Points To Watch When Insulating Wood-Framed Flat Roofs

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Points To Watch When Insulating Wood-Framed Flat Roofs

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NATIONAL RESEARCH COUNCIL

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DIVISION OF BUILDING RESEARCH

HOUSING NOTE

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1963, P. 66

B U I L D I N G RESEARCH

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Ottawa, May 1963

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Points to watch

when insullatiing

wood-framed flat roofs

There are two ways to imulate a flat or a flat-pitched roof.

The insulation can be installed be- tween the roof joists, usually with the space above the insulation vented to the exterior, or it can be installed on top of the roof deck immediately under the roofing, in which case no roof ventilation is provided.

To provide guidance about which method is preferable the following discussion has been prepared outlin- ing the advantages and disadvantages of each and summarizing recommend- ed practices to be followed when employing either system.

Between the roof joists

Insulation installed between roof joists has some important advantages. The first of these is cost. Batt-type mineral wool insulation is usually used in this appli- cation and is relatively very cheap and easy to install. Because of its relative cheapness it can be economically in- stalled in thicknesses of 2 ins., 3 in., or 4 ins., or even greater, to provide a high

degree of insulation which not only re- sults in greater fuel economy during the heating season, but also insulates the house from the heat of the sun in summer.

With this type of installation, it is important to have adequate vapour barrier protection on the room side of the insulation. In addition, the space between the insulation and the roof deck must be adequately vented to the ex- terior so that any water vapour that enters the roof space can escape to the exterior.

One of the most difficult problems with this type of installation is that of preventing water vapour from the living quarters entering the roof space and con- densing on the underside of the roof deck. After an extended period of cold weather a considerable amount of mois- ture may accumulate in the form of frost and may be suddenly released as

By A. T. HANSEN Housing Section,

Division of Building Research, N.R.C.

water as the weather moderates.

Where the moisture comes from

Part of the moisture which escapes into the roof space is the result of actual water vapour diffusion through the ceiling and vapour barrier. Where a reasonably good vapour barrier is in- stalled in the ceiling, the vapour trans- ferred into the roof space in this manner is usually relatively insignificant. Water vapour is also transferred into the joist spaces by currents of warm air from the living quarters through any openings into the roof. It is believed that this is the major source of moisture and is re- sponsible for most of the complaints re- ceived regarding condensation in roof spaces in residences.

Openings into roof spaces occur most commonly around electric fixtures in the ceiling, around plumbing stacks or vent pipes or any place where air movement from the living space into the roof space is relatively unobstructed (Figure 1). The possibility should also be considered of air leakage paths through the wall spaces into the roof space. The room air is constantly being forced into the roof space by pressure similar to that which causes air to rise in a chimney. In addi- tion, wind action may lower the pres- sure in the roof space below the pre- vailing pressure in the living areas which will further increase the flow of humid air into the roof space. It is therefore extremely important to seal up any open- ings between the living area and the roof space. This cannot be over-empha- sized.

In many cases, plumbing vents or stacks are installed in such a way as to permit a relatively unobstructed flow of air from the basement into the roof space. Often the vapour barrier in the ceiling is at best, loosely fitted around the stacks and does not adequately re- strict the flow of moist air into the roof space. The resulting air flow can be reduced by tightly packing mineral wool insulation around the stacks or vents where they pierce the wall plates. This should be done at the top and bottom plates. In addition to providing protec- tion against condensation, this practice provides fire stopping between floor lev- els which is also highly desirable.

A further source of air leakage into the roof space occurs at ceiling fixtures where the vapour barrier must be cut to permit fixture installation. Ideally, it would be desirable to have a light fix- ture which, when installed, would form an airtight seal between the living area and the roof space above it. Because of the potential difficulties with conven- tionally installed ceiling fixtures they should be avoided whenever possible. If ceiling fixtures arc used, emphasis must be placed on obtaining a good air seal

E 5 S E N T l A L TO USE PROPE4 as well as a vapour seal, and the barrier

should be cut to fit tightly around the junction box.

What is really adequate for roof ventilation?

