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Busting 7 Insulation Myths.

August 09, 2021

By Steve Maxwell

“It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.”– Anonymous

A few winters ago, I got an email from a contractor looking for advice on an unusual situation. A homeowner had pulled the baseboard off a wall and a steady flow of water streamed out onto the floor. Lots of water. As it turned out, there was no plumbing or siding leak involved. The water accumulated because of mistakes made when the home was insulated during construction. This is just one example of how fundamental errors can be made when it comes to insulation. Bust the seven most persistent insulation myths and you’ll enjoy better energy efficiency and a better building life.

Myth #1: Internal wall and attic condensation is caused by a lack of ventilation.

This is true in a way, but there’s more to it than this. Although the right kind of building envelope ventilation can allow condensed water to leave walls or attics harmlessly, lack of ventilation is not the root cause of condensation. The real cause is air leakage at times of year when outdoor air is colder than indoor air.

The ability of air to hold moisture is directly proportional to its temperature. The warmer the air, the more moisture it can hold. So, if warm, indoor air is allowed to make its way into wall cavities or attic spaces, that air will slowly cool as it moves deeper into the building envelope. Eventually, the air loses the ability to hold the moisture it used to be able to handle at higher temperatures, causing water to appear out of nowhere in the form of tiny droplets within your walls or attic. The level where temperatures drop to the point of causing condensation is called the dew point.

That distressed homeowner with water running out from behind the baseboard had a 2x4 wall insulated with fiber-based batts and an improperly installed vapour barrier. The air leaks and sharp temperature gradient between interior and exterior wall surfaces led to massive internal wall condensation and leaking during winter. This condition is actually quite common, though not often as extreme. Not that it matters much. A little water in wall cavities is as bad as a lot.

Myth #2: R values accurately reflect real-world energy performance.

R value and the metric equivalent RSI are the yardsticks by which all insulation products are measured in Canada. Professional builders, homeowners and building inspectors all use and rely on R and RSI values because there are no other number out there to measure insulation performance. Trouble is, these numbers are highly flawed yardsticks. They change length, so to speak, depending on what you’re measuring. That’s because there can be (and often is) a big difference between insulation values as determined in the lab, and the real-world energy performance that different insulations deliver.

The problem with lab analysis of R values is that it completely eliminates air movement from the results. This matters a lot with fluffy insulation materials that allow air movement through them. Air movement greatly lowers real-world insulation performance, and everyone knows that drafts and air currents often happen within wall cavities and attics. On the other hand, insulation products that don’t allow air movement through them (spray foams and rigid foams, for instance) have real-world insulation values almost identical to the lab results you see on packaging and advertising. Their performance doesn’t decline. Air-impervious insulations can be twice as effective as air-porous insulations of the same R value under certain conditions.

Myth #3: Vapour barriers trap moisture

It’s natural to believe this myth when you’re staring at a clear vapour barrier with moisture behind it. Trouble is, reality is not always as it seems. The purpose of a vapour barrier is to stop warm, moist, indoor air from infiltrating fiber-type insulation during cold weather. If infiltration occurs, the indoor air cools during winter, loses some of its ability to hold moisture, and this moisture comes out in the form of droplets. Visible moisture on the inside of a vapour barrier is either caused by a leaky vapour barrier or moisture migrating into the wall cavity from the outside. Leaky siding can cause this, but it can also happen in basements that are apparently leak free. Vapour barriers are essential for any kind of insulation that air can pass through. Never do the really foolish thing of slashing a vapour barrier that you find has moisture behind it, or failing to install a vapour barrier in the first place.

Myth #4: Spray foam is bad.

Back in the 1970s, urea formaldehyde spray foam was used to insulate homes. This proved to be a problem because authorities didn’t fully understand the dangers of urea formaldehyde off-gassing. Just don’t let your clients think that all spray foam is harmful today. That’s too simplistic. Despite media reports that conjure fear, properly applied spray foam delivers safe results and outstanding air sealing. Nothing else works as well as closed cell spray foam for sealing and insulating.

Myth #5: Stud wall R values are the same as insulation R values.

Just because you use, say, R24 batts in a stud frame wall, it doesn’t mean you get an R24 wall. There are two reasons why. The first is that fiber-based insulations only deliver their stated performance in the lab. Drafts and convection air currents within stud frame cavities lower real-world performance significantly. At least as important is the fact that 15 to 20 percent of every stud frame wall is made of pieces of wood that go right through the wall, extending from inside surface to outside. Wood has an insulating value of only R1 per inch, so for a typical 2x6 stud frame wall, one fifth of the wall area is only functioning at about R5 or R6. This situation is made worse by the fact that many stud frame wall cavities require custom-cut batts. In the time-is-money building world, precise fitting of batts doesn’t always happen whenever it should. Accurate insulation R values only come from a complete, real-world assessment of an entire building envelope, not just the nominal rating of one insulation component.

Myth #6: Stud Frame Walls Are OK for Basements

People have been turning basements into living spaces for more than half a century, and during most of this time batt-insulated stud frame walls were the only way to create warm, finished basements. We have more choices these days and that’s a very good thing. The fact is, hollow stud frame walls are vulnerable to internal moisture coming from two sides, precisely because basement walls are underground. Moisture can migrate into walls from the finished basement space if warm air is allowed to enter the wall because of flaws in the vapour barrier. That’s why you need a vapour barrier on the warm side of every exterior basement wall. Trouble is, moisture vapour can (and often does) migrate through masonry basement walls from the outside and into the stud frame wall cavity, even if no liquid water is visible. Musty basement odours and low indoor air quality throughout the house can happen because of hidden moisture and mold trapped behind drywall.

Rigid foam insulation systems are best for basements because they’re less vulnerable to air movement, they don’t allow internal condensation and they don’t absorb much moisture.

Myth #7: Insulation Alone is Fine for Cathedral Ceilings

Stuffing batts between rafters in a cathedral ceiling and covering them with vapour barrier is easy but risky because there’s no place for condensation to escape if it builds up within the roof structure. And build up it probably will. Unlike walls, roofs are more likely to have air leaks that lead to internal condensation and frost. That’s why it’s not unusual for water to leak down through cathedral ceilings in spring, as a winter’s worth of frost melts and trickles out. Cathedral ceilings insulated with batts need open vent space from eaves to peak to prevent cold weather condensation, or they need to be insulated with spray foam.

Insulating properly is more difficult than it looks. That’s why the reality of insulation often falls so far short of the potential. Besides the fact that the building industry is slow to adopt change, the dynamics of insulation and energy efficiency aren’t easy to see. Heat loss is invisible, insulation products are usually hidden, and the physics of air movement and condensation are widely misunderstood. Let facts bust the seven insulation myths and you’ll be much closer to optimizing the energy performance of your next building or renovation.


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