Building Science – Part 1

I want to help you understand building science and what the building envelope is, so today, I’m going to speak a little about it.

What is Building Science?

  • It’s the collection of knowledge that focuses on analyzing and controlling physical phenomena affecting buildings.
  • The study includes:
    • Material science
    • Thermodynamics
    • Hydrodynamics
    • Fluidics
    • Architecture
    • Engineering
    • Construction sciences
  • It’s really the study of structure construction, safety, maintenance and more recently – energy efficiency
  • To do this, we break down the house into systems, components and items such as
Roofi g 
Drain ge 
Struct re 
PI mbing 
I te ior 
Air Co ditioning 
  • It specifically studies these three major factors:
- Resisting heat, and how heat aff Cts the building 
- Temperature control Insid h b ilding 
How air and w' d affects the exterior 
- How understand air flo and ventilatio ofthe interior 
- Ho well the exterior resists water a d T0isture 
Controlling the •n erior environmeot
  • Since the dawn of humankind, we’ve 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 gatherings, places to increase various knowledge, and to commemorate our past.
  • So, what factors effect this environment we’ve created?
    • These are things out of our control, like:
      • Wind
      • Rain
      • Sun
  • Alright, but what can we control?
    • Inside the home we can effectively control:
      • Air flow
      • Heating / Cooling
      • Lighting
  • The building envelope is the combination of systems that provide the separation between inside and outside.
    • Roof
    • Walls (including the exterior and interior wall coverings)
    • Insulation
    • Foundations
    • Windows and Doors
  • Now, remember those three major environmental factors we must control?
  • These three things are also what is most responsibility for the deterioration of materials.
  • All these factors add energy to the building, but this energy works against our efforts to control the interior.
  • Not only that, the effects are exponential and synergistic in nature.
    • That means that the effects grow quickly, and they love interacting 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
    • Toss some moisture into that, and we get increased speeds of degradation and increases biological organism growth (mold, wood rot and such)
    •  For every 18% increase in relative humidity, the vapor pressure is doubled, and the life of wooden building pressure is doubled, and the life of wooden building materials is decreased by 50% materials is decreased by 50%
    •  For every 20% increase in ultraviolet radiation (sunlight) there is a 50% decrease in the life of building materials building materials.
    • Water and moisture are the most harmful damage factor to buildings.
    • Buildings will retain water and moisture, at differing rates, depending on the materials involved. 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’s maintained by two factors:
    • The 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 out, and the moisture levels (humidity/rain) effect the heating of the building.

Let’s talk about heat loss.

  • Air leakage is the most common source of heat loss. However, we loss a percentage from
    • the attic/roof – which is usually a pretty easy fix – add more insulation
    • Exterior walls – these can be tricky to add insulation.
    • Basements, and even more with crawlspaces
    • Windows/Doors – which are usually easy to fix – higher quality windows/doors
    • Something also to note, is that the more airtight it is, the more problems you face with proper air exchange and ventilation. This can be helped by installing and operating an HRV.
  • With heat loss, there is this phenomenon that happens called “stack effect”
    • Air flows from high pressure areas to low pressure areas.
    • In homes, crawlspaces/basements are colder (which is higher air pressure) and the ceilings/attics are warmer (which is a low air pressure)
    • Air flows in at the bottom of the house and out the top.
    • Warmer air carries moisture dissolved in the air.
    • As the cooler air rises and it absorbs the warmth, it also absorbs the moisture.
    • So now the air has “heat” and “moisture” in it.

This brings us to what’s called “Hygric Buffer Capacity”.

  • This is the capacity of building materials ability to store water.
  • Now, most materials will retain some moisture.
  • And this is alright if it’s not exceeding its maximum capacity.
  • The building materials are the buffer between the inside and the outside, different materials have different hygric buffer capacities.
  • Modern construction tries to utilize multiple layers of different 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, For a 2000 sqft building
    • Masonry holds about 2273 liters
    • Wood siding/framing/sheathing is about 227 liters
    • Metal siding/framing w/ gypsum sheathing – 22 liters
    • If you want more information about that check out
      • Moisture Control for Buildings, Joseph Lstiburek, Ph.D., ASHRAE 2002, February 2002 ASHRAE Journal
  • Drying happens differently in different places:
    • 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 doesn’t 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.

Let’s talk about vapor barriers, what are they, what do they do?

  • They’re 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.
    • Different 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 different types of vapor barriers:
    • Impermeable
    • Semi-Permeable
    • Permeable
  • Moisture moves through building materials in three different 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 significant 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’s 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 causes 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.

Let’s talk about moisture.

  • Wood framing is still very 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.
  • Basically, water can move in many directions through the building’s exterior envelope, inside the building.
  • We control water in a few ways:
    • We design our buildings to shed water from roof to the footing.
    • The roof isn’t waterproof, but it’s 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.

To be continued…