Adding Attic Insulation Can Make Your Roof Wetter

Why a colder attic is a wetter attic

Warm air holds more water as vapour. Vapour is water in gas form, invisible, the thing your breath and your shower and your cooking put into the air. When that warm indoor air reaches a cold surface, the vapour turns back into liquid water on that surface. That is condensation, the same thing that happens on a cold glass in summer.

In a house, warm moist air moves upward and finds every hole in the ceiling. Pot lights, plumbing stacks, the gap around a bathroom fan, the top of an interior wall, a wiring hole. The air goes into the attic and meets the roof sheathing, which is the plywood or OSB panel nailed across the top of the rafters and carrying the shingles. In winter that panel is cold.

Insulation slows heat from leaving the house. That is the point of it, and it works. One side effect on a house that was never air sealed is that the attic now runs colder than it did before, because less heat is escaping up into it. The sheathing is colder, so condensation starts sooner and there is less warmth to dry it out again afterward.

The same amount of moist air is still leaking up through the ceiling, because insulation is not an air barrier. Blown fibre lets air pass straight through it. So the upgrade changes one half of the equation and leaves the other half alone, which is how a job sold as an energy improvement ends up as a moisture problem.

On the North Shore the outdoor air is wet for months at a time, which means a damp attic has fewer dry days to recover on. That is the reason ventilation and air sealing carry more weight here than they would in a dry interior climate.

What Section 9.19 requires

The BC Building Code deals with attics in Section 9.19, Roof Spaces. The first article sets up everything that follows. Article 9.19.1.1 states that except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the transfer of moisture from the space to the exterior.

Read the purpose in that sentence. The vents are there to move moisture out. They are a drying system for the roof, and the Code treats them as a companion to the insulation.

Article 9.19.1.3 puts a number on the space. Not less than 63 mm of space shall be provided between the top of the insulation and the underside of the roof sheathing. That is the clear gap the air travels through, measured from the surface of the insulation up to the wood.

The same article closes the loophole that would otherwise swallow the rule. Sentence (3) states that ceiling insulation shall be installed in a manner that will not restrict the free flow of air through roof vents or through any portion of the attic or roof space. So the requirement covers the whole path air takes, from one end of the roof to the other.

The eaves are where this goes wrong

Picture the edge of your roof. The rafters slope down and meet the top of the exterior wall, then carry on past it to form the overhang. The underside of that overhang is the soffit, the horizontal panel you see when you stand outside and look up at the roof edge. The vents in that panel are the intake, the place fresh air enters the attic.

That junction is the tightest space in the whole attic. The roof is lowest there, the wall top plate is in the way, and a person crawling in the attic cannot easily reach it. It is also where a blowing hose is pointed when someone wants the insulation to look even across the floor.

So the insulation gets pushed into the eaves and covers the soffit vents from the inside. From the attic hatch everything looks correct. From outside the soffit vents are still visible. The air path is closed, and nobody can see it.

The Code answer is in Article 9.19.1.3(2). At the junction of sloped roofs and exterior walls, where preformed baffles are used to contain the insulation, those baffles shall provide an unobstructed air space between the insulation and the underside of the roof sheathing that is not less than 25 mm in dimension, and of sufficient cross area to meet the venting requirements of Article 9.19.1.2. The baffles shall also extend vertically not less than 50 mm above the top of the insulation.

A baffle is a rigid tray, usually cardboard or plastic, stapled to the sheathing between the rafters at the eave. It holds the insulation back and keeps an open channel from the soffit vent up into the attic. The 50 mm height above the insulation is the part installers forget, and it matters because blown insulation settles and gets pushed around. A baffle that only reaches the top of the insulation on installation day is buried the first time somebody crawls past it.

• Baffles go in before the insulation, in every rafter bay that has a soffit vent under it.
• The air channel behind the baffle stays open at not less than 25 mm across.
• The baffle stands not less than 50 mm proud of the insulation surface so settling does not close it.
• Check them from inside the attic with a light, at the edge, not from the hatch.
• On a re-roof the baffles are reachable from above while the sheathing is off, which is the cheapest time to fix them.

