Life Cycle Costing of Log Walls for the National Energy Code for Houses

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Life Cycle Costing of Log Walls for the National Energy Code for Houses

Swinton, M. C.

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Life Cycle Costing of Log Walls for the National Energy Code for Houses

Swinton, M.C.

A-3107.1

NATIONAL

ENERGY CODE FOR HOUSES

-

M.C. Swinton

Approved

_-

&A. tcarakat, Ph.D.

Head, Building Performance Laboratory

Report No: A3107.1

Report Date: January 5,1996

Contract No: A3107

Reference: Letter of Agreement dated September 30, 1994 Laboratory: Building Performance

14 pages

Copy No. 2 of 4 copies

This report may not be reproduced in whole or in part without the written consent of

Report A31 07.1 Page 1 introduction

At

Rs

3Mh meeting, the Standing Committee on Energy Conservation in

Buildings requested that a study be undertaken of the life cycle cost of log walls (solid wood walls), to assist

in

determining what should be the appropriate

prescriptive requirements for such walls in the National Energy

Code

for Houses. In response to this request, the Building Performance Laboratory, Institute for Research in Construction, performed life cycle cost analysis of log walls in ccnsultation with the Canadian Manufactured Housing Association, with funding from Natural Resources

Canada.

The study consisted of 3 main tasks:

1. Costing of Solid Log

Waf1

Assemblies

2. Determining Assembly R-values for Solid Log Walls 3. Life Cycle Costing for 34 Zones, for each fuel type. Tfw following describes the approach and resuRs of each task.

Costing

of Solid Log Wall Assemblies

The Energy Bw ilding Group, which had previously costed opaque envelope components for the National Energy Code for Houses, was retained to survey the construction costs of:

rectangular-milled log walls round-scribed log walls

The costing method was relatively straight forward: a number of manufacturers and builders

were

suweyed

across

the country to obtain the cost of building log walls in $/m2, for various wall thicknesses (rectangular-milled) or for various mean log diameters (round-scribed).

Five manufacturers of rectangular-milled lag homes were surveyed. Wall thicknesses ranging from 100 to 305 mm were costed. All surveyed

manufacturers were found to offer a 200 mm wall, and most offer at

least

one

other

size as well, usually I50 mm.

Six builders of round-scribed log homes were

surveyed.

Walls with

mean

jog diameters ranging from 305 to 500 mm were costed. Most suweyed builders

were found to offer a 355 mm diameter

log

wall, and most offer at least one other size.

A report was issued by Energy Building Group1 which records the results of that survey.

Determining Assembly R-values for Solld

Lag

Walls

To facilitate

the

calculation

of the assembly RSI of a log wall using a simple hand

calculation, a new RSI

adjustment

factor called 'profile

factof

was

introduced. The profile factor is defined as the ratio of actual assembly RSI (including air

films) as determined by detailed heat transfer analysis, to the assembly RSI (including air films) of a hypothetical solid wood wall of the same wood species having a rectangular cross-section, no joints and a thickness equal to the

nominal

thickness

of the wall (rectangular-milled)

or

to the mean diameter of the

logs (round-scribed).

A solid piece of wood would have an RSI

value equal to its thickness times the thermal

resistivity of the w o d . A log wall built within

the confines of the cross section of the solid

wood piece would be expected to have some

fraction of that RSI, depending on how much

wood is missing due to the profile, and

depending on the joint detail and insulation material used.

Detailed 2-dimensional computation of heat transfer through log walls was undertaken to derive appropriate prome factors for both types of log walls.

Appendix A describes this approach in more detail. Examples of calculated heat flux lines (generally horizontal) and constant temperature lines (isotherms,

generally vertical) are shown in Fig. 1 for a rectangular-milled log wall and Fig. 2 for a round-scribed log wall. Typical profile

factors

were found to be 0.97 for

rectangular-milled walls (i.e.,

close

to 1 as would be expected}, and 0.77 for round-scribed (i.e., considerably

less

than t

because

the thickness of the wall at the joints is less than the mean diameter of the logs, which leads to short-

circuiting). The profile factorof the round-scribed wall includes the effect of

insulation in the grooves. As a _{result of the insulation, the profile factor is not}

directly proportional to the average thickness of the wall. The profile factor of

0.77 pertains to logs that are shaped

in

accordance to the Log Building

Standards,

and

allows for a proportionately similar amount of insulation in the larger lag sites.

