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Canadian Building Digest

CBD-16. Thermal insulation in dwellings

Originally published April 1961

W. H. Ball

Thermal insulation is installed in a building for several reasons. All of these relate to the primary characteristics of a thermal insulating material; it provides relatively good resistance to the flow of heat. The reduction of heat loss in winter provides savings in heating costs and the reduction of heat gain in summer will reduce the cost of cooling where summer air-conditioning is provided. Insulation in walls, ceilings and sometimes floors is also desirable in all buildings intended for human use because comfort conditions are more easily achieved. For buildings in which moderate to high relative humidities are to be maintained in winter, insulation is necessary to prevent surface condensation on walls, ceilings and floors.

With few exceptions the basic constructions employed in dwellings do not of themselves provide sufficient insulating value. It is necessary or desirable in most cases to add a layer of insulation to improve the over-all resistance to heat flow. Ideally it would be best if this insulating layer could be applied to the building in a manner similar to that of clothing on a person. In this way the insulation would be continuous over the building and its structure would be protected from the extremes of temperature both winter and summer.

In wood-frame dwelling construction, it has been the custom to place insulation in spaces created between the framing members. With solid masonry walls it is common practice to create spaces for insulation with wood furring strips where batt or blanket insulation is to be used. Board types of insulation such as cork and foamed polystyrene can be pinned or glued to masonry and can also act as a base for plaster thus making it possible to achieve an uninterrupted insulating layer. Insulation may also be placed in cavities in certain types of masonry walls.

Lack of care in the design and installation of insulation arrangements can create performance problems that are a nuisance to building occupants and can result in the loss through increased maintenance costs of much of the savings achieved by an over-all reduction in heat loss. Problems arise initially because of lack of continuity in the insulating laver and the difficulties can be multiplied by careless installation and by lack of care in the selection of the insulating material. It is the purpose of this Digest to discuss a number of details relating to the use and installation of insulation that are important for good performance. These will apply primarily to dwelling construction but may have application also to other types of buildings, particularly those made of wood or of a combination of wood and masonry.

While the primary objective in using insulation is to reduce the over-all rate of heat flow, it is also important that the over-all resistance obtained should be uniformly good. Unless this is achieved, the variations in heat flow characteristics may lead to proportionately high variations in surface temperatures. Such variations cause dust patterns on walls and ceilings. The lath marks noticeable on the plastered finish of old uninsulated houses result from this. Extreme variations in surface temperatures may lead also to wetting by condensation or to the formation of frost in winter. It is scant reward for a building owner to know that he has paid for insulation to reduce his heating costs when he may be forced to redecorate walls and ceilings at frequent intervals, perhaps as frequently as once a year, to maintain an attractive interior for his premises.

Some of the variations in surface temperature may arise from the particular construction used and may be difficult to avoid. Others, however, may be related to the way in which the insulation is installed and may be avoided with a little care during construction. Faulty installation can also at times lead to a serious reduction of the over-all effectiveness of the insulation in reducing heat flow.

Insulation should fit snugly against framing members over its full thickness. If this is not achieved because of reduced edge thickness or inadequate width of the insulation, as shown in Fig. 1, the rate of heat loss and thus the surface temperature variations at framing members will be increased.

Figure 1
Figure 1.   Faulty fit of insulation at framing members in walls or ceilings

The question of the best location for batt or blanket insulations which do not completely fill wall spaces is often raised. The "warm side" location of Fig. 1 is generally used because the vapour barrier normally provided with the batt or blanket can be lapped over the interior faces of the framing members. But the alternative location, with the batt or blanket placed toward the cold side produces more uniform surface temperature patterns, and is less subject to the influence of the faults shown in Fig. 1. The framing members are then mainly on the warm side of the insulation and have less effect on the warm side surface temperature. With this installation it is essential that a separate vapour barrier be applied over the interior face of the framing members since the vapour barrier provided with the insulation cannot readily be sealed against the sides of the framing members.

Batt, blanket, or membrane-type insulations located midway in a space can create performance problems if not carefully installed. It is essential in developing the full potential of such arrangements that they be sufficiently well sealed to the framing members at the sides as well as at top and bottom so that air cannot circulate from one air space to another. Gaps at one level only need not be serious but gaps of from 1/8 to 3/8 inches in width at top and bottom of wall spaces can allow almost the full normal air circulation which would occur if the insulation were not present, as illustrated in Fig. 2. A similar situation can arise in the case of ceiling insulation in winter when air tends to rise from below the insulation, or in floor insulation in summer when the cooler air above the insulation tends to fall, thus transporting heat around the insulation and reducing its effectiveness.

