It’s February and we’ve just had another seasonal round of chocolates and hearts. Now it’s time for a shower of ‘attic rain’ articles.
Attic rain happens in cold climates, in the depths of winter.
Attic rain happens when the exterior temperature warms up suddenly - like when there’s a Chinook in Calgary. Or when there’s a break in a Maritime winter. You know, when it’s -26°C without wind chill one day and the next it’s +8°C. Both days can be gloriously sunny. Both days can have very different effects on attics.
Understanding Attic Rain: Construction Causes
We know the climatic conditions that can lead to attic rain.
But what about the construction conditions that allow it to occur and why is it primarily newer construction that’s affected?
Newer houses have less uncontrolled air leakage than older houses. Often referred to as being airtight. They’re not airtight. They have a reduced amount of air leakage. In building science terms, we’re aiming to minimize the impact of the wind and the stack effect, so we can control the air movement inside the house and into the building envelope. That’s a good thing.
The attic rain problem is driven by WHERE the air leakage happens in a better-controlled building envelope.
Let’s break it down.
Air movement in a building is driven by two mechanisms:
The Wind Effect
Dynamic pressure differences on the outside of the house drive infiltration and exfiltration. Air is pulled in on the windward side at the same rate that it’s pulled out on the leeward side. The speed and direction of wind changes constantly. The faster the wind, the stronger the infiltration/exfiltration rate.
The Stack Effect
The vertical movement of air in a house, caused by air buoyancy and pressure differences, drives exfiltration separately from the wind effect. This means warmer air is typically at the top of the building because it’s less dense/more buoyant than cooler air. Without air sealing and insulation, the stack effect is encouraged, bullied, and bossed around by higher wind speeds and/or colder exterior temperatures.
There’s a third air movement mechanism that has to do with the effect of exhaust appliances. Mechanical systems can depressurize the house, creating negative pressure in the lower levels and creating more pressure in the upper levels. Still, it’s not the driving factor as it’s the ability of warm air from the living space to move into the attic that causes attic rain.
What else do we know?
The Science of Moisture Movement in Buildings
Moisture movement in a building tags along with air pressure differences as water vapour. Warmer air can hold more moisture than cooler air.
Let’s look at new construction, with its lower air leakage rates and higher insulation values. What happens inside the building? For starters, more warm air stays inside the conditioned space for a longer time.
That’s a good thing.
At the same time, that warm air holds more moisture which stays inside the conditioned space for a longer time. That’s not a good thing. That’s why new construction requires Controlled Mechanical Ventilation.
We know we need mechanical ventilation in new construction in cold climates. We’ve known it since the 1970s. Whole-house balanced systems that use HRVs and ERVs provide tempered, controllable, filterable fresh air while exhausting moisture and odours. Handily, they scavenge energy from the air in the exhaust stream (that’s how the supply air gets tempered)
But that’s not what drives attic rain. Mechanical ventilation is used to improve the air quality of the living space.
Misconceptions and Conflations
So what’s my gripe with the articles that get paraded out at this time of year? They conflate two things and don’t address the real issue.
Here’s an example from a 2023 article from the CBC: “Attic Rain” is on its way… increase[d] insulation levels and an overall non-breathable building envelope, and you get a higher risk of attic rain.”
NOPE.
IF THIS WERE A NON-BREATHABLE BUILDING ENVELOPE,
WE WOULD NOT HAVE ATTIC RAIN.
The stack effect is the culprit. Warm, moist air at the top of the house is pulled into the attic by pressure differences. It rises to the underside of the roof deck, which is, in most of Canada in the winter, a Very Cold Surface. As the warm air moves into the attic, it loses heat energy, then it bumps into the Very Cold Surface of the roof deck where it condenses and freezes. It melts when the roof deck warms up as the outside temperature bumps up past zero.
Attic rain can be thwarted by a continuous air barrier. One that stops the air flow into the attic. One that stops the stack effect.
Attic rain can be further thwarted by adequate attic ventilation. Or a completely non-vented roof with the attic inside the conditioned space. Either one. Don’t care. Your choice.
Please don’t blame the building envelope if exhaust devices like bath fans, range hoods, or clothes dryers are ported into the attic. That’s all about the mechanical system design/install. And whoever was in charge of that should be tarred and feathered.
