Ventilating Cathedral Ceilings

Back in 1990, a friend of mine moved into a stately old home built in the small town near where I live, and since then he’s learned first-hand about the importance of cathedral ceiling ventilation. The house was built in 1890, by a commercial fishing tycoon whose family pioneered the area, and like many old homes of that era, no consideration was given to attic ventilation. The place was sealed tightly, from eaves to peak, and that worked fine in the 19th century. Nothing else was necessary during the era when wintertime heating was minimal, and indoor humidity levels remained low thanks to leaky windows and limited domestic hot water use. But as electricity, running water and a whopping-big oil furnace were added to the house over the years, moisture production skyrocketed, and so did the need for attic ventilation.

This was pretty easy to add in most places, through the use of rooftop vents and gable louvres. But not so in one second-story sun room with an insulated cathedral ceiling covered in vee-matched, tongue-and-groove pine. With the rafter cavities stuffed full of insulation and entirely sealed, there was no easy way to create an outlet for the moisture that condensed within the ceiling during long cold spells. And condense it did. So much so that every spring two or three gallons of brown, dripping water have to be collected in buckets as a winter’s worth of frost melts in a few days, runs down walls and drips through the board-to-board gaps. The problem remains to this day because it’s so difficult to fix properly.

If this sounds like just another old-house horror story, think again. Many Canadians still construct new cathedral ceilings in exactly the same way as in the bad old days, building headaches for themselves that go beyond just soppy ceilings on warm spring afternoons. Perennially-wet roof structures rot and encourage mold growth, promoting low indoor air quality as it does. I know from the spike in reader emails I get during warm, spring weather that poorly constructed cathedral ceilings are still being built and still causing headaches. The good news is, there’s a way around the problem. You can have a frost proof cathedral ceiling as long as you’re willing to understand the design features necessary for success and make them a reality. As usual, it takes a little more effort, but it’s all worth it in the end.

Cathedral Ceilings That Breath

More than 10 years ago I developed an approach to cathedral ceiling ventilation that I now know works well over the long haul. It encourages the movement of air between rafters and sheathing in roofs framed with lumber as narrow as 2×6. And it does this while also offering the opportunity to add reasonable levels of thermal insulation, too.

The system is based on the idea of maintaining an eaves to peak ventilation channel between each rafter, and this requires three elements:

  1. Site-cut foam spacers and baffles that maintain a 1 1/2-inch deep ventilation channel underneath all roof sheathing.
  2. Fiberglass insulation and vapour barrier filling the remainder of the inter-rafter space.
  3. Another layer of rigid foam placed over the vapour barrier, fastened to the underside of the rafters with nails and metal washers.

When this design is connected to perforated eaves soffit on the bottom, and gable-end louvres at the top, a free-flowing channel for air movement is preserved. You can actually feel this movement with your hand, even on calm days. And this not only leads to a reliable, drip-free cathedral ceiling, but also extends shingle life by lowering roof temperatures during summer.

Step #1: Install Spacers and Baffles

Although it’s possible to buy ready-made foam baffles to fit between rafters and maintain a ventilation channel, I prefer to cut my own on site because they work better. Ready-made baffles don’t offer a ventilation channel across the full width of rafter cavities, a feature that I believe is key.

Start by cutting some 1/2-inch thick x 1 1/2-inch wide strips of extruded polystyrene foam on your tablesaw, to act as spacers. Most of my system can be constructed with less expensive expanded polystyrene (that’s the beady white stuff), but the spacers should be cut from the stronger, extruded type, to make installation easier. It’s frustrating when the spacers snap during installation, and extruded foam is stronger and resists breakage better.

The purpose of the spacers is to hold the baffles away from the underside of the roof sheathing, and 1-inch long asphalt shingle nails do a great job securing the spacers. Nail them to each side of all rafters, tight against the underside of the sheathing, running the full length of each rafter. Baffles come next.