With these

profile

factors, the Assembly RSI of log walls can be calculated as

Report A31 07.1 Page 3

Assembly RSI = [(mean diameter x resistivity of

wood)

+

RSI air films] x 0.77

Assembly RSI

=

[(nom. thickness x resistivity of wood)

+

RSI air films] x 0.97

As listed in Appendix C of the National Energy Code for Houses, the resistivity of

cedar is 0.0090 RSllrnm and for pine it

is

0.0081 RSHImm, for equilibrium moisture contents. The

totai

RSI for air films for walls is 0.15.

Appendix A presents the details of the derivation procedure followed for determining the profile factors.

Results of Cost

Survey

and RSI Evaluation

The assembly characteristics reported in the cost

survey

by Energy Building Group, and the applicable profile factors were used to calculate

assembly

RSI. The cost were then plotted against assembly RSI, as shown in Fig. 3

In general, the incremental costs of rectangular-milled walls were consistent from manufacturer to manufacturer, even though the absolute costs differed

significantly. There was more variation in the round-scribed results. One builder reposted very high incremental costs (closer to rectangular-milled construction than round-scribed), whereas another builder reported negative incremental costs with increasing log diameter. It was concluded that no single set of

consistently costed walls from one builder could be used to represent all surveyed costs.

These results posed a problem for the life cycle costing approach used for NECH. The incremental costs of all other components evaluated for NECH, e.g. wood frame walls, were determined by selecting actual assemblies in the dataset to represent the trend of all surveyed costs. This could not be done for log walls, since there was no standard method of costing Fog walls according to log size. Thus, picking representative assemblies from several builders would not yield incremental

costs

of log sizes as much

as

differences in costing approach from

one

builder to the next.

It was thus decided to assess the trend in incremental cost of the log walls as a

function of RSI by fitting straight lines through each data set using

standard

statistical techniques. Each cuwe fitting exercise (one

for

rectangular-m illed and one for round-scribed) made use of all data points in each data set. This

curve

fitting approach

was

consistent with the approach used for life cycle costing for

the '83 Measures2- the predecessor to the NECH. The curve fitted result thus gives

a

measure of the average incremental cost of building with thicker

logs

to

achieve greater R-values.

The slope of the incremental cost per RSI for rectangular-milled walls was

$220/m2/RSI, as illustrated in Fig. 4. For round-scribed log walls, the slope of the incremental cost was $102/m2/RSI, as shown

in

Fig. 5. These compare to about $8/m2/RSI for wood frame walls. The incremental cost of the three wall systems are compared in Fig. 6. The

data

points for the two log wall types are the fitted costs

of

log walls for each thickness or diameter reported in the survey. Since these are now plotted as 'incremental costs' in Fig. 6, each cost curve stam at zero. For each curve, the reference cost used to calculate the increment was the lowest fitted cost in the dataset.

Life Cycle Costing

for

34 Zones, for Each Fuel Type.

The fitted incremental costs per RSI per m2 shown in Fig. 6 for the

log

walls were used in ths Eife cycle cost analysis. Life cycle costing was performed using the

same computer program, LCCH" and

using

same

economic assumptions as used for life cycle costing of

wood

frame walls for the National Energy Code for Houses.

Rectangular-milled Resuks

Fig. 7 shows the result of life cycle cost evaluation of rectangular-milled log walls

for an electrically heated house in Ontario Zone B. This zone and fuel was picked because in combination, the cold climate and high

cost

of fuel should result in relatively stringent requirements, as was the case for all other

components, such as wood frame walls. However, as can be seen from Fig. 7, in

spite of the high cost of heating for this fuel and location, the high incremental cost of construction of 150 mm rectangular-milled log wall relative to a 100

rnm

wall is much greater than the present value of energy costs that would be saved. The 100 mm wall was therefore the 'optimum' for this case. This was the result for afrnost all of the populated zones in each province and territory.

Round-scri bed

Resutts

Fig. 8 shows similar

results

for round-scribed

log

walls. The 'optimum' mean log

diameter is 305 mm for the case shown. This was also the result almost all of the populated zones in each province

and

territory.