So why is it that newer houses are showing attic rain?
The ceiling/roof/attic plane doesn’t have a continuous air barrier and the neutral pressure plane lays in the upper part of the house, close to the top-most ceiling. This results in a stronger ‘push’ of warm, moist air against the ceiling. There are few exfiltration points in the wall area, and some of them, like holes through the top plate and unsealed junctions in the framed cavity, likely feed directly into the attic anyway.
The warm moist air that collects at the top-most ceiling in a house with lower air leakage rates has nowhere else to go but up and into the attic.
In a house with a higher air leakage rate, on the other hand, the neutral pressure plane lays somewhere lower in the house. This means there are many exfiltration points below the top-most ceiling, mainly in the wall area:
- Rough openings at windows and doors
- Floor header/rim joist areas
- Exposed floors
- Penetrations for mechanicals and services
- Connections between living space and attached garages
Yet, when articles are written about ‘unbreathable’ envelopes and correlate them to attic rain, the logical (!) conclusion is not to do a better job of air sealing the attic and providing good air flow via attic ventilation, it’s “…something something houses need to breathe.”
NOPE.
Lemme tell you, in no uncertain terms that a house does not need to breathe. You need to breathe. Construction assemblies need to be protected from moisture flows and they need to be able to dry.
The Building Science and Solutions for the Attic
Above the insulation and below the roof deck, the attic is a buffered space with it’s own little climate going on. We install a continuous air barrier to stop exfiltration of warm moist air from the conditioned space, and we install passive venting to even out the temperature differences between its buffered self and the great outdoors.
Attic rain only happens when ice and frost, formed due to condensation driven by exfiltration from the conditioned space, melts. Why does the ice and frost melt? Because the outside temperature goes up. What happens to air when the temperature goes up? It’s buoyancy increases and it rises. You get convective currents in the attic. That’s where attic ventilation comes in handy. Soffit to ridge. Pull the warmed air out the top.
Soffit to ridge ventilation helps keep the attic dry by setting up a way to exfiltrate warm air via natural convection and the stack effect within the attic. The warmest air in the attic always has a way to leave at the ridge, and is being offered it’s hat and coat by the soffit vent.
Gable end ventilation doesn’t do as good a job, as the vents are often situated lower than the peak, where the warmest air in the attic will end up. In addition, soffit to ridge ventilation has the advantage of being exposed to wind coming from many more directions than the gable ends.
Here’s why gable end attic venting doesn’t perform as well as soffit-ridge: If the wind is blowing end-to-end, then there’s a good chance of good air flow due to windward/leeward pressure differences. If the wind blows crossways to the gable end, while the air pressure may be strongest at the corners of the house, it’s highly unlikely that one corner will decide to set up a negative pressure zone in front of one gable vent and the other corner will set up a positive pressure zone in front of the other. Turbulence abounds. And then the wind stops or changes direction all together. Fickle.
There are plenty more factors to consider in terms of attic ventilation and the behaviour of wind, but I can hear your eyes glazing over from my desk.
This is why we study building science and get familiar with fluid dynamics. Air is a fluid. Water is a fluid. Air and water vapour are a fluid couple. They do a mean tango.
Living Space and Attic Space: It’s Definitely Not Supposed To Be Connected
<RANT>
I would like to see zonal blower door testing done on houses that experience attic rain compared to similar age/construction specs that don’t experience attic rain. Zonal testing indicates the level of ‘connection’ between conditioned and unconditioned space. It’s an inexpensive, foolproof way to hone in on thermal bypasses as well as general air leakage locations. It allows you to put your air sealing efforts to best use by showing you where the biggest problems are. Zonal testing is the next step after a walk-around air leakage location inspection.
BUILDERS AND RENOVATORS: It’s a really easy pro-active fix to have an extra zonal test done when the blower door is running.
What’s that?
You don’t ‘need’ a blower door test? You good?
Please let me know how you know where you might be growing unintended consequences.
You can’t manage or solve a problem if you don’t measure it. As we get into higher performance code requirements, the tolerances for moisture loads in houses go down.
You need an energy advisor with a blower door.
You need this testing so you can avoid callbacks. Save yourself the annual headache of cranky clients with attic rain problems with this one step: a pre-drywall blower door test.
<END RANT>