The idea here is to cut lengths of 1-inch thick foam so it fits between the rafters and stays there by friction. You’ll find that trimming them 1/8-inch wider than the rafter space does the trick. A drywall saw is great for custom-fitting the foam baffles around plumbing stacks, wiring and other obstructions. Just be sure to extend the baffles high enough so they clear the upper extent of the insulation layer, near the peak at the place where the insulation stops. This may be at a point where collar ties exist, or all the way up to a ridge vent. Whatever happens, there must be a place at or near the peak where air can get in and out.

Step #2: Fiberglass Insulation and Vapour Barrier

At this stage you’ve got rafter cavities that are partially filled with foam, and now’s the time to fill them completely with fiberglass batts. If you’re dealing with 2×8 rafters, go ahead and install R-20 batts; 2×6 rafters should still be able to accommodate R-12 batts, plus the foam baffles that are already there.

You’ll find the batts will stay put even on a fairly low-slope roof, but probably not for long. Slight movement of the baffles from wind could cause the fiberglass to fall out, so don’t put much up before supporting it underneath. That’s why it makes sense to staple a vapour barrier to the underside of the rafters, sooner rather than later. And as you do, remember that part of the success of this approach is to be diligent about sealing your cathedral ceiling, and that means using acoustic caulking on all vapour barrier joints. You could use regular caulking, but an acoustic formulation is best because it never hardens. That’s good on a vapour barrier because lumps of hardened caulking can interfere with the installation of finished wall surfaces later. You’re aiming to create a completely impervious membrane with your 6 mil poly, so take your time. The job always takes more caulking than you think, so have plenty on hand.

Step #3: Add More Insulation

The insulation you’ve installed so far won’t be prone to condensation build up, but it doesn’t offer enough thermal performance, either. That’s why you need to layer more rigid foam on the inside edges of the rafters, over top of the vapour barrier. This may sound like suspicious advice, but it is within code. Specs state that it’s acceptable to have a vapour barrier as far as 1/3 of the way into an insulation layer, based on R-value numbers, not material thickness. This means, for instance, that as much as 1 1/2-inches of foam (for a value of R-7.5) can be safely installed on the inside edges of a cathedral ceiling framed with 2x8s and insulated to R-25 (that’s 6 inches of fiberglass, plus the 1-inch thick baffle). You could add even more insulation than this, and probably should, though you’d have to move the vapour barrier further towards the interior surface of the insulating sandwich to do it safely. A vapour barrier that’s too far into the insulating layer might get cold enough that condensation can occur on it, though the risk is low in this case because the foam you’re using over top is impervious to moisture vapour.

Ceiling Finishes

Although you could install wood directly over the foam, as a finished ceiling surface, drywall is a better choice. It’s easier to secure than wood, and it forms a better vapour seal because of the tape, mud and paint that’s applied to it. Since a foam surface is not as physically solid as wood, it’s especially important to install 5/8-inch drywall in this application. All you need are screws long enough to penetrate at least 1 1/4 inches into the underlying rafters. If you have trouble finding drywall screws 3 or 3 1/2 inches long (and you certainly might) I know from experience that regular #10 wood screws work fine. Your roof will, of course, be shingled while you’re completing the cathedral ceiling, but when it comes time to reshingle, remember to use a pneumatic nail gun. Pounding nails with a hammer could knock the mud off the screw heads as vibration is transferred directly to the drywall surface.

As you install gypsum board, intentionally leave a 1/4-inch caulking gap around all framing elements that penetrate the cathedral ceiling. It’s not unusual to have exposed collar ties or beams in buildings like this, and these breaches of the air barrier membrane will lead to moisture accumulation within the ceiling unless you take precautions. When you’re all done drywalling, lay a bead of caulk in the gap you left, tooling it with a 3/4-inch diameter dowel. Polyurethane caulking is best because it remains highly flexible, yet can be painted.

Share on FacebookShare on Google+Tweet about this on TwitterPin on PinterestShare on Reddit

Posted on November 19th, 2010

Got a Question for Steve?