Design - Roof and Cathedral Ceilings

The design of cathedral ceilings is an interesting, complicated and confusing issue and one that has caused much waffling on my part.  Apparently, the majority of cathedral ceilings extant today have been constructed with the top-to-bottom combination of roofing, sheathing, rafter/insulation and drywall.  And the outcome has been that most have failed, are failing or are destined to fail because of moisture condensation at the top of the insulation. So what should do we do different?

Red Flags

The entire roof for the living quarters of our house will be shed type with cathedral ceilings (no attic).  My research on cathedral ceilings has led me to conclude that the sheathing should not be applied directly to the tops of the rafters.  The emerging consensus is that 6 mil moisture barrier on the bottom side of the rafters is no answer (actually should be avoided in our climate) but that there should be ventilated space between the rafters and the sheathing -- a "mini-attic" if you will. 

And there are alternatives to a mini-attic .  A good one, if you can afford it, is to fill the entire space between sheathing and drywall with spray foam insulation, as opposed to using compacted fiberglass or compacted cellulose.  Spray foam completely blocks air infiltration and the moisture that it carries.  Another approach that apparently works as well or nearly as well is to hold conventional insulation back from the top few inches under the sheathing and fill the space with spray foam insulation.  This would seem to be an economical alternative to all spray.  Most of the other solutions that crop up online are mostly iterations on the mini-attic approach.

We plan to use rice hulls for insulation for both the walls and the ceilings,  And, since I have had no information on rice hulls as an air barrier, I plan to equate them to dense-packed cellulose and dense-packed fiberglass.  This means that, at the 15" thickness of our walls and ceilings, the hulls will probably stop most of the moisture-bearing air infiltration that penetrates beyond the air-tight drywall detailing. 

For more on the thinking that went into the final design for the roof, check out an earlier post:  Timeline - Design Evolution - Energy Efficient Roof.

Solving the Moisture Problem

In order to create a ventilated "mini-attic" above the insulation and avoid the moisture problem, I plan to fasten salvaged 2 x 4s on top of  the 2 x 12s rafters then nail the sheathing to the the 2 x 4s.  When insulation is added later, it will be held level with the tops of the rafters to create a 3 1/2"  space between them and the sheathing. Ventilation will occur when the "mini-attic" allows convective air movement between vents in the soffet at the lower end of the shed roof and vents at the upper end located next to the wall between rafters.

The ceiling that will support the rice hulls will be wood (see below).  It will have to be installed a little at a time and insulation blown in.  The question becomes, how do I keep from filling the 3 1/2" mini-attic with insulation instead of holding it back level with the tops of the 2 x 12s?  The best answer so far seems to be stapling some sort of strong fabric, such as fiberglass screening or weed barrier, to the tops of the 2 x 12s before adding the 2 x 4s.  It would not impede air and moisture transfer through the ceiling but would be strong enough to control the rice hulls.  

The disadvantage of using fabric is that the 2 x4s would have to be installed from above while balancing on the rafters rather than from below from a scaffold or ladder.  Or the fabric would have to be installed one rafter at a time as 2 x 4s were fastened.  This choice could be avoided altogether by using sheet goods, such as plywood or OSB board, over the the rafters instead of fabric.  The moisture accumulating at the top of the insulation would be pulled through the sheet goods by the air movement in the mini-attic but not quite as fast as through fabric.  The sheet good approach would have one other perk that interests me.  Air-sealing tape could be used over the junctions between sheets for an easy way to eliminate air-infiltration through the ceiling or, failing that, caulk could be used from below. If I did this in conjunction with our plan for something similar with the exterior walls, the entire envelope would be sealed.

If it weren't for having to maintain the space for the mini-attic, 16" tall I-joists could be installed to give an R-48+.  But, again, how do we maintain the attic space?  If I decide in favor of I-joists, it would have to be with a 12" height with the addition of screening or sheeting on top then 2 x 4s on edge.

