Construction - First Cathedral Ceiling

The first cathedral ceiling presented an opportunity to apply the concepts detailed in the two posts on vapor and air barriers.  And it is interesting how much the final design of the ceilings deviates so much from the original design that I so naively and confidently detailed in a prior post before fully understanding vapor and air barriers.

I am deliberately inserting this post between the two posts on vapor/air barriers in order to reference it while discussing air barriers in the second of the two posts.

Original Design
My original design called for a 3 1/2" tall "mini-attic" between a fabric stapled to the tops of 2 x 12 rafters (with which to confine the blown-in rice hull insulation) and the roof sheathing. After more insight into moisture control and air infiltration/exfiltration in wall and ceiling assemblies, and, after a meeting with the consultant who will certify our project for energy efficiency, a different design for the mini-attic emerged. Also, instead of using 2 x 12s, I opted for trusses but only after thoroughly parsing I-joists as well. 

Perhaps the most succinct article on the subject of cathedral ceilings that I have seen is How to Build An Insulated Cathedral Ceiling on the Green Building Advisor site.  It clearly informed me that my original design would have been a disaster, that air sealing the ceiling/roof assembly is much more important than ventilation between the insulation and the sheathing, but that a dedicated air space sandwiched between a double layer of sheathing as described below for our project would be an advanced design worth the additional time and expense.

Roof Trusses
Two-by-twelves seemed a bad choice for three reasons.  First, 11 1/2" of insulation would yield an R-factor of only 35 when our target was at least 45.  Second, the thick 2 x 12s would allow considerably more energy-robbing thermal bridging than either trusses or I-joists. (Ever notice how easy it is to identify cathedral ceilings vs. conventional attics by the snow melt pattern on the roof?  Striations appear over cathedral ceilings because melting is faster over the 2-bys than over the insulation between them, whereas melting over conventional roofs is more uniform.) The third reason for avoiding long 2 x 12s is that their length and girth make it more likely that they come from old growth trees while I-joists and trusses have certifiably sustainable sourcing.  

I-joists 16" tall would have been a little cheaper than 16" trusses but would have required considerable job-site customization.  They would have

Roof trusses resting on truss walls (click on the picture to enlarge)

had to have been plumb-cut and reinforced on both ends and, since bird-mouths cannot be cut into the lower chords, the top plates of the walls would have had to have been fitted with wedge-shaped support boards.  And, while the trusses could be customized at the factory to fit flat on both 15" wide walls, the I-joists would have rested on the inside top plate on one wall and the outside top plate of the other. These considerations made the minor up-charge for the trusses a good trade-off.

The one downside to the decision, though, is that, while the trusses are significantly less thermal bridging than 2 x 12s,  I-joists would have been even better.  In hindsight, I would have used trusses 18" tall instead of 16".  The difference in cost would have been manageable and the extra two inches would have boosted the R-value by at least 7 points which would presumably off-set the loss from thermal bridging.

Mini-Attic Design
The air barrier for a conventional attic must be done at the level of the drywall as discussed in the recent post on vapor/air barriers.  In our case, however, the barrier will move to the level of the tops of the trusses due to our choice of tongue and groove pine ceilings that will be more permeable than drywall.  Instead of mesh on top of 2 x 12s as originally envisioned, I installed 1/2" plywood sheathing as the first of two layers of sheathing. The first layer will serve as the "floor" for the mini-attic; the second layer will double as its "ceiling" and as decking for the roof.  Since vapor passing through a wall or ceiling largely depends upon moving air, air sealing the floor of the mini-attic as described below will virtually eliminate vapor penetration through the pine ceilings.

At the time of this writing, I had covered the plywood with 6 mil sheet plastic anchored with batten boards to protect it for a few months until the mini-attic could be completed in conjunction with roofing the rest of the house.  And, for whatever it is worth, the first attempt to protect the plywood was a failure.  I conscientiously anchored the plastic with batten boards fastened with nails.  However, it took only a short time for wind blowing across the surface and coming up through the spaces between the sheets of plywood to heave the plastic enough to work the nails loose from the relatively thin (1/2") plywood.  The battens either blew off the roof or clung loosely to the plastic.  In either case, the nails protruding from them ripped holes in the plastic to the extent that I had to recover the roof with new plastic after taping the seams between the plywood sheets and filling the nail holes (last photo below).  This time I screwed the batten boards to place.  The moral is "use screws"; do not depend on nailed battens and don't even think that staples alone will work.

Just before the final roofing goes on, I will use construction screws to fasten 2 x 4s on edge on top of and fastened to the roof trusses through the first sheathing. I will then nail the second layer of sheathing to them.  The result will be a 3 1/2" ventilation space -- mini-attic -- that communicates with the outdoor air via continuous soffit vents in the north eave and a continuous ridge vent between the south edge of the roof and the overhangs for the second story windows.  

