What Happens to Your Home in Extreme Cold?

As I’m writing this blog post, a very cold air mass typically found at the North Pole has begun to move southward, into more temperate climates.  This latest polar vortex is forecast to affect much of the continental US with below average temperatures.

Here’s a question, what effect does that very cold air have on our homes?  Well, it depends!

Let’s start with the most obvious, extreme cold effects how long heat stays inside a home.  We pay to condition the air inside our homes, heating and cooling, humidifying or dehumidifying, and filtering takes energy, and energy costs money.  The better insulated and air sealed a home is, the longer it can hang onto the conditioning.

Heat wants to move from someplace warm to someplace cold.  Have you ever stood in front of a window on a cold day?  You feel chilled, that is heat leaving your body and moving towards the cold glass, hot to cold.  This heat transfer is by way of radiation, heat moving through a space, from one object to another without contact.  If two objects are in contact with one another, the heat transfer is conductive.  The third way heat moves is by way of convection or heat that moves through a fluid (believe it or not, air is a fluid).  A forced air furnace heats air and then distributes it through a home by way of convection.

One final comment on heat, we as humans are most comfortable when all the surfaces around us are at a comfortable temperature, air temperature alone does not create comfort.  This is known as the mean radiant temperature.  We could have an air temperature of 70°F, but if we are standing next to a large picture window with a glass temperature of 30°F, we still feel cold.  The radiative properties of any cool surface around us can cause comfort issues.

Delta T (also known as ΔT) is the temperature difference between inside and out a home.   ΔT is also part of the equation used to calculating heat transmission through a building assembly.  Looking at the temperature forecast in the photo above, the low temperature for my hometown on the 20th is expected to be -27°F.  If we use 70°F for our temperature inside the home, the expected ΔT will be 97°.  The higher the ΔT, the faster the heat moves from inside to out or hot to cold.

Here’s an example.  The standard code-built home in my market uses a simple cavity only insulation strategy, typically insulated only in the wall cavities with a R-21 fiberglass batt.  The weighted average R-value for that wall is closer to R-15 (due to the lower resistance to heat flow of the wood framing members, commonly called a thermal bridge.  Windows have an even lower resistance, further speeding up the loss of heat).  The formula for calculating the hourly heat transmission through an assembly is Q = U x A x ΔT.  Q is the hourly heat transmission (in Btu’s/hr), U is the U-factor for the assembly (U-factor is the inverse of R-value, 1/R-value equals the U-factor), A is the area of the assembly, and ΔT is the temperature difference between inside and out.  Let’s calculate the heat transmission for 100 square feet of wall with a weighted average R-value of 15, or U-factor of 0.0667.  Q = 0.0667 x 100 x 97, or 647 Btu’s/hour.  If we lower the ΔT to 50°F, the heat transmission becomes 334 Btu/hr.  The colder it is outside, the more heat that is needed to maintain comfort, which of course is affecting heating costs.  You can read a more in-depth blog post on this topic at: Building Science-R-value – Northern Built.   Heat loss through the building encloser is one way we lose heat; another is losses by air leakage.

Air leaks into or out of our buildings in three ways, by mechanical means (exhaust fans and/or leaky ducts that are outside the building envelope), through the effects of wind, or by stack effect.

If you have a poorly air sealed enclosure and a large ΔT, you could have stack effect moving conditioned air out of your home.  Simple physics tells us that for every cubic foot of air that leaves a structure, an equal amount of air enters (air pressure is just like heat flow, moving from more to less).  Stack effect is created by the buoyancy of warm air, air that is rising inside a home.  When that air reaches the top, it puts a positive pressure on the ceiling.  If a ceiling is poorly air sealed, the conditioned air moves into the attic and/or to the outside.  To equalize the pressure difference created by the escaping air, cold air is drawn into the home, usually at the lower levels.  That air is in turn heated and rises.  It’s a viscous cycle that happens all winter.  The higher the ΔT and the taller the structure, the more stack effect pressure is exerted on the home.  You can read how to calculate the cost of an air leak here: Energy Audit-Calculating the Cost of a Home’s Air Leaks – Northern Built.

Stack effect forces warm air to leave our homes, that exiting air contains some level of humidity or water vapor.  When air containing water vapor comes in contact with a cold surface, condensation will form.  This is a photo I took a couple weeks ago, a home with a poorly air sealed attic.  The homeowners had water dripping from their ceiling, a quick trip to the attic showed the reason, attic frost.  This is a well vented attic, attic ventilation is not the cure, sealing the space between the conditioned living area and the unconditioned attic is.

Wind is another way we lose heat from our buildings.  Wind blowing against a wall will put a positive pressure on that wall, air will be forced into the structure.  Remember when I said a cubic foot of air out of a building will result in an equal cubic foot of air entering, well, that air that was pushed into the building on the windward side will result in air leaving the structure on the leeward side (the opposite side the wind is blowing).  The effect is cold air entering, and warm air leaving.

One additional effect wind can have, it can strip away the effectiveness of insulation, lowering its performance.  This phenomenon is called wind washing and I see it often.  The thermal image below shows my personal home on a windy day, wind is blowing through my soffit area, cooling the small air pockets between the fibrous insulation and lowering the insulations performance.  You can read more on wind washing here:  Construction Design-Wind Washing – Northern Built

The last way we lose conditioned air from our homes is by way of mechanical ventilation.  Bath fans, kitchen range hoods, and dryers are all exhausting appliances, pulling air out of a home.  The same is true of atmospherically vented appliances and open-hearth fireplaces or woodstoves that use interior air for combustion.  All create a negative pressure which results in uncontrolled air infiltration.  Again, one cubic foot of air leaving is replaced by one cubic foot entering, someplace.

Leaky ducts of forced air heating and cooling systems that are outside the building envelope create a similar issue.  Leaks to the outside in the supply ducts cause a house to become depressurized.  Leaks to the outside in return ducts cause the home to become pressurized.  Neither is what any time of the year, but especially when very cold temperatures are present.  Ducts should be inside the building, if they have to be outside, they require meticulous air sealing and good levels of insulation.

What about heat recovery ventilators (HRV) or energy recovery ventilators (ERV), aren’t they bringing cold outside air into the home during cold weather?  Yes, but both have heat recovery, some of the outgoing warm air’s heat is transferred to the incoming cold air.  Also, most modern ERV’s and HRV’s have sensors that will prevent operation or enter a recirculate mode without bringing in the cold during events like a polar vortex.

At the beginning of this blog post, I mentioned that there are factors that change the effects of cold air on a home, the “it depends” comment.  If you have a well-insulated home, with a jacket of continuous exterior insulation, and the home is well air sealed, the loss of the conditioned air is slowed.  We hold on to the expensive air longer.  Costs are reduced and comfort is improved.  We have the ability to fight back against a polar vortex.  We simply need to build smarter and build better.

2 Replies to “What Happens to Your Home in Extreme Cold?”

  1. Excellent overview of heat loss – especially describing Delta T. I like the low Delta T of finished basement spaces with relatively warm soil on the lower portions of basement walls which we often design for when clients have land that slopes to the south to also allow for passive solar gain in a walkout/daylit/garden basement. For a three bedroom home, we often put bedrooms two and three there along with a bath, common room, and mechanical/storage. That allows for a more compact footprint and eliminates the need for a conventional second floor where the walls have high heat loss with high Delta T. Our website has examples of such designs. Thanks for the well-written, tv educational article. We plan to reference it in our Spring Equinox newsletter.

    1. Thanks Debra! This blog post will be edited over time. I have a lot more info on what happens to a home during the winter that I will share.

      Randy

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