Exposed trusses and cathedral ceilings are incredibly common in cabin construction. Unfortunately, the construction details of these are often incorrect and lead to poor energy performance, ice dams, leaks, mold, mildew, and rot. We’ll look at several correct construction assemblies for unvented insulated ceilings.

What is a Cathedral Ceiling?

A cathedral ceiling is a type of vaulted ceiling – often with the two terms being interchanged. We’ll leave the discussion on semantics to someone else to debate.

Most simply – a vaulted ceiling follows the profile of the roof line – creating a high ceiling that often adds light and ambiance to a space. Cathedral ceilings are the type of vaulted ceiling that matches a gable roof most common in cabins.

All roof assemblies – vaulted or otherwise – must either be vented or unvented. The issue with most vaulted and cathedral ceilings is that they’re unvented (no attics) and require a unique construction assembly to avoid moisture issues and sheathing rot. Incorrectly detailed roof assemblies are also the root cause of ice dams.

We’ll continue to use the term cathedral ceiling – but everything discussed here applies to any insulated sloped roof surface (or wall – like an a-frame roof).

Insulated & Unvented Roofs

Design Considerations

Rafter Depth

Prescriptive code requires significant R-values in ceilings. This is often difficult or impossible to achieve within the rafter cavities. Said another way, a 2×10 rafter is not deep enough to meet the minimum required insulation values in most climate zones.

This means that to get the insulating value required – you’ll have to either fur out the rafter from the interior to provide more depth; and/or add exterior insulation.

Air & Moisture Management

Unvented assemblies cannot use air-permeable insulation (fiberglass batts, dense-packed cellulose, or blown-in fiberglass) alone because these insulation types allow moist indoor air to reach the cold roof sheathing. This leads to condensation or moisture accumulation on the sheathing creating mildew, mold, rot, etc.

Two things need to happen instead.

1. Air sealing between the interior space and the unvented assembly cavity. Special attention should be paid to junction boxes, fixtures, or any other penetration or seam in the barrier.
   Note that unvented roof assemblies are designed to dry to the interior. That’s why an unvented roof assembly should never include interior polyethylene. If you choose to use a vapor barrier – it needs to be a smart vapor retarder.
2. Non-air-permeable insulation should be used adjacent to condensing surfaces to prevent moisture issues from forming. Once in place – air-permeable insulation can be used in conjunction to more economically meet prescriptive r-values.

Construction Assemblies

Option 1 – Don’t Use an Unvented Assembly

The first option is to forgo an unvented assembly and instead vent the ceiling space following traditional construction methods. You’ll need at least a one-inch gap between the air-permeable insulation and the roof sheathing along with intake and exhaust vents.

The compromise here is that often there is not the rafter depth to provide the required r-values and adequate ventilation; requiring the ceiling to be pushed down into the living space. This assembly also does not work if there are any hips, valleys, dormers, or skylights. These constraints often make an unvented assembly the only path forward.

It’s also worth pointing out a few “truths” about unvented assemblies in general:

• Roof ventilation does not significantly lower the temperature of the roof surface. This is especially true during the summer months.
• Properly detailed roof ventilation can lower the risk of ice damming. However, the flow of heat into roof ventilation channels should be reduced as much as possible. This is often difficult to achieve.
• Roof ventilation can help dry out damp roof sheathing. However, moisture escaping from the home should be limited and managed to a point so that the roof sheathing never gets damp in the first place.
• Roof ventilation often does more harm than good allowing ceiling leaks to create unwanted air and thermal leaks.

Option 2 – Exterior Rigid Insulation

Installing continuous rigid insulation on the exterior surface of your roof sheathing allows for air-permeable insulation to be installed on the interior surface of the roof sheathing between the rafters. The exterior rigid insulation also creates a wonderful thermal break.

As noted above, you’ll still want to pay close attention to your interior air barrier to prevent moisture from moving from the indoor space into this cavity. How much rigid versus air-permeable insulation you’re allowed to use will depend on your enforced building code version and climate zone.

Option 3 – Closed-cell Spray Foam

This is the most expensive, and thus rare, option. Closed-cell spray foam alone can be applied on the interior side of the roof sheathing between the rafters to meet the required R-values.

Since closed-cell spray foam packs the greatest R-value per inch – this can be a great option for conditions where rafter depth is a limiting factor. It does not, however, solve the thermal bridging issue solved with other assemblies.

Option 3.5 – Closed-cell Spray Foam & Rigid Insulation

That said, there’s no reason Option 3 could not be paired with rigid insulation on the exterior of the roof sheathing to both solve for thermal bridging and make it more economical. Just make sure the interior r-values meet the prescriptive code minimums.

Option 4 – Closed-cell Spray Foam & Air-permeable Insulation

Like Option 3.5 closed-cell spray foam can be used to meet the interior minimum R-values for non-air-permeable insulation and the difference then be made up with air-permeable insulation. In a retrofit situation – a Flash-and-Butt method can be used.

This is more economical than spray foam alone or spray foam and rigid insulation. It, however, does not address the thermal bridging issue. And again, with air-permeable insulation, you’ll need to have excellent detailing on your interior air barrier.