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Attics & Roofs
For
Northern Residential Construction
For
Northern Residential Construction
by
Extension Energy and Housing Specialist
In this publication:
2.1 COLD ROOFS
3 AIR-VAPOR BARRIER DETAILING FOR CEILING ANDROOF
4.1 CEILING SAGGING
5.1 RAISED HEEL TRUSSES
8 INTERIOR PARTITIONS AND CEILINGS
9.1 PLUMBING
Ceilings are usually the best-insulatedpart of conventional houses. Attic spaces are easy to insulate withlow-cost blown or batt insulations. The insulation levels in theattics of energy-efficient houses usually reach R-values of 48 to 60(12 to 18 inches of blown or batt insulation). We emphasize that theinsulation quantity alone does not determine its effectiveness.
The energy efficiency of roofs and attics can usually be improvedby
- installing a wind barrier at the eaves to prevent wind penetration into the roof cavity.
- air sealing to eliminate or reduce air movement through the ceiling around plumbing stacks, electrical outlets, at partition-wall top plates, chimneys, flues, and around attic access hatches.
- increasing insulation thickness in cathedral ceilings.
- eliminating gaps in the insulation, particularly at truss struts.
- increasing insulation thickness at the eaves.
- reducing thermal bridging through structural members.
1 MOISTURE FORMATION
Moisture formation can occur in ceilings for two reasons. First,and most important, moisture accumulates when warm moist air risesinto the attic space through air leakage paths and condenses on coldsurfaces (see figure 1). Second, vapor diffusion will cause watervapor to move into the attic. During the winter in colder regionsfrost may build up in the insulation, which leads to problems in thespring when the frost melts. Moisture problems can be greater in awell-insulated attic because the air immediately above the insulationis much colder than in poorly insulated attics. The cooler air willnot absorb much moisture.
Figure 1 Air and Heat Leakage in Conventional Attics
The only effective way to reduce condensation in roof and atticspaces is to prevent it from entering in the first place. This isdone by installing a continuous air and vapor barrier. The colder theclimate, the more important the quality of the installation of theair-vapor barrier.
Ventilation of the attic and roof space is generally required. Ithelps exhaust any moisture or water vapor in those spaces and aidssummer cooling. This is usually accomplished by having a continuoussoffit ventilation strip as well as ridge ventilation. The commoncommercially available ridge-cap vent can suffice, but this can beblocked by snow in winter. The practical roof ventilation solution isthe use of gable-end vents in roof systems where this is feasible,rather than ridge-cap vents.
In colder regions of Alaska, where fine, blowing snow is aproblem, it may be better to seal the attic. This means "hot roof"design may be necessary, that is, a roof without ventilation. Ifmoisture from inside the house is prevented from getting into theroof cavity, and there is sufficient insulation to keep snow on theroof surface from melting, there is no reason for roof ventilation.
The amount of snowfall and the wind conditions of a particularlocation should be primary factors to consider when deciding whetheror not to build a hot roof. The builder should be aware that theinsulating value of snow is approximately R-1 per inch Where thesnowfall accumulation on the roof can exceed 8 inches, the surfacetemperature of the roof might result in melting the snow andsubsequent ice damming.
In western and northern coastal Alaska, wind is a dominant factorand may regularly clear the roofs of snow. Local wind conditionsshould be considered when deciding if a hot roof is appropriate for agiven location. Figures 2 and 3 show the effect of differentinsulation thicknesses in hot roof designs, and the subsequentoutside temperatures at which melting will occur at the roof deck forvarious depths of snow to 48 inches.
The design question of when and where to use a hot or cold roofdesign has always been a subject of consternation, controversy, anduncertainty in Alaska. For these reasons, a diversion into morediscussion of these designs is warranted here. First the definitionof hot and cold roofs needs clarification.
Figure 2 Deck Temperature, Warm Roof, 12 inches FiberglassInsulation
Figure 3 Deck Temperature, Warm Roof, 18 inches FiberglassInsulation
2 HOT VERSUS COLD ROOFS
There are two basic types of roofs used in Alaska. They arecommonly called a cold roof and a hot roof.