Figure 2
Figure 2.  Effects of gaps in blankets installed midway in stud spaces

Fill Insulation

Fill insulation is seldom used in the walls of new wood-frame structures because there is difficulty in controlling its density, and, unless this can be done, there is always the possibility of settlement of the insulation. Settlement of insulation in wall spaces would leave the upper portions of walls with no protection against high heat loss. While the increased heating cost due to small settlements might not be significant, the variation in interior surface temperatures could be troublesome.

Fill insulation can be applied readily over most ceilings between joists. Some materials are specially prepared or are naturally suited for simply pouring them from the shipping containers. Other materials require that they be "fluffed" or expanded, either mechanically or pneumatically, and are often placed by blowing into the attic. Whichever method of placing the insulation is used it should always be distributed over the ceiling to ensure that a uniform layer of insulation of adequate density is provided. This may require that the surface of the insulation be levelled either by hand or with a special rake designed for the purpose. Unless the insulation is uniformly applied dust marking and surface condensation can result. Serious settlement of ceiling insulation may also reduce its over-all effectiveness through reduction in thickness, and the density in place should be sufficient to ensure maintenance of the desired minimum thickness. The increased density brought about by settlement will usually improve the insulating value per inch of thickness, but this will never be great enough to offset the effect of decrease in thickness.

The recent trend toward low-slope roofs has increased the difficulty of making a good installation of fill insulation in many attics. The insulation must be literally "shot" onto some portions of the ceiling and it is most difficult to achieve a proper distribution even with the use of long-handled rakes. In addition it is difficult in such cases to make a good application at the junction of the ceiling and outside walls, so often the insulation is reduced in thickness near and over the top of the wall.

The Insulation of Flat Roofs

Insulation for flat roofs may be placed either between joists as illustrated in Fig. 3 (a), or on top of the roof deck as in Fig. 3(b). Variations of both methods are also used. That of Fig. 3(a) may, for example, have batt insulation placed between furring strips running at right angles, below the joists.

Figure 3
Figure 3.  Roof insulation arrangements

The methods of installation similar to that of 3(a) are sometimes criticized because it is often difficult to provide adequate ventilation above the insulation in large roof areas, and in locations where buildings are erected side by side. Condensation problems have been reported in constructions of this type even where an attempt has been made to control it by the use of a vapour barrier and ventilation. As a result there has been a tendency to favour the type of installation illustrated by Fig. 3 (b).

Installation of insulation above the roof sheathing has the advantage that the roof structure is protected from large variations in temperature. In addition, water piping and electrical wiring can lie installed within the structure without interference with the insulation or the vapour barrier. The main disadvantage is that the insulation is sealed between the vapour barrier and the built-up roofing (also a vapour barrier). If moisture is present in the insulation when it is placed or subsequently enters it through the vapour barrier it may cause blistering of the built-up roof covering due to the expansion of the water vapour, on heating by the sun. Some protection against this type of roofing failure might be achieved by strapping over the insulation of Fig. 3 (b) to create a ventilated air space, and adding roof sheathing to carry the roof covering. This type of construction is recommended particularly for those cases where the insulation layer consists of material such as lightweight concrete which has a high moisture content when placed, and for cases where it is impossible to ensure that the insulation will be roofed in a dry condition.

Insulation of Floors in Basementless Houses

Heat loss from floors in basementless buildings can be controlled in two ways: the foundation wall may be insulated either on the inside or on the outside, or the floor structure itself may lie insulated.

Good floor insulation practice is illustrated in Fig. 4(a). The chief disadvantage of this is that the crawl space follows outside temperature; water pipes have to be protected against freezing and beating ducts must be insulated to limit heat loss to the crawl space.

Figure 4
Figure 4.  Methods of insulating floors of basementless houses

The disadvantages of a cold crawl space are overcome in the method shown in Fig. 2 (b) in which the foundation wall is insulated. It allows the use of almost any type of board, batt, or blanket insulation on the foundation walls. The application of a proper vapour barrier on the warm side of the insulation is difficult, particularly between the joists, and some wetting by condensation in winter may have to be tolerated. Alternatively, insulation may be applied on the outside of the foundation walls, extending for 6 inches or more below grade. This has the advantage of keeping the foundation walls and footings warmer. But the requirement for exterior protection will add to its cost and the insulation must be water- and rot-resistant.

A recommended method of insulating the edge of a concrete slab foundation is shown in Fig. 4(b). Here, as in the case of insulation placed on the outside of a foundation wall, the kind of insulation that should be used is limited to one which is not affected by moisture and contact with the ground.


The statements and recommendations made here are supported by observations of both satisfactory and unsatisfactory thermal performance in buildings, particularly houses, by staff members of the Division of Building Research and others. In several instances the problems observed in buildings have been studied in the laboratory so that the phenomena involved are known, and the suggestions made are based on this combination of experience.

Date Published: 1961-04-01
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