Necessity for Adequate Ventilation

It is normal practice in roof ventila- tion to provide vents to the exterior equal in area to 1/300 of the floor area of the building. Although probably ade- quate to dispose of the relatively small amounts of moisture which migrate through the vapour barrier by diffusion, this amount of ventilation may not al- ways be adequate for taking care of the concentrations of moisture transferred into the roof space by actual air leakage from the living space. The higher the prevailing relative humidity in the living space, the greater will be the tendency for this condensation accumulation.

In addition, since venting the roof space to the exterior may increase the difference in air pressure between the living area and roof space, thereby in- creasing room air leakage into the roof space, it is conceivable in some cases that very poor or inadequate roof space venting may be worse than providing no ventilation. It is therefore important when insulation is to be installed be- tween roof joists that as efficient venti- lation as possible is provided above the insulation.

Ventilation of flat or flat-pitched roofs is usually not as efficient as the ventilation that can be achieved with gable or hip roofs. Because of the orientation of the roof joists, the roof receives ventilation from two sides only, and because the vents are usually all at the same elevation, the rate of ventila- tion is dependent on wind action. With conventionally vented gable or hip roofs, the vents are distributed between the eaves and the ridge. This permits some circulation of air due to convection cur- rents in addition to the ventilation due to wind pressure.

In many flat-pitched roofs (cathedral ceilings) the joist spaces will be separ- ated from each other and there will usually be a ridge beam at the peak that prevents cross ventilation, further lowering the vent efficiency. This ob- struction to ventilation can be overcome by drilling large holes in the ridge board to connect opposite spaces, or by installing a ridge beam 2 ins. to 4 ins. shallower in depth than the roof joists, leaving a gap between the ridge board and the peak (Figure 2).

A preferred method for all flat or flat-pitched roofs would be to apply 2- by 2-inch furring at right angles on top of the joists for the support of the roof sheathing (Figure 3). This would inter- connect all the roof joist spaces and ventilation from four sides could be achieved. With flat-pitched roofs these

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furring strips would have to bc offset from the ridge board to permit air move- ment across the peak. This natural ven- tilation can be assisted further by the installation of vent stacks, mountcd on the roof and opening into the roof space. These stacks must be suitably designed to prevent entry of rain or snow and screened to keep out insects.

Above t h e roof deck

Insulation used above the roof deck is usually the rigid- or board-type and is somewhat more expensive than the conventional mineral wool batt or blan- ket insulations.

Board-type insulations are more com- monly made from mineral wool, fibre- board, cork, and, more recently, ex-

panded polystyrene. This type of in- stallation is of particular advantage where it is desired to use the roof decking as the finished ceiling as is sometimes done with plank and beam roofs for rustic effect.

Insulation applied to the roof deck has a major advantage in that it elimin- ates the transfer of moist air into the roof space which is believed to be the cause of most moisture problems in roofs. Some of the rigid insulations are either vapour barriers in themselves (such as expanded or foamed polysty- rene) or incorporate a vapour barrier in their construction. Those which do not must, of course, be applied over a va- pour barrier. This permits a more com- plete vapour protection for the insu- lation than is usually provided if the batt or blanket insulation is installed between the joists and the vapour barrier is pierced by electrical fixtures or plumb-

ing (Figure 4).

If the insulation is the type that re- quires a separately applied vapour bar- rier, the insulation on the deck is in effect sandwiched between two vapour barriers since the roof covering will also act as a vapour barrier.

In such cases any water that does enter the insulation due to roof leakage is trapped and subsequent drying will be very slow. This is not a problem with insulations such as expanded or foamed polystyrene which are not capable of asborbing much water. Even with the other types of insulation the problem of moisture collection should not be sig- nificant, provided a suitable vapour bar- rier is installed on the warm side of the insulation and the roofing is carefully applied to eliminate the possibility of roof leakage into the insulation.

It should be noted that roofing applied directly over insulations is usually lim-

ited to built-up roofing since the nail- holding power of most insulations is not sufficient to permit the application of shingles.

Summary

As this review suggests, insulations applied either between the joists or above the deck can provide satisfactory per- formance if suitable practices are fol- lowed.

,Because of the elimination of air leak- age into insulation applied over the decking, this type of installation seems potentially freer from condensation prob- lems.

When insulations are installed between the joists, special care must be taken in installation of ceiling fixtures or plumb- ing vents so that the vapour barrier is damaged as little as possible and ade- quate ventilation to the exterior must be provided above the insulation.