Structural screws are available in a
variety of lengths

Fastening the 2 x 4s on edge to provide space for the mini-attic is structurally feasible and DIY-friendly due to the advent of structural screws and impact drivers.  Where long lag screws would have been used in pre-drilled holes in similar situations in the past, self-threading, "star"-driven construction screws are used.  Not only are they faster to use but are also considerably stronger at much smaller diameters than lag screws.  (The International code now even allows construction screw connections between rafters and the top plates in lieu of rafter ties.)

Increasing the Thermal Efficiency of the Roof
We can increase the thermal efficiency of the roof in two ways:  (1) top-to-bottom design that minimizes heat loss in winter then (2), adding foil-backed sheathing that also reduces heat gain in summer.

With rice hull insulation, at R-3+ per inch, flush with the tops of 2 x 12 rafters, the R-value for the roof would be 36+ which is 6 over the recommended minimum for our climate zone. However, buying good quality 2 x 12s in the lengths necessary to span the open areas of the house may be impossible or impossibly expensive.  Our original budget assumed that we would be using salvaged lumber* and I had not comparison-shopped dimension lumber vs. I-joists at the time of this writing.

In the unlikely event that another salvage opportunity comes up or 2 x 12s of sufficient length can be bought at prices that are considerably cheaper than I-joists, I would go ahead and use 2 x 12s. In which case, I am toying with the idea of adding edgewise 2 x 4s to the bottoms of them before attaching the tongue and groove pine ceiling.  The additional 3.5" of depth for the cathedral ceiling would increase the R-factor at least by 10, giving us a total R-factor approaching 50. (Parenthetically, the pine ceiling is both an aesthetic choice and a structural one -- to support to the rice hull insulation which is a mite heavier than fiberglass or cellulose.)

While salvaged 2 x 12s would have been about as green as it gets, engineered I-joists are greener than new dimension lumber because they minimize thermal bridging, are available in heights taller than dimension lumber to accomodate more insulation, are available in very long lengths and, best of all, are made from sustainable plantation trees. Moreover, they are manufactured to exacting standards. But their use requires special knowledge that experience with dimension lumber does not automatically impart -- a challenge I will have to meet if we use them.

While the "mini-attic" approach should solve the moisture problem, it does nothing to prevent the conduction of heat in and out through the rafters, i.e., thermal bridging.
I-joists 12" tall would be the ideal way to hold thermal bridging to a minimum because

Man-made I-joists

their vertical components are so skinny compared to dimension lumber. But, in case the I-joists are too expensive, I have been thinking about ways to avoid thermal bridging through conventional 2 x 12s. One solution would be to sandwich insulating shims between the bottoms of the rafters and the 2 x 4s installed below them. The shims could be cut from extruded polystyrene insulation boards and glued to place temporarily until the 2 x 4s could be fastened with long construction screws. When it comes time to do the final comparison shopping, I would not be surprised if the combination of dimension lumber, insulation and the pricey construction screws might make I-joists a reasonable choice after all.  More on this at the time of construction.

Actually, for our passive solar sun-drenched home, our worry is as much about heat gain through the roof in summer as heat loss in winter.  So I plan to pay a little more for OSB sheathing having a foil backing that will help to deflect the sun's radiant heat before it can raise the temperature in the mini-attic and challenge the R-factor of the rafter/insulation complex further down.

Steel Roof

Our roof will be a highly reflective light colored steel roof which, compared to asphalt shingles that are made from petroleum, lasts longer, has a recyclable end-life and is cheaper upfront.  I will underlay it with 30#felt then, instead of self-adhering bitumen-type material for the eave edges to prevent damage from ice dams, I will substitute unused roll roofing that was a Craigslist find at nominal cost. Unfortunately, our budget dictates exposed fasteners for the metal panels, as shown in the photo, instead of the more desirable concealed fasteners. For a more complete discussion of our choice of steel roofing, check out a prior post on roof cladding.
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*  Originally, the rafters were to have been 2 x 12s 22 feet long salvaged from an old implement shed at about half the cost of new ones.  Until the "rafter fiasco" (discussed in another post on Craigslist shopping), all of my experience with Craigslist has been nothing but positive. However, the outcome that I have to live with is that all of the rafters will probably have to be store-bought.