2.1 COLD ROOFS FOR ALASKAN HOMES
Cold roofs have a ventilated cavity (attic) above the insulation. It generally consists of lumber rafters or trusses with a 4/12 to 6/12 slope and a flat ceiling. The roof is commonly insulated with 6 to 12 inches of fiberglass batts or blown-in fiberglass or cellulose. A 6-mil polyethylene air-vapor barrier is installed under the insulation. For the roof to be classified as cold, continuous 2-inch vent slots must be installed at the eaves for natural eave-to-eave ventilation. Also louvres are installed at the gables. In some situations it may be acceptable to use ridge vents.A hot roof may be a flat, cathedral, or shed roof with no natural ventilation in the roof cavity. In a commercial roof, rigid insulation may be placed on the top of the decking. In a residence, the rafters may be packed with insulation, leaving no air space for natural ventilation of the cavity. Even a gable roof maybe classified as a "hot" roof if it doesn't have adequate ventilation. (Axel Carlson, Roof Ice May Create Safety Hazards, March l988.)
The important lesson and distinction between these two roof typesis in their application.
In new construction a properly designed and constructed cold roofwith a well ventilated roof cavity should have no ice dams or iciclesalong the eaves (see figure 4). The cold roof design is the bestoption for the railbelt, southcentral, interior, and southeast Alaskaclimatic zones.
Figure 4 A Functional Cold Roof Design
A hot roof causes accumulated snow to melt gradually. The waterwill then flow toward the eaves on a sloping roof. When the waterreaches the eaves it will refreeze and build up as an ice dam. As theice dam increases in height, the surface of the water may becomedeeper and broader (see figure 5).
Figure 5 Ice at Eaves of a Hot Cathedral Roof
(ACHP Building Manual, 1988)
Water formed behind ice dams may leak under asphalt shingles orover the flashing, accumulate, and freeze in the roof cavity. Alsothe water may leak through any seam in the roofing. Obviously this isless desirable design for the regions where a cold roof design isappropriate.
It is important to note that architectural detailing and designwhich employ multiple levels and roofs can lead to roof sectionswhich are not easily vented. This can be avoided by using verticalvent ducts to other vented roof sections from the top of unventedsections to assure full natural ventilation. All reasonableefforts should be made to avoid hot roof conditions in all new ACHPresidential construction, and to eliminate it whenever possible inretrofit construction.
If this is not done possible consequences are:
- Over a period of five to ten years excess moisture in an improperly ventilated roof cavity may cause delamination of plywood deck. Over a longer period, it may even cause rotting and structural deterioration of the rafters. During a period of abnormally deep snow fall, the roof could collapse.
- Water leaking into the roof cavity may seep through the insulation and holes in the vapor barrier and cause water stains on the ceiling.
- Water leaking into the roof cavity may become trapped as a result of inadequate roof ventilation. With higher levels of insulation, the dew point temperature at the deck will become lower and moisture in the roof cavity may form heavy masses of ice on the bottom of the plywood deck. At times it has been observed that the rafters of a gable roof were completely filled with frost and ice.
- In the spring or during unseasonably warm weather the large accumulation of ice will suddenly melt. The water will seep through the insulation and accumulate on the air-vapor barrier. This can ultimately cause the ceiling to collapse with a flood of water.
2.2 APPROPRIATE USE OF HOT ROOF DESIGNS
Faro hotel roomsIn western and northern coastal Alaska and the Aleutians a hotroof design may be the best alternative. However, it must be avery tight hot roof design to be effective and must be used with thefollowing cautions and constraints:
- Blowing snow must not penetrate the hot roof for the same reason it shouldn't get into a cold roof. The roof must be tightly constructed.
- The design strategy relies on wind to clear the roof of accumulated snow. This strategy requires an average annual wind speed above 10 mph to be effective.
- In the event that a large snow accumulation does occur, the snow must be removed.
3 AIR-VAPOR BARRIER DETAILING FOR CEILINGAND ROOF
The air-vapor barrier must be continuous. Basic considerationswhich must be addressed include:
- Carrying the air-vapor barrier over the top plates of partition walls.
- Sealing around chimney and flue penetrations.
- Providing flexible air- and vapor-tight seals around plumbing penetrations.
- Placing attic access hatches on a gable end of the roof as an option.