How much vent area, and where it goes

Article 9.19.1.2 sets the amount. The general rule in Sentence (1) is that the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area. Unobstructed means the actual open area of the vent after the screen and louvres, which is smaller than the size of the hole cut in the roof. Vent products publish this number, so ask for it rather than measuring the opening.

Sentence (2) doubles the requirement in two cases. Where the roof slope is less than 1 in 6, or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area. A slope of 1 in 6 means the roof rises 1 unit of height for every 6 units of horizontal run, a shallow pitch. Roof joists means the roof is framed with single members that also carry the ceiling below, which is how a vaulted or cathedral ceiling is built.

Both cases are common here. The flat and low-slope roofs on post and beam houses through Edgemont and Deep Cove fall under the first. The open vaulted ceilings on the same houses fall under the second. If either one describes your roof, the vent area doubles.

Sentence (3) governs placement, and this is the part that decides whether the vents work. Required vents may be roof type, eave type, gable-end type or any combination, and shall be distributed uniformly on opposite sides of the building, with not less than 25% of the required openings located at the top of the space, and not less than 25% located at the bottom.

The reason is that air only moves through the attic if it can enter low and leave high. Warm air rises to the ridge and leaves through the upper vents, and that pulls cool outdoor air in through the soffits. Put all the vent area at the ridge and there is nothing feeding it. A roof with 20 new ridge vents and buried soffits moves less air than a roof with fewer vents that are balanced properly.

Vaulted ceilings, purlins, and mansard roofs

A vaulted ceiling has no attic. The drywall is fastened to the underside of the same joists that carry the roof, so each space between two joists is a long narrow channel with a closed top and bottom. If the insulation fills that channel there is nowhere for the air to go, and there is no way for one channel to feed another.

The Code handles this in Article 9.19.1.2(4). Except where each joist space is separately vented, roof joist spaces shall be interconnected by installing purlins not less than 38 mm by 38 mm on the top of the roof joists. A purlin here is a strip of lumber laid across the tops of the joists before the sheathing goes on. It lifts the sheathing off the joists and leaves a continuous gap running sideways, so air can cross from one bay into the next.

That is a framing decision. It happens when the roof is being built or rebuilt, and it cannot be added later without taking the sheathing off. So if you are planning to open a ceiling into a vault, or re-roofing a house that already has one, this belongs in the drawings before anyone orders material.

The alternative in the same sentence is to vent each joist space separately, with its own intake at the bottom and its own outlet at the top of that single channel. It is more work per bay and it gets used where purlins would change the ceiling height.

There is one exception worth knowing. Article 9.19.1.4 says the lower portion of a mansard or gambrel style roof need not be ventilated, and that the upper portion shall be ventilated in conformance with Articles 9.19.1.1 to 9.19.1.3. A mansard or gambrel roof has two slopes on each side, a steep lower one and a shallow upper one. The steep lower part is exempt. Everything above it follows the normal rules.

What has to happen before the insulation goes in

The ceiling plane is the whole surface between the heated house and the cold attic. Air sealing it means closing every hole through that surface so indoor air stops leaking into the attic. This is the work that makes an insulation upgrade safe, and it has to come first, because once there is half a metre of loose fibre on the attic floor nobody can find the holes.

The list is short and it is the same on most houses. Pot light housings. Plumbing vent stacks where they pass through the ceiling. The bathroom fan housing and its duct. The tops of interior walls, where the gap between the two top plates runs the length of the wall. Wiring and pipe penetrations. The chimney or flue chase, which needs a non-combustible seal and its own clearances.

Check where the fans actually go. On older North Shore houses a bathroom or kitchen fan often ends in the attic with a loose duct, or with no duct at all, so the wet air it removes from the bathroom is delivered straight onto the cold roof sheathing. Every fan needs a solid duct that runs to a hood on the outside of the building, insulated along its length so the moist air inside it does not condense before it gets out.