Report A31 07.1 Page 5 Recording the Results in National Energy Code for Houses

The life cycle cast results were recorded in the prescriptive tables for each zone

of the National Energy Code for

Houses4

-

34 zones in total. An example table is attached, with the log wall entries bolded.

Since

consultation with the Standing Committee on these results was not possible before publication of the second public review draft of the NECW,

a

note explaining the status of these results

was

included in the published tables, as shown,

All generated graphs

and

tables for 34 zones and for up to four fuels were recorded

in

a permanent record. These results were made available to the Standing Committee on

Energy

Conservation in Buildings, to assist in the

assessment of what should be the appropriate RSI to be prescribed

for

log walls in the National Energy Code for

Houses.

Acknowledgment

The author wishes to thank Cliff Shirtliffe for implementing IS02 and perForming the heat transfer analyses of tog walls, and for his assistance in documenting that work.

References

1. Construction Costs for Solid Wood Walls in Houses. Contract report by the Energy Building Group for the National Research

Council,

February 1995. 2. Measures For Energy Conservation in New Buildings 1983. Issued by the

Associate

Committee

on the National Building Code, National Research Council of Canada. NRCC No. 22432.

3. Swinton, M.C.; Sander, D.M. S~ecification of a Method for Life

Cycle

Cost

Analvsis for the Eoerav Code for

Houses,

July, t992.(26th meeting

of

the Standing Committee on Energy Conservation in Buildings, Canadian Commission on Building and Fire Codes, Appendix

0.)

4. National Energy Code for Houses 1995

-

Public Review Draft

#2.

Canadian

Commission

on Building and Fire Codes, National Research Council Canada. March, 1995.

1. #I u I* n _{IW u. IH}

*m1*. a : ) an

n

-

:

W - t a c * bn- 1-t-1 I C D ~ m c w - bn- h a t I l u x l r r w m 0 . 1 m n

Report A31 07.1

Page 7

I

f c m l r l 1 1 1 . 1 1

m u m - b r mr r o l l r r n . a I t o h ~ m hn.m w r I lur !.rn.: 0 . l w n

Fig.

2. Heat

Flux Lines

and

Isotherms in

Round-scribed

Log Walls

Report A31 07.1

Fig.

7.

Life

Cycle

Cost

of

Rectangular-Milled

Log Walls

Report A31 07. I

Page 13

Fig.

8.

Life

Cycle

Cost of

Round-Scribed bog Walls

Ontario

-

Zone

B, Electric Heating,

2

Story Full

Bsmt

RSI

Mean Log

Tables of Prescriptive Requirements

Phndpal Hmtinq Source

Ele~wrY, Propane, 011 Mat. Gas Other Meal Pump (as)' Heat Pump(gs)'

*[as3 -air source heat pump; (qs)

-

qround sour- heat pump

Table 3.3.1 .A Mintmum Effective h m a l Res~stana

F m i n g Part of Sentenca 3.3.1.1 . ( I ) _{RSI-value (mZ*"cAV)}

, Abweqround Building Assemblies

&&: I I

Type I

-

attfetype, parallel-ehord trusses and

plywood I-joists

S@I

-

AI wans except tog wans 4.7

Type 11

-

Log w&~s**

-

rsetangular-mnm 1 0 . 9

1

7.1

1

4.6 4.6

.,

I

T h m RSF vduee shown for log wells are fhm nrsulC of EIfb cycle oosr studtm c a m i d ouf En coopwafton

with the log constru~flon Industry, These studies were completed ImmedIatdy prior to publlcatlot~ of

thEa dmft. The vaIues shown have not been rcvlBWBd by the Standing Cornmiflee on Energy Consematfun In Buildings and fherefom do not refleet the Standlng Committes's vlews.

Fixed glatlng without sash

Farming Part of Sentem:es 3.3.2.1 .[I) wd (2) and RSI-value ( m 2 * " ~

Trpe 1

-

with imbedded h M n g ducts, cables

Table 5.3.1 .A

Forming Part of Sentence 5.3.1 .I.(?)