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Update:  Winter 2021

This only one of several posts have dealt with the evolution and final design for the roof.  To fast-forward, the one we built was supported by roof trusses and double-sheathed so as to create a dedicated 3 1/2" mini-attic space between layers and provide a space above the ceiling that was tightly-packed with insulation that was either 16" or 18" thick (+/- R-60).

Timeline - Craigslist Shopping (Cont'd) - Tools and Equipment

Last Five Years

Trolling Craigslist has taught me that most tools and equipment are best bought under two diverse conditions.  There are those individuals who inherit tools, have only a vague notion as to their worth and want to convert them to cash.  Then there are the knowledgeable equipment owners who are retiring or upgrading, have high-end tools and tend to price them at fair depreciated value.  But they also tend to work with someone who has serous intentions with the tools and values quality.

Portable Tools

Most of the portable tools I still needed for construction and interior finish work fall into the first category mentioned above whereby I sorta made a rule to consider only well-

Fiber-cement nippers, right-angle drill and worm saw

maintained tools and pay no more than half the price of a new tool.  Accordingly, I was able to pick up such things as a reconditioned worm-drive Skilsaw for $80, a half inch Milwaukee right-angle drill for $80 (inheritee) , a 5 hp 20 gal air compressor for $90 (inheritee) and a free pair of wall jacks. And, from another inheritee, 26 various-sized woodworking clamps for $50. Although, not exactly a portable tool, a like-new 10" professional wet saw was posted for $375 by a homeowner who paid over $500 for it at the tile store, used it for one small project and posted it immediately. It is the same saw we had rented from the store on several occasions.

Woodworking Tools

Due to lack of space previously, my meager inventory of woodworking equipment was limited to portable tools and a radial arm saw which was outfitted with a 16 ft table and

kept finely tuned so as to be able to do reasonably accurate work.  But I knew it would not be precise enough for the kind of woodworking our building project would require.  But, thanks to Craigslist, I am now pretty well situated. 

I acquired a Delta Unisaw for $1,300, which is about a third the price of a comparable new one, from a guy in Kentucky who found it too cumbersome for his shop.  In another case, I benefited from another guy's misfortune--after owner-building a McMansion, he divorced and had to downsize (I hope there is no cause and effect between owner-building and

divorce!).   He was willing to part with an 8" Grizzly jointer and a Delta Industrial band saw for the total of $800.  From another avid woodworker who inherited a better model from his Dad, I picked up an "industrial-strength" Belsaw 12" thickness planner for $200 and he threw in a combo belt-disc sander for $25.  A 12 speed drill press for $75 was Craigslisted by an inheritee. A like-new Jet dust removal unit at $220 was not a bargain but certainly priced fairly.

As things worked out, I ended up with a large workshop that put the tools into play quicker than originally planned.  As discussed in the post, Unexpected Hiatus, we moved to the house next door to the building site and converted the large garage into a nice workshop.

Outdoor Equipment

With over two acres to mow (originally, and now over three), suddenly we needed the riding mower that I found on CL in mid-winter for less than half the price of a new one.  It has done its job for five years now with only minor DIY repairs and maintenance.  A rugged, old-school 3 bag electric concrete mixer was had for just $40 which figured heavily in the construction of the solar collector.

Not on Craigslist but online, I bought a low-hour (1,200 hours) Takeuchi TL130 track loader sight unseen from Arkansas and had it delivered to the local Bobcat-Takeuchi dealer for checking and tuning.  It turned out to be an excellent buy and, after investing $4,000 in upgrades and servicing, I expected to use it for

the construction then sell it for a $5,000 profit.  Wrong!  After using it for 40+ hours at my step-son's construction site, the engine blew.  

