R38 Roof Construction Detail

If we decide to use plastic foam insulating materials, a further look at the manufacturers' catalogs reveals the following possibilities:

Extruded polystyrene R-5.6 per inch Isocyanurate foam R-7.2 per inch Phenolic foam R-8.3 per inch

We also discover from ASHRAE Fundamentals that the R-valuc of a gypsum board ceiling and interior air film is only a bit over 1. The R-valucs of the ventilating airspace, sheathing, roofing, and exterior air film cannot be taken into account because the ventilating airspace is assumed to be at outdoor temperature. Therefore, we must find space for R-29 of insulating materials between the airspace and the gypsum board.

Structurally, the roof will consist of wood rafter pairs tied at intervals with horizontal wooden members. The building is 16' wide, so each rafter must span about 8', as measured in horizontal projection. We consult Span Tables for Joists and Rafters to find the necessary size for the rafters, reading from the table that gives values for members that carry a sloping gypsum board ceiling and a 30 psf snow load.

2X6 rafters at a 16" spacing can span more than 9', so they would be more than sufficient for this building. We recognize, however, that 2X6 rafters will probably not provide sufficient space for insulating materials, so we use the table to verify some other structural options to keep open:

2X8 rafters (o 24" spacing 2X10 rafters (a 24" spacing 2X12 rafters (a 24" spacing

From a structural standpoint, rafters deeper than 2X8 could be spaced more than 24" o.c., but we do not want to exceed this spacing for two reasons. One is ease of insulating: Standard insulating batts are made only for 16" and 24" spacings. The other is that the required thicknesses of plywood roof sheathing and gypsum board ceiling panels become excessive at rafter spacings greater than 24".

As an alternative to solid wood rafters, we could use manufactured

wood I-joists as rafters to create the depth we need. These come in standard depths of 9Vi\ 11 14", and 16". The load-and-span tables in the manufacturers' literature tell us that I-joists in any of these depths could serve as rafters for this building at a 24" spacing.

Now we return in our thinking to the thermal insulation problem: What are some insulation options that would achieve an overall rating of R-30 for the roof construction? We list a few possibilities:

2. 6.25" batt + 2" of any foam plastic (R = 30.2 to 35.6)

3. 6.5" batt -f- 1" of isocyanurate or phenolic foam (R = 29.2 to 30.3)

Option the 8.5" batt, has the advantage of simplicity. If we add the required 2" airspace below the roof sheathing, a rafter depth of 10.5" would be required, which is less than the actual depth of a 2 X 12. The disadvantage of this option is that long 2 X 12s are heavy and hard to handle at roof level during construction (Parts That are Easy to Handle). Manufactured wood I-joists 11%" deep might be a good alternative because they arc somewhat lighter, but they require a more elaborate, hard-to-make detail where they rest on the wall frame, so we decide to search for a solid-lumber solution if possible.

Looking at Options 2 through 4, we see that a 6.25" or 6.5" batt plus 2" airspace would require a minimum rafter depth of 8.25" to 8.5". We round this up to the nearest standard lumber depth, 9.25", for nominal 2 X 10 rafters. The foam panels could be nailed across the underside of the rafters, and the gypsum board could be attached with long screws that would pass through the foam and penetrate into the rafters about Ya", the depth recommended by gypsum board manufacturers. For 2" thick foam, a bit of arithmetic shows us that the screws would have to be about 3.25" long to achieve a %" penetration into the rafters. We find from the Gypsum Construction Handbook that the longest standard drywall screw is only 3", so Option 2 is not feasible. Option 3 would work, however, because, with 1" of foam insulation, a standard 2lA" drywall screw would just achieve the necessary penetration. A side benefit of this construction would be that the foam panels would insulate the rafters as well as the spaces between them, acting as a Thermal Break for the more conductive wood.

Option 4 could be created by installing 2 X 4 furring on edge across the

undersides of the rafters, as shown. This shares with Option 3 the advantage that thermal bridging through the wood of the rafters is minimized, and glass fiber insulation is generally cheaper per unit of thermal resistance than plastic foam. But this cost advantage would be negated by the additional expense of the 2 X 4s, and the toenailing of the 2 X 4s to the rafters would be somewhat difficult because of the awkward overhead position [Accessible Connections).

The options that involve using only foam plastic insulation (5 and 6) are problematic because it is difficult to fit the rigid foam panels tightly enough between rafters to eliminate thermal leakage. This problem could be eliminated by spraying polvurethane foam in place rather than using prefoamed panels. This involves another subcontractor, however, and would probably be rather expensive for so small a building.

Thus, we tentatively adopt Option 3, consisting of 6.5" batts between nominal 10" rafters spaced 24" o.c., with a 1" thick layer of isocvanurate or phenolic foam panels attached across the bottom of the rafters. We will proceed with the detail on this basis and see if everything works out satisfactorily.

Option 3

Option 4

BwP sheathing*

rafters

insulation-

Foam panel Gypsum t rafters

BwP sheathing*

Wood soffit

Level cut

frirdsmoitth

Detailing the Exterior Features To create the fascialess eave that we sketched earlier, we will ask the carpenters to make a level cut on the bottom end of the rafters. For the moment, we draw this with a full 18" overhang on the rafter itself; later we may adjust this dimension if the finished overhang dimension is too large or too small.

Next, we add roof sheathing panels. Plywood or oriented strand board OSB) panels for 24" rafter spacing can be as thin as 7/16", but experience with other buildings has shown us that a thickness produces a roof plane that is less prone to show sagging between rafters.

The lower edges of the sheathing panels will need to be supported at the eave to prevent an unattractive wavi-ness from showing along the edge of the roof. The soffit can provide this support if it is stiff enough (Small Structures). It could be made from plywood and/or nominal 1" boards. (Again, experience comes into play here. Engineering analysis might show that a thinner plywood soffit would be strong enough and stiff enough, but hands-on experience and field observation tell us that thinner plywood will be too flimsy.) We could make the soffit from a single strip of plywood, but we would have to cut into it for ventilation openings, and the wide pieces of plywood would be heavy and hard to lit accu

Soffit supports lower edqe of roof she4thing-^

45° bevel

Wood soffit rately into place (Parts That are Easy to Handle). Furthermore, the 45° (12/12) bevel on the outer edge would be hard to cut in a perfectly straight line with a hand circular saw, and its knife edge would be fragile because of the layered construction of the plywood (Clean

Edge). If we adopted a square edge rather than a 45° edge, the exposed edges of the laminations in the plywood would not be vcrv attractive, especially because there tend to be voids in the interior lavers. O

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