Heat Remvery

Heat remverj on principal exhaust pottion d the I '

t-twchanid ventilation system in dwsllbng units resuld requlred required

Report A31 07.1 Page A1

Appendix A

Determination

of

Profile

Factors

for Calculating

Effective Thermal

Resistance

of

Round-scribed

and Rectangular-milled Log Walls

0 biect ive

The objective of this Task was to develop a means of calculating assembly RSl's

for solid m o d walls (log walls) using a simple formula, by pre-evaluating the effect of the profile and the joint on the thermal performance of the log wall.

Approach

The following steps were taken to meet the objectives

of

this task:

1. Define a 'profile factofto account for the effect of the profile of the log wall and the joint details on the effective RSI of the wall assembly.

2. Select and implement a two-dimensional finite-element or finite-difference model to determine the effect of profile and joint details on the heat loss characteristics of the wall.

3. For various round-scribed and rectangular-milled profiles

used

by industry, determine the profile factor by relating the effective thermal resistance of the log walls found through the detailed 2-dimensional heat transfer analysis to a reference thermal resistance found by simple calculation.

4.

Recommend

appropriate profile factors to use for the National energy Code

for

Houses.

Definition

The profie factor is defined as the ratio of actual assembly RSI (including air

films)

as

determined

by detailed heat transfer

analysis,

to

the

assembly

RSI

(including air films) of a theoretical soiid wood wall of the same wood species having a rectangular cross-section, no joints and a thickness equal to the

nominal thickness of the wall (rectangular-milled) or to the mean diameter of the

In general, the profile factor can be determined by resolving

the

following equation:

Once

the appropriate profile factor is determined through analysis, a formula for calculating the assembly RSI of a log wall is developed by reversing the above formula:

Assembly RSI = [(reference thickness x thermal resist iviity-)

+

RSI

,,,,,I

x profile factor ₍₂₎ where:

the reference thickness is the mean diameter of the logs for round-scn'bed log walls

a the _{reference thickness} is the _{nominal thickness} of _{the wall for rectangular-milled log}

walls

It was postulated that the profile factors would be similar enough for a range of log thicknesses and joint configurations, that a single factor could be used for a broad range of log wails; is., one profile factor for all round-scribed log walls and one for all rectangular-milled log walls. The less the variation in profile factor for different designs and sizes, the broader would be the applicability of the factor. Determining

the

Profile Factors

The most accurate way of assessing the thermal resistance of jog walls would have been to measure the thermal resistance of each size of log wall available. The measurement data

was

not available and would have been expensive and time consuming to obtain. Modelling of the walls with finite difference or finite element mathematics is the next best method to determine the thermal

resistance of the wall configurations

2-0 Finite Difference

Heat

Transfer Analysis Program

A 2-dimensional finite difference heat transfer analysis program

-

IS02

-

was

selected to develop profile factors for a number of round-scribed and rectangular-milled log wall assemblies. It is a Windows or DOS-based program for analyzing the steady state temperature field in 2 dimensional bodies made up of solid sections and air cavities. The program is specifically designed to handle sections of the building envelope. A library of material properties is specific to building constructions. The air spaces and air

surface films relate to building type environmental conditions. Radiation and convection conditions are approximated by surface resistances.

Representation of Log Walls in IS02

Representation of the construction elements is achieved with a series

of

Report A31 07.1

Page A3

and Y axis. A series of steps can be used to produce a sloped boundary. In certain axisymmetric cases, the program's ability to handle circular

boundaries can be utilized. A rectangular mesh

is

overlaid on the drawing of the building element. The mesh divides the element into rectangular cells of

different sizes. The rectangular elements can be used to assemble more complex or symmetrical constructions. The program can zoom in on details to allow accurate dimensioning. A mesh generator selects the mesh size based on the wishes of the operator of the program. The operator can request a

finer mesh or can manually add mesh lines to areas that are of special interest.

The centre of each cell is termed a node. The

number

OF nodes is used as a rough measure of the precision of the model of the element. Tbe finite

difference mathematical routine models the nodes as small concentrated lumps of

material

having the mass and heat capacity of the cell and there are connecting spokes that conduct the

heat

in the _{2 dimensions at a rate equal}

to what would flaw through the model at that location.

The

basic mathematical model produces a mesh that resembles a 'Tinker Toy' or

'molecular' structure that has the overall form of the construction element. (If the mesh were

extremely

fine it would produce a rational thermal model of the actual molecular structure of the element).

