a percentage from the attic/roof – which is usually an easy fix – just add more insulation. The exterior walls, basement, and crawlspaces – these can be tricky to add insulation.

Windows and doors, which are also, typically easy to fix – install higher quality windows and doors.

There is also something else to note, that the more airtight a building is, the more problems you face with proper air exchange and ventilation. This can be helped by installing and operating a Heat Recovery and Ventilation (HRV) system to the HVAC.

Stack Effect

When experiencing heat loss, there is a phenomenon that occurs called stack effect.

Air flows from a high-pressure area to low pressure area. In homes, crawlspaces and basements are colder (which has a higher air pressure) and the ceilings and attics are warmer (which has a lower air pressure), air flows in at the bottom of the house and out the top. Warmer air carries moisture that has dissolved into the air. As the cooler air rises and it absorbs the warmth, it also absorbs the moisture. Now the air has heat and moisture in it.

This brings us to an important fact about materials, it is called the Hygric Buffer Capacity. This is the capacity of building materials ability to store water. Most materials will retain some moisture, and this is all right, if it is not exceeding its maximum capacity.

The building materials are the buffer between the inside and the outside, varied materials have different hygric buffer capacities. Modern construction tries to utilize multiple layers of varied materials to maximize the hygric buffer capacity. The exterior wall coverings will determine how much can be absorbed before the hygric buffer is saturated and deterioration begins.

For example, a two thousand square foot building:

  • Masonry holds about 2273 liters of moisture.
  • Wood siding/framing/sheathing holds about 227 liters.
  • Metal siding/framing with gypsum sheathing can hold about 22 liters.

If you’d like more information, I suggest reading Moisture Control for Buildings, Joseph Lstiburek, Ph.D., ASHRAE 2002, February 2002, ASHRAE Journal.

Drying happens differently in different climats:

  • Hot/Dry areas typically have low moisture levels and low drying needs
  • Hot/Humid areas with high moisture levels have high drying needs
  • Marine areas have high and varied drying needs
  • Cold areas with medium moisture levels and variable temperatures have variable drying needs
  • Very cold areas with low moisture levels have low drying needs

But, drying does not always require heat. Keep these things in mind:

Sunlight dries, but also heats the brick which causes moisture to migrate inwards. Rain, drawn by capillary action, also migrates inward. Wind drives the rain, but also can create pressure differences Wind together with humidity, creates a vapor pressure gradient which can further cause moisture migration.

Air gaps can slow this moisture movement. Sometimes heat and sun can slow drying down. Drying can occur more efficiently in cold and very cold areas. We must build building that use material best suited for each of these areas.

Vapor barriers, what are they, what do they do?

They are used to control the flow of moisture into the building envelop. Moisture can be vapor/humidity/liquid. Modern construction has something call “house wrap”, house wrap lets some moisture through, but stops both liquid water and air. Several types of house wrap have different properties. Older homes used to use building paper, but it had a lot of seams and would compromise the envelope.

There are three types of vapor barriers:

  • Impermeable
  • Semi-Permeable
  • Permeable

Moisture moves through building materials in three ways:

  • Capillary Action: when the surface is wet
  • Air: warm moist air
  • Vapor diffusion: This is part of the second law of thermodynamics:

Moisture will flow across a concentration gradient as a heat gradient. More flows to less.

Proper control of moisture usually requires both an air barrier (house wrap) and vapor barrier. Uncontrolled air infiltration into the building because of poor air barriers can lead to considerable damage. In different areas, the requirements for vapor barriers are different. They must be installed for the type of climate. Cold areas, we must make it as difficult as possible for the building materials not to get wet from inside (plumbing). The air and vapor barriers both go in interior building walls. A permeable vapor barrier installed on the exterior will allow it to dry to the outside.

In hot/humid areas we must make it as hard as possible for the building materials to get wet from the outside (rain/humidity/marine). The air and vapor barriers are installed on exterior to let building materials dry to the inside. Also, these buildings need to have a positive pressure with dehumidification (air conditioner).

In mixed climates, like here in Ottawa – it is a little complicated. Flow through approach:  Use permeable or semi-permeable material on interior and exterior.  Requires both air pressure control and interior humidity control. Install vapor barrier in the approximate “middle” of the wall with insulated sheathing on the exterior.  Air barrier can be either towards the interior or the exterior.  Air pressure and humidity control must be utilized. Humidity flows inwards in the warm, summer months.  Must be stopped at the exterior or at the sheathing. Humidity flows outwards in the cold, dry months.  Must be stopped at the interior walls. Both air and vapor barriers should be employed.

Wood rot (mold) spores will activate when the moisture content raises about 19 to 20%. To prevent mold and accelerated deterioration of materials, the moisture content must be kept below 16%. Even with low indoor humidity levels cold weather can cause condensation, which will increase moisture content in small areas. Usually around doors and windows and any weak areas of the envelope. We know we must protect the home from water, rain, but most importantly humidity (water vapor). Water, in the form of vapor, has a much greater effect on the building and building materials than liquid water.


Wood framing is still quite common. When wood gets wet it gets wider, but not longer. When it dries out, its slimmer, but not shorter. People created manufactured wood products to try and reduce that. They use techniques like cross graining and fiber orientation. Resins and water proofing materials. They do tend to expand in all directions, instead of just one.

Now, this expansion and contraction will cause a separation and let water/vapor to move through the gaps. There are rules for water though

  • Water always runs downhill.
  • Water vapor travels from high pressure to low pressure areas.
  • Water vapor travels from warm to cold areas – by heat.
  • Water vapor travels from wetter to dryer areas by capillary action.

Water can move in many directions through the building’s exterior envelope, to the inside the building.

We control water in a few ways:

We design our buildings to shed water from roof to the footing. The roof is not waterproof, but it is water resistant. The roof works with the gutters, downspouts and exterior wall covering to keep water and moisture away from the inside of the building.

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