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Technical Note (National Research Council of Canada. Division of Building Research), 1962-09-01
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The Thermal Conductivity of Brick Walls - A Summary of Literature Reviewed
Brown, W. P.; Wilson, A. G.
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DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
'f
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JHI
N II
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AIL
NOT FOR PUBLICATION
NOTJE
No.
380
FOR INTERNAL USE
PREPARED BY
w.
P. Brown andA. G. Wilson
CHECKED BY KRS and HED APPROVED BY NBH
PREPARED FOR
Central Mortgage and Housing Corporation
SUBJECT
September 1962
THE THERMAL CONDUCTIVITY OF BRICK WALLS - a summary of literature reviewed
The Division has been asked by Central Mortgage and Housing Corporation for assistance in establishing appropriate values of the thermal conductivity of brick masonry for determining the
compliance of a brick masonry wall system with the thermal reqUire-ments of the Corporation, as given in the Housing Standards.
Specifically, the Corporation reqUires a rational basis for establishing the relative thermal properties of clay brick and concrete ｢ｲｩ｣ｫｾ
particularly that made with slag aggregate. For this purpose a review has been made of North American and European literature on thermal
properties of brick masonry and lightweight concrete. This note gives a summary of the results of this review.
There are extensive data available on the thermal conductivity of lightweight concrete, including that made with slag aggregate.
There are also considerable data on the thermal conductivity of clay brick and the apparent conductivity of mortared clay brick walls. This includes solid, cored and perforated brick. There is little information on the apparent conductivity of mortared slag brick
walls. Published data on the thermal conductivity of mortar alone are also quite limited. Only a few test results are available for any
of these materials produced in Canada.
Most of the published records are for dry or air-dry materials. There has been some notable work on the effect of moisture on the heat
- 2
transfer properties of masonry, particularly in Germany, and this too has been reviewed. It is difficult to apply the results directly to canadian conditions, however, since the increase in thermal
conductivity depends on moisture content. The moisture content of masonry in service is a function of weather conditions, exposure and material properties; very little information is available on the in situ moisture content of masonry for ｃ｡ｮ｡､ｩｾｮ conditions.
The thermal conductivity data for lightweight aggregate
concrete were plotted against density and good agreement was obtained with thermal conductivity - density data for ligtitweight concrete
given in the 1961 ASHRAE Guide and Data Book. The figures in the Guide have been plotted as the "brick only" curve in Fig. 1 of this note. Most of the values for slag aggregate concrete fell slightly below this curve.
The data for the thermal Cellductivity of solid clay brick
(no mortar) were also plotted vs density and a good correlation
obtained with the Itbrick onlytt curve. Figures for cored or perforated brick can be similarly plotted if ttapparenttt thermal conductivity is used. Apparent conductivity refers to the thermal conductance of the brick multiplied by the thickness. The correlation with the "brick only" curve is poorer for cored or perforated brick than for solid brick and depends on whether ttbulk" or 'tmateriallt density is used.
ＧｾｵｬｫＢ density is that obtained when the weight of the brick is divided by the total volume (volume based on over-all dimensions). ttMaterial" density is the weight of the brick divided by the volume of the material (total volume less volume of cores or perforations). When bulk density is used to predict the apparent thermal conductivity of cored brick the value obtained is likely to be lower than the real value; when material density is used the predicted value is likely
to be higher than the real value. In using the "brick onlylt curve to predict the apparent conductivity of cored or perforated brick it would appear reasonable to use the average of bulk and material density.
The apparent thermal conductivity data for mortared brick walls were also plotted vs density and a correlation similar to the
Itbrick only" curve was obtained, but with many of the points falling above the curve. It is assumed that this was due largely to the influence of the mortar. The effect of the mortar on the apparent conductiVity of a brick wall depends on the conductivity of the mortar and the relative areas of mortar and bricko The 1961 ASHRAE
Guide and Data Book gives a value of 5 Btu per (hour) (sq ft) (OF per inch) for mortar. !!he literature indicates a large variation in mortar conductivit,y, depending upon oomposition, but recommended ｾ･ｳ are generally equal to or greater than the Guide value.