According to Joe Lstiburek at Building Science Corp., plywood for the first layer of sheathing is a better choice than OSB because it will allow water vapor to pass through it should vapor escape the living space, negotiate the less-than-impermeable wood ceiling and rise through the insulation. By contrast, the impermeability of OSB would block vapor which then could harbor mold, rot the sheathing, if not the trusses, and degrade the R-value of the insulation. OSB for the second layer of sheathing is acceptable however because any vapor from below will be vented from the mini-attic through the soffit vents and does not have to find its way through the second layer of sheathing.

The code calls for 1" minimum ventilation space between the insulation and the sheathing of a conventional cathedral ceiling.  Lstiburek suggests at least 2" for the air space while questioning the efficacy of any air space directly in contact with the insulation.  Our mini-attic will not only provide 3 1/2" instead of an inch or two but will also have sheathing separating the air space from the insulation.

The roof will overhang the walls 24".  I plan to extend the edgewise 2 x 4s that carry the second sheathing outward as support for the overhangs.  As discussed below, the 2 x 4s will not complicate air-sealing as would rafter tails extending from the roof trusses.

Sheathing the Short Truss Wall

The trusses are plumb cut flush with the short wall, i.e., there
 are no rafter tails extending from the trusses to interfere with
sealing the junction between the wall and the roof with a
continuous run of flashing tape

For the same reason I used plywood instead of OSB under the mini-attic, I used it for sheathing the short truss wall (and plan to use it for all of the exterior walls). It is important to note that, by plumb-cutting the roof trusses and leaving off the rafter tails, all of the wall sheathing could be abutted against the roof sheathing in order to simplify air sealing at the junction between the two. If rafter tails had been present, the sheathing would have had to have been cut and fitted around them -- a tedious job with a less-than-ideal outcome when it comes to air-sealing.  I was able to use a continuous run of flashing tape to seal the junction between roof and wall whereas, with rafter tails, tape, caulk and spray foam on the

 inside would also have been necessary for the inevitable gaps between the  tails and the wall sheathing.  

Air Sealing the Roof Sheathing

Blocking between trusses to stiffen the junction of the roof

sheathing and the wall sheathing and to facilitate caulking it
 from the interior, in addition to having taped the junction on
 the exterior (click on photo to see detail)

The clips used between sheets of plywoodsheathing are spacers to allow for expansion without buckling.  However, the space also would allow air infiltration and exfiltration that would be totally unacceptable.  Using caulk in the cracks would be counter-productive eventually since it loses its flexibility with age.  Spray foam would be rigid from the git-go.  So thank god for flashing tape. I used it not only to close the gaps left by the clips but also where the sheets of plywood met over the trusses. The nice thing is the tape will remain flexible indefinitely.



After the front wall and the rake walls for the second story have been sheathed withplywood, the junction between them and the roof sheathing will be handled in the same manner as the junction between the short wall and the roof. Then, considering that (1) the roof-wall junction is sealed with tape on all four sides, (2) the cracks between sheathing panels of both the roof and the walls are sealed with tape and (3) proper air-sealing is done around the windows when they are installed, the second story would theoretically be ready for a blower door test well in advance of the drywall stage. 

Design - Vapor Barriers and air barriers

There are myriad materials marketed for controlling the flow of moisture and air through wall and ceiling assemblies.  However, it doesn't take much research to become confused about which to use where.  For example, the nearby map shows that, for our lower Midwest climate zone, we need an interior vapor barrier.  As we will see, this would be an unwise choice. When step-son, Keith, and I were building his house in a nearby county, even the building inspector was sufficiently ambivalent to accept the wall construction with or without a polyethylene sheet plastic vapor barrier.


This post is limited to those materials used on the inside of assemblies. The next post will tackle those used exteriorly. Furthermore, this post emphasizes vapor barriers for wall and ceiling assemblies while the next post completes the story by discussing air barriers.

The Problem
Anyone who is involved with building a house would do well to read "Understanding Vapor Barriers" by Joseph Lstiburek on which the following discussion is based.  He says that "Vapor barriers are...a cold climate artifact that have diffused into other climates more from ignorance than need. The history of barriers itself is a story based more on personalities than physics.....It is frightening indeed that construction practices can be so dramatically influenced by so little research...  Incorrect use of vapor barriers is leading to an increase in moisture related problems. Vapor barriers were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying. Vapor barriers installed on the interior of (wall or ceiling) assemblies prevent assemblies from drying inward.  This can be a problem in any air-conditioned enclosure. This can be problem in any below grade space.  This can be a problem when there is also a vapor barrier on the exterior.  This can be a problem where brick is installed over building paper and vapor permeable sheathing."