- Minimizing electrical boxes in the ceiling. When using a polyethylene air-vapor barrier, cross-strapping will provide a cavity for wiring and electrical boxes on the inside of the air-vapor barrier. Always be certain that seams in the polyethylene ceiling air-vapor barrier are supported with solid backing. No seams or joints in the polyethylene air-vapor barrier should ever be made without this backing support.
- Whenever possible, avoid using recessed light fixtures.
4 MOISTURE DAMAGE
Sagging of ceiling finishes and truss uplift can occur in bothconventional and energy-efficient houses.
Ceiling sag is caused when the drywall on the ceiling has anelevated moisture content and, as a very viscous fluid would , itsags. The rate of sag is a function of the moisture content of thedrywall and the weight the drywall is supporting. Consequently, theamount of sag is time dependent, but it is also related to thespacing of supports and inversely to the thickness of thedrywall.
In order to keep sag to a minimum, when the drywall is installedit should be completely dry and the following precautions taken tokeep it dry:
- Keep the drywall warm: provide heat and insulation.
- Keep the relative humidity low.
- Don't install heavy insulation until drywall is dry.
- Keep the spacing of supports close together.
- Use thicker drywall: 5/8 inch minimum; 3/4 inch desirable.
- Avoid the use of water-base texture finishes.
- Provide ventilation during construction. In the winter months, this can be done with a large heat recovery ventilator to minimize heating costs.
- If a polyethylene air-vapor barrier is used, the attic must be insulated before the heat is turned on. Otherwise, the water vapor will condense on the polyethylene.
In some houses with roof trusses, an upward movement of theceiling occurs during the winter months resulting in damage tointerior finishes, particularly to interior walls. This can resultfrom:
- Different moisture contents in the lower and upper chords of the truss.
- Some pieces of lumber expand and shrink much more than the average. If the upper chord of a truss is made of a piece of lumber that has high expansion and contraction characteristics (reaction wood), truss uplift can occur every winter.
- The drying out of the lower chord. In this case, the truss uplift will usually occur only once.
Proper grading of lumber and proper drying of wood can eliminatetruss uplift, but the following steps can be taken by the builder tominimize the effects of truss uplift:
- Provide adequate ventilation to the attic area.
- Locate attic vents to ensure good air flow.
- Ensure that the soffit vents are not blocked by insulation.
- When applying drywall, connect the ceiling drywall to the partition-wall top plates with drywall clips. Fasten the drywall to the ceiling at a distance far enough away from the partition so that, in the event the truss rises, the drywall can absorb the deflection without cracking (see figure 6).
- Buy trusses that are dry.
- Keep the trusses dry.
Figure 6 Precaution against Truss Uplift
4.3 FLOOR TRUSSES AND PERMAFROST
In areas with permafrost where the house is built on pilings,opposite effects can occur when parallel-chord floor trusses are usedfor the floor and the roof: the floor may bow downward while the roofbows upward.
5 ROOF TRUSSES
Most builders use prefabricated roof trusses. One of the majorweaknesses of conventional roof trusses is that space for insulationis minimized at the eaves.
The use of raised heel trusses (see figure 7) will solve thisproblem. While costs associated with a raised heel truss(manufacturing cost, extra siding, extra insulation,) may total from$100 to $300 per house, energy savings and the prevention of surfacecondensation can justify the investment.
Figure 7 Raised Heel Truss
Advantages
- Allows full insulation depth in all areas of the ceiling.
- Provides a clear span and allows for a continuous air-vapor barrier.
Disadvantages
- Slightly higher cost than conventional truss, but often competitive.
- More siding will be required because of larger soffit.
This truss consists of a conventional truss onto which is hung asecond lower chord (see figure 8).
Figure 8 Drop Chord Truss
Advantages
- Can help reduce truss uplift, although it does not eliminate concern about the reaction of the wood.
- Provides full depth insulation up to the perimeter walls.
- Provides a clear span ceiling and the installation of a continuous air-vapor barrier.
Disadvantages
- Requires taller studs.
- Extra siding is required.
- Requires blocking at the ceiling and wall junction for air-vapor barrier attachment.
The lower chords of a scissor truss are sloped rather thanhorizontal. This allows the builder to construct a house withcathedral ceilings without the need for a central load-bearing beamor wall. Scissor trusses can be modified to accommodate higherinsulation levels by constructing them with a raised heel (see figure9).