Coupled with the low hours, the new engine increases the value of the machine when we get ready to sell it but probably not enough to cover its cost.  In the meantime, it is nice to know that we have an extremely reliable piece of equipment. But, in addition to the smooth bucket for it, we needed a tooth bucket and a forklift, both of which were had for a total of $800 from a Craigslist posting and a little haggling. (The Bobcat dealer wants $1,200 for used tooth buckets; forklifts start at around $800.)  However, this is not the end of a the track loader story.  For more, check out an earlier post, Track Loader Adventures.

Timeline - Craigslist Shopping - Materials

Last Five Years

I guess a $34/sq ft building budget helped to make me a Craigslist junkie.  Until actual construction crowded my schedule, I tried to find time each day (early mornings and weekend days are best), to check out two categories --"Materials" and "Tools".  Even then, I have missed more opportunities than I have hit because I contacted the seller nanoseconds too late.  I limit my searches to individuals instead of dealers (or dealers who pose as individuals) because most individuals are not into it for money and are not always privy to the real value of tools and materials.

Masonite for air-tight backing behind salvaged 1x sheathing

Materials Purchased

It is hard now to recall all of the good buys but here are some examples that come to mind.  From several tear downs , I have plenty of free salvaged one-by lumber for sheathing the exterior walls, installed old-school on a 45 degree angle. However, in order to pass the blower-door test for air infiltration, the walls will have to be sealed with some sort of sheet goods under the 1x's.  For $3 per sheet, I was able to find 40 sheets of quarter inch Masonite for this purpose (by being recycled, it should no longer be out-gassing VOCs).

I was able to pick up two tempered glass panels for $40 that will serve as two walls of the shower stall.  A high-end screen door will be perfect for the screened porch ($20).  A metal utility shed that retails for $2,500 still in the box for $500 made for a finger-numbing, assembly project a couple of winters ago.  For $25, I found more than enough mint quality lever-style bright brass interior "door knobs" for the entire house.  To our surprise, we were able to find a vintage cast iron farmhouse kitchen sink with right and left drain boards in mint condition for $150.  A cast iron bar sink was $40. (We are still looking for a similar find on a cast iron bathtub and another kitchen sink to use as a "slop sink" in the laundry.)

Freebie Materials -- Some from Craigslist, Some Not

A neighbor contributed six wooden patio doors with insulated glass that will make a south wall should we ever build a greenhouse.  A nearby business provided three 12 foot trailer loads of short 2 x 4's and 3 x 4 sheets of plywood.  Another business had two trailer loads of recyclable lumber and commercial quality solid core doors which will be useful for tables, benches and replacement doors for some of the family's rental properties.  A 1.6 g/f one-piece toilet was a free Craigslist find.  The first tear-down I did was the result of my "Wanted" posting on Craigslist.  

Win Some, Lose Some
One unfortunate Craigslist mistake was enough to offset, in one fell swoop, many of the other good buys so painstakingly accumulated over several years.  I made the mistake of paying for a large, partially-dismantled implement shed then dilly-dallying before tearing it down.  The purchase was made during the worst of the record-breaking cold weather a couple of winter ago.  The building was on property that was in the path of expanding residential development on which a sale was supposedly pending.  The seller assured me, though, that closure of the sale was not imminent so there would be no reason not to postpone the tear-down until the weather moderated.  Unfortunately, the sale morphed into a foreclosure before I could claim the building and the new owner said "we did not sell you anything and we do not give you permission to trespass."   Not only did I lose $800, I squandered the opportunity to obtain enough very long 2 x 12s to do most of the roof of our house as well as shorter 2 x 12s and various other dimension lumber--all better quality than is available now and at half the price.  Now, of course, all of that good lumber will end up in the landfill.

Don't Hesitate--But..........
I would heartily recommend shopping Craigslist.  Not only is it possible to do well financially, but doing so is a green alternative to buying new.  However, be cautious: If possible go immediately to the seller, pay the cash, get a receipt signed, and be prepared to take the purchase with you.  If claiming the purchase will take time, simply pass up the deal.