Recessed lights need a decision before they are buried. A fixture that is not rated for insulation contact overheats when covered, and that is a fire hazard. IC-rated means the fixture has been tested and listed for insulation contact, so insulation can sit against it. Confirm the rating stamped on each housing. Where it is missing, absent, or unreadable, replace the fixture with a sealed IC-rated unit rather than building a box around it, because a box is one more hole in the air barrier.

Then look at the attic itself before adding anything. Dark staining on the sheathing, rusty nail tips, mould on the wood, compressed or wet insulation, daylight where there should be none. Adding insulation over an active moisture problem hides it and makes it worse. Fix the cause, let the wood dry, then insulate.

• Seal every ceiling penetration: pot lights, stacks, fan housings, wall top plates, wiring holes.
• Duct every bathroom and kitchen fan to an exterior hood, and insulate the duct.
• Confirm each recessed fixture is IC-rated, or replace it with a sealed IC-rated unit.
• Install eave baffles in every bay before the insulation arrives.
• Inspect and photograph the sheathing for staining, mould, or rot, and resolve it first.
• Seal and insulate the attic hatch, which is a hole in both the air barrier and the insulation.

The hatch, and why the Code cares about it

Article 9.19.2.1 requires an access hatch to every attic or roof space where the open space measures 3 square metres or more in area, 1 metre or more in length or width, and 600 mm or more in height over at least that area. If the space meets all three, it gets a hatch.

The size is set in the same article. The hatch shall be not less than 550 mm by 900 mm, except that where the hatch serves not more than one dwelling unit it may be reduced to 0.32 square metres in area with no dimension less than 500 mm. Sentence (3) adds that hatchways to attic or roof spaces shall be fitted with doors or covers.

Now put the hatch next to the air sealing discussion above. A hatch is a rectangular opening cut through the ceiling. On the day it was framed, somebody cut the drywall, the vapour control layer and the insulation, and set a piece of plywood loosely in the hole. Warm wet air rises to the highest point in the house, which on most floor plans is the hallway ceiling where the hatch sits.

So the hatch is one of the largest single air leaks in a typical house, and it is also a bare spot in the insulation. Good practice is to fit weatherstripping around the frame so the cover seals when it is closed, add a latch or clips that hold it down against the seal, and fasten rigid insulation to the top face of the cover to a depth that matches the rest of the attic. This is practical work that follows from the air sealing the insulation depends on.

A pull-down attic stair needs the same treatment, using an insulated box that sits over the whole opening from the attic side.

How much insulation, and which number to trust

Metro Vancouver sits in Climate Zone 4, the mildest of the zones used in BC energy requirements. Published BC government guidance on energy efficiency for houses in Climate Zone 4 gives a prescriptive minimum effective thermal resistance for ceilings below attics of effective RSI 6.91, which is about R-39 in the imperial units printed on insulation bags.

Those units are worth explaining once. RSI and R-value both measure how well a material resists heat flow, with a higher number meaning more resistance. RSI is the metric version and R-value is the imperial one, so RSI 6.91 and R-39 describe the same performance. Effective means the number for the whole assembly including the wood framing, which conducts more heat than the insulation between it. The figure printed on a bag is for the insulation alone, so it is always higher than the effective value of the assembly built with it.

Treat effective RSI 6.91 as the prescriptive minimum from the Climate Zone 4 guidance and confirm the current requirement for your own project. Energy requirements are updated with Code cycles, municipalities set their own timelines, and your project may be on a BC Energy Step Code path where the house is assessed by its modelled performance and a blower door test rather than by a table of minimum values.

Ask your building department, or ask the energy advisor on the project, which path applies before you buy material. On a Step Code path, the air tightness result usually has more influence on whether you pass than another 50 mm of loose fibre in the attic does, which brings the argument back to sealing the ceiling plane.

Beyond the required minimum, attic insulation is one of the cheaper places to add depth, because the space is open and the material is inexpensive. The limit is the 63 mm clearance in Article 9.19.1.3 and the free airflow requirement in Sentence (3). Depth that closes the channel at the eave costs more than it saves.