Figure 9 Scissor Truss
Advantages
- More easily insulated to high levels than other types of cathedral ceiling.
- In many cases, can be insulated with blown insulation.
Disadvantages
- Costs may be higher than other methods of construction.
- Difficult to insulate between chords, unless using blown insulation.
Parallel chord trusses consist of parallel chords of wood whichare joined by an open web of wood, or steel braces, or a solid web ofplywood (see figure 10). This type of truss permits high levels ofinsulation in cathedral ceilings.
Figure 10 Parallel Chord Truss
Advantages
- Allows for large amounts of insulation in cathedral ceilings and also can provide ventilation without purlins.
- Can provide large, clear spans and allow for application of a continuous air-vapor barrier.
Disadvantages
- Higher cost than dimension lumber.
- With a web of steel braces, heat losses due to thermal bridging can be high.
- Difficult to insulate between chords, but blown insulation may help with this problem.
6 ATTIC SPACE FRAMING
A method of framing an attic space using standard dimension lumberis illustrated in figure 11. Note that the ceiling joists must extendbeyond the wall and to the rafter at the eaves to produce atriangulated structure. However, this may not be required in allcases.
Figure 11 Stick -Framed Raised Heel
The recommended Canadian rafter roof design is described in thefollowing quotation and shown in figure 12. From Harold Orr, Canadian Wood-Framed House Construction (CMHC Pub NHA 5031M, May1985):
In a rafter-framed attic with a slope less than 4/12, the roof loads are carried by beam action and not by truss actions. The roof rafters and the ceiling joists must be sized to carry the vertical loads imposed by snow and other loads. With this system the loads from the rafters are carried to interior partitions by braces at angles greater than 45°, dwarf walls, and ceiling joists. "These methods of support reduce the outward thrust of the roof, continuous ties between the lower ends of opposing rafters are not necessary."
Figure 12 Canadian Rafter Roof Design
From Canadian Wood Framed House Construction (CMHS Pub NHA5031M, May 1985)
Advantages
- Allows higher insulation levels at the edge of the ceiling.
Disadvantages
- In most cases, more expensive to construct than trusses.
7 CATHEDRAL CEILING FRAMING
A method of framing a cathedral ceiling is illustrated in figure13. Ceiling joists of 2x12 are used as rafters but if the two layersof R-11 batt insulation are desired, the entire rafter space will befilled with insulation. To allow for the proper ventilation spaceabove the insulation, 2x2s are first nailed to the top of each rafterand parallel to them, to give at least 11/2 inches of air space abovethe insulation. Then a second layer of 2x2 (2x3s or 1x4s can also beused) are nailed to the rafters and perpendicular to them to allowfor attachment of sheathing and roofing. This alternative assures acold, ventilated roof with adequate ventilation parallel to eachrafter space when tied to appropriate eaves and ridge ventilation,and still allows a cathedral ceiling design.
Figure 13 Stick -Framed Cathedral Ceiling
Advantages
- In some areas, lower cost than parallel chord trusses.
Disadvantages
- Limited to a maximum of R-40
- Reduced insulation values at the ceiling joists.
8 INTERIOR PARTITIONS AND CEILINGS
Interior partition construction can have a large effect on theenergy efficiency of a ceiling. Because much air movement can occurat the junction of the ceiling and partition wall. This problem canbe corrected by incorporating a good air barrier. The two primarymethods of making the air barrier continuous over partitionsare:
- The application of polyethylene and drywall across an entire clear span ceiling before erecting interior partitions (see figure 14).
Figure 14 Interior Partition Walls: Framing after Drywall
- Standard length studs can be used for interior partition walls if a 1x4 shim plate is placed on top of the exterior wall top plate. This raises the trusses and allows enough room for 5/8 inch thick drywall and for tilting the partitions in place (see figure 15). Alternatively, studs will need to be undercut by 5/8 inch. Drywall can be taped before the wall is raised.
Figure 15 Interior Partition Walls: Drywall Air Barrier
- Placement of a 24-inch wide strip of 6-mil polyethylene between partition-wall top plates. The polyethylene strip is later sealed to the ceiling air barrier in each room (see figure 16).