3
-It is accepted engineering practice to establish the apparent thermal conductivity of brick walls by calculation using known values of thermal conductivity and re lative area of brick and mortar0 Such
calculations were made to adjust the ttbrick onlytt curve for the effect of mortar, using a mortar thermal conductivity of 5 and a mortar area of 22.5 per cent of wall area. ｾｩｳ corresponds to a
standard 2 3/8- by 8-inch brick with 1/2-inch thick mortar joints o
ｾ･ results of these calculations are shown in Fig. 1 as the "brick wall" curve. A slightly lower curve results if 378-inch mortar joints are assumed. ｾ･ curve is considerably higher if the mortar ｣ｯｮｾ
ductivity is taken as 6.5 Btu per (hour) (sq ft) (OF per inch), one of the recommended values in the literature.
Results of thermal conductance tests on slag and clay brick walls using brick obtained from the Toronto area have been made
available to the Division on a private basiso ille apparent
conductivities obtained from these tests show quite a wide scatter with respect to the calculated "brick wall" curve. All but one of
the poin ts fallon or above the curve when -bulk density is used while some fall below the curve when material density is used o It is difficult to make a distinction between the apparent ｣ｯｮ､ｵ｣ｴｩｶｩｴｹｾ､･ｮｳｩｴｹ
relationships for slag and clay brick walls based on these resultso It should be emphasized that Fig. 1 considers only dry values of apparent thermal conductivity. In practice, outside masonry
walls will never be dry. AI though specific information on actual
moisture contents of masonry in service is lacking for Canadian conditions, it can be safely stated that the increase in apparent conductivity due to moisture will be significant in most parts of Canada. Without such detailed information on moisture contents in service, however, there is no adequate basis for determining what this increase in conductivity should be for any given case o
Conclusion
On the basis of the information available at the present time as summarized in this note, it is concluded that:
(1) The presence of moisture has a significant effect an
the conductivity of clay and concrete masonry materials and it would be desirable ultimately to take this into account in assigning
thermal values to these materials for purposes of the Housing Standards. Before this can be done, however, moisture conditions of the masonry in situ must be establishedo
(2) Apparent thermal conductivities for clay and concrete brick masonry should be based for the present on values for these materials in the dry state.
- - -
4
-(3) In the absence of tests on specific materials the thermal conductivity-density relationship for lightweight concrete jiven in the 1961 ASHRAE Guide and Data Book and plotted as the
brick onlytt curve in Fig0 1 provides a reasonable basis for
establishing the conductivity values for concrete bricko
(4) !!he test results reported in the literature show this curve to be a reasonable basis for clay brick as well o
(5) In the case of cored or perforated brick some improvement in prediction of conductivity values can be obtained by using the
average of the bulk and material densities o
(6) The effect of mortar on the apparent thermal conductivity of a masonry wall Should be considered by adjusting the ttbrick onlyU curve to obtain the "brick wall" curve shown in Figa 10 In calculating the over-all heat transmission coefficient of walls incorporating
brick and mortar, the apparent conductivity of brick and mortar combined to be used in these calculations is then 0 btained from the
"brick wall tt curvea !!he "briok wall" curve has been calculated from
the "brick onlytt curve assuming a conductivity value for mortar of 5 Btu, and a mortar area of 2205 per cent of the total wall - the latter representing the use of standard brick with t=inch thick mortar joints, which might well be used as the basis for correcting for the mortar in all cases a
I
1/1
/
V'
V ' /."
/ ｾ./;
Vｾ
V .......
j,ooo""V
BRICK WALL (22·5% MORTAR 1---' " , " '
V
V AREA, MORTAR K= 5·0)-'"
.""
--/V " ,ｾ
---
V--
-"'
V
_l-I - --
V
V--
-I--1--'"ｾｂｒｉｃｋ
ONLY -I--ｾ ｾe
z
8 a:: I.LI a.. ... ｾ 7 -ｾ 6-
a:: :x:ｾ
5....
(Xl -;: 4 ->....
ug
3 z o u ;i 2 ｾ a:: I.LI :x:....
･ｾ
I.LI a:: c:x ｾ 0 c:x 20 30 40 50 60 70 80 90 100 110DENS I T Y OF MASONRY UNIT, LB/CU FT
120 130 140
FIGURE I
CURVES OF APPARENT THERMAL CONDUCTIVITY VERSUS DENSITY FOR
CONCRETE AND CLAY BRICK MASONRY
(BASED ON RELATIONSHIPS FOR LIGHTWEIGHT AGGREGATE CONCRETE GIVEN IN THE 196\ AsHRAE GUIDE AND DATA BOOK)
BIi' 27(,2
OTTAWA DIVISION OF BUILDING RESEARCH NATIONAL RESEARCH COUNCIL CANADA •