Simplified Terminology
Lstiburek proposes simplifying terminology.  He suggests that all of the materials used in wall and ceiling cavities that are capable of influencing the behavior of moisture vapor, such as house wraps, polyethylene sheeting, felt paper, OSB, plywood, foam insulation board with or without foil backing, drywall, latex paint, vinyl wallpaper and all cladding materials should be called "vapor retarders" because they all have the capacity of retarding the movement of water by vapor diffusion.  Vapor retarders should then be sub-classified into four groups according to the rate at which vapor diffuses through them as measured by their vapor permanence or "perm" as follows:

• Class I Vapor Retarder:        0.1 perm or less
• Class II Vapor Retarder:       Between 0.1 and 1.0 perms
• Class III Vapor Retarder:      Between  1.0 and 10 perms

Then Lstiburek goes on to categorize materials generically into four groups based upon the above three classes as follows:

• Vapor impermeable:                Class I Vapor Retarder       (vapor barrier)
• Vapor semi-impermeable:        Class II Vapor Retarder      (vapor retarder)
• Vapor semi-permeable:            Class III Vapor Retarder    (vapor retarder)
• Vapor permeable:                    Greater than 10 perms

Air moves through wall and ceiling assemblies due to differences in air pressure and contains varying amounts of water in the form of vapor.  All of the materials listed below as vapor retarders have some capacity for blocking air movement and, in so doing, might be called "air barriers".  

Examples of Vapor Retarders
The following list comes from Energy.gov:

• Class I:  Glass, metal, polyethylene sheeting, rubber membrane
• Class II:  Unfaced extruded (XPS) or expanded polystyrene (EPS), 30# felt (asphalt coated paper), plywood, bitumen coated kraft paper
• Class III:  Gypsum board, unfaced fiberglass insulation, board lumber, concrete block, brick, 15# felt (asphalt coated paper), house wrap

Choosing a Vapor Barrier

Lstiburek is quite clear as to best practices for choosing vapor barriers  Paraphrased from his work, they are as follows:

• Avoid using vapor barriers where vapor retarders will suffice; avoid vapor retarders where vapor permeable materials will work; thereby "encouraging drying mechanisms over wetting prevention mechanisms
• Avoid vapor barriers on both sides of an assembly so as not to block drying in at least one direction
• Avoid installing on the interior of air conditioned space such vapor barriers as polyethylene sheeting, foil faced batt insulation and reflective radiant barrier foil
• Avoid vinyl wallpaper on the interior of air conditioned spaces

Interior Vapor Barriers

A reasoned summary for when to use an internal vapor barrier like polyethylene sheeting is found in Green Builder and reproduced verbatim below:

• Most buildings don't need polyethylene anywhere, except directly under the concrete slab or on a crawl space floor.
• The main reason to install an interior vapor retarder is to keep a building inspector happy.
• If a building inspector wants you to install a layer of interior polyethylene on a wall or ceiling, see if you can convince the inspector to accept a layer of vapor retarder paint or a "smart" retarder (for example, MemBrain or Intello-Plus) instead.
• Although most walls and ceilings don't need an interior vapor barrier, it's always a good idea to include an interior air barrier.  Air leakage is far more likely to lead to problems than vapor diffusion (the italics are mine).

Both Lstiburek and Energy.gov say that drywall with latex paint on it, when installed

correctly, forms an adequate semipermeable vapor retarder for most of the US. As we sit at the junction of the mixed-humid and hot-humid zones depicted on the above map, this is obviously the best option for our project as well.  The only caveats that make our project different is that (a) we will not have conventional air conditioning for cooling and humidity control as would be expected for our area and (b) our earth sheltered design means our living space is partially below grade.  However, the energy recovery ventilator that we have planned should adequately replace air conditioning for humidity control and the earth contact walls will be not be in contact with moisture because of the French drains and the insulation/watershed umbrella. And the earth contact walls will not be subject to sweating because the earth behind them will be warmed by the AGS system (for info on AGS, click on "Featured Post" in the column to the left).

Vapor Control Varies According to Climate and Assembly Components
Despite the above advice for avoiding interior vapor barriers, there is no universal solution to vapor control for all situations.  Lstiburek's paper discusses various scenarios for exterior wall assemblies and specifies the best climate zone(s) for each (not only for zones depicted by the map above, but for severe cold climates further north as well).  In northern climates, for instance, the best practice is in fact to use an interior vapor barrier.  But, in another paper on air barriers, he warns against using them even in cold climates for air conditioned spaces.

Even though the primary function of air barriers is to limit air infiltration and exfiltration, they are also vapor retarders in that they control the movement of moisture-laden air through an assembly -- as we shall see in a subsequent post.

In the meanwhile, I will devote the next post to the newly-built cathedral ceiling so as to be able to reference it later for the follow-up post on air barriers.