Figure 16 Interior Partition Bearing Wall
In the case of the drywall air barrier, the interior partitionwalls can be cut 3/4 inch short and temporarily held in place.
After the drywall is slipped through the gap, the wall isanchored.
9 CEILING PENETRATIONS
9.1 PLUMBING
Place as much plumbing as possible in interior partition wallsrather than exterior walls. By minimizing plumbing in exterior walls,it is easier to produce an airtight assembly, and allow moreinsulation. To prevent air leakage where plumbing stacks penetratethe ceiling at partition wall top plates, an air- and vapor-tightflexible seal must be provided. The reason for requiring flexibilityin the sealing of plumbing penetrations is that plumbing stacks movebecause of thermal expansion and contraction as well as shrinkage andsettlement of the house frame. Plumbing expansion joints willminimize thermal expansion and contraction.
One of the most effective ways to handle this is to pass theplumbing stack through a rubber gasket such as EPDM, which is sealedto the air-vapor barrier with acoustical sealant or gasketing andclamped down with a plywood collar (see figure 17). The hole in therubber is cut 11/2 inches smaller than the plumbing stack diameterand forms a tight friction fit when the plumbing stack is forcedthrough.
Figure 17 Plumbing Stack
An equally effective option involves a permanent rigid seal of thestack to the air-vapor barrier and a more flexible plumbing system.This may involve an expansion joint in the stack or several bendsclose to the vent in the attic.
Wiring penetrations of the ceiling often occur throughpartition-wall top plates and can be sealed directly to the airbarrier and top plate with acoustical sealant. Some caulkingmaterials may react with the wiring sheathing and for this reasonthis method may not be acceptable by electrical inspectors, in whichcase a rubber gasket, as described for plumbing stacks, could beused.
Electrical boxes for lighting fixtures can be sealed in one of thefollowing ways:
- Placing the electrical box in a prefabricated polyethylene envelope (poly hat) (see figure 18).
Figure 18 Ceiling Electrical Box
- Placing the electrical box in a site-built wood box wrapped in 6- mil polyethylene.
- Placing the electrical box within a strapped cavity.
- Using a pancake surface-mounted electrical box (see figure 19).
- Do not use recessed lights.
Figure 19 Pancake Surface Mount Electrical Box
The most effective method for constructing an attic hatch accessin an energy-efficient house is to locate the attic access in a gableend. If the access must pass through the ceiling air barrier (as in ahip roof), the hatch frame must be sealed to the ceiling air barrierand the access door weather-stripped, insulated, andlatched.
REFERENCES
Carlson, A.R., Heat Loss and Condensation in Northern ResidentialConstruction. The Northern Engineer. Vol. 2, No. 2, Instituteof Arctic and Environmental Engineering, University of Alaska, Summer1972.
Dickins, H.B., et al., Moisture Accumulation in Roof SpacesUnder Extreme Winter Conditions. National Research Council,Division of Building Research, Technical Paper 228 (NRC 9132),Ottawa, Canada, 1966.
Finne, Eirik. (Det Kalde Taks Funksjon) The Function of a ColdRoof. Norwegian Building Research Institute, Publication 77,Oslo, Norway, 1963.
Graee, Trygve. (Luftlekkaisier Og Snosmelting Pa Tak) Air Leaksand Snow Melting on Roofs. Institute of Agricultural Structures,Agricultural College of Norway. Reprint 132; As, Norway.
Sturman, Gary G. A Discussion of the "Upside Down" Roof System.Term Paper for C.E. 603, Arctic Engineering I. University ofAlaska, December 14, 1970.
Tamura, G. T., et al. Air Leakage and Pressure Measurements ofTwo Occupied Houses. National Research Council Research Paper 207(NRC 9648). Ottawa, Canada, 1963.
Alaska Craftsman Home Building Manual. University of AlaskaFairbanks, Cooperative Extension Service and Alaska Craftsman HomeProgram. 1988.
Canadian Wood-Framed House Construction. CMHS PublicationNHA 5031 M, May 1985.
Measures to Control Condensation. Canadian Central Mortgageand Housing Corporation, Builders' Bulletin No. 220, Ottawa, Canada,1972.
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