The program

can

handle meshes that have up to 16,000 nodes. The

surfaces can have constant temperature with air films,

constant

heat flow rate, insulated (adiabatic}

surface

or contjnuity boundary conditions. Heat sources and sinks such as heating wites can be modeled.

Other Features of IS02 Used for the Analysis

The outputs from the program include tabulated resuks and color graphical plots with materials, isotherms, heat flow lines identified. Profiles of the

temperature can be plotted in color. (This can be used to display the potential

for surface condensation and mold growth, for example, for given indoor RH). Integrated heat flow that

crosses

a section of the boundary, the maximum and minimum temperatures on a section of the boundary, selected individual node temperatures, heat flow and its direction at a node, all are plotted as

required.

IS02

also performs an independent calculation of the simple I-dimensional

thermal transmittance (conductance) from inside air to outside air. This allows easy comparison of the 1-dimensional calculation to the detailed 2-

dimensional results. For the purposes of the calculation of profile factors this eliminates any inconsistencies and added errors.

Implementation Issues for Modelling Thermal Resistance

of

Log

Walls Cog Dimensions

Accurate modelling requires access to the actual dimensions of the log walls and the range of variation of the log dimensions. It was decided to model log walls of the various

sizes

that are designed according to the Canadian and American Log Builders Association 1 993 Log Building Standards. These define the limits on the overlap of round logs and the site of the groove.

Groove

Types

for Round-scribed Log Walb

While a number

of

groove types for round

logs

are allowed

in

the standard,

the

design

of

the groove does not widely affect the thermal resistances of the walls. The Norwegian V-groove and a simplified square groove were

modeled for 203 mm diameter logs. The difference in thermal resistance for logs with similar grooves was not

large.

The choice of a full width square

edged groove represents 'the maximum thermal resistance of conventional groove

designs

so these would be fair

or

generous to all round log

manufacturers,

Representation of Round Logs With Rectangular Slices

The

decision was made to model the logs as a series of horizontal slices that for a log of circular shape instead of as logs with an exact circular shape. Smaller slices were used in locations of high curvature in the vertical direction, i.e. at the top and bottom of the logs. There was generally four sizes of

slices

each

thinner than the previous by a factor of two. This

produced the stepped boundary shown in Fig A1

.

Calculations were done to show that the stepped shape of the surface added no significant error to the heat transfer results.

Insulation in the Grooves

A variety of insulations can be used in the

grooves

in the actual

logs

systems but in the great majority

of

cases glass fibre is used. Glass fibre insulation was modeled in our calculations.

Representation of Rectangular-milled Logs

The squarelrectangular logs were simpler to model and the most significant approximation in the shape was the corners on the profile.

Seven

height to width ratios were modeled. Three

sires

of 45 degree

chamfers

were used for

203 mm wide logs; 203 rnm being the predominant size

for

square or

rectangular

logs.

Each chamfer was modeled as a series of 10 steps, as

Report A31 07.1 Page A5 Figure At - m a m "7 C I I ' N u - e u Y I om1 n r rrt r a nt rr H n 1 4 4

u n o ) !i5 F3 w * 2 LI i Z 4

3::

g s g

Report A31 07.1

Page A7 effect

of

log width on the profile factor. The effect of surface film resistance was checked

for

three chamfer sizes and three height to diameter ratios.

Treatment of Air Films

The heat flow through a wail can be calculated by taking the product of the temperature difference across the wall, the cross-sectional area and the inverse

of

the total thermal resistance from air to air. The total thermal resistance accounts for the air films, the thickness of the wall

and

the shape

of _the

_logs.

_{The heat flow is strongly 2-dimensional, especially for round-}

scribed logs, so the air films form an inherent part of the flow pattern and the overall thermal resistance. ?herefore, the films cannot be removed from the model without

loss

of accuracy. This is also the reason for including the air films in the profile factor calculation. (Calculating the thermal resistance of the wall first without films

and

then adding air films was judged to be an inappropriate representation of the heat transfer).

Wall Profile

Factors

As indicated above, the equivalent shape of the log

and

the films can be represented as the product of a %a! profile

factor"

and the thermal resistance of a square log with the air films.

However,

the profile factor varies according to the number of logs in a wall for a given height of wall, so

that we had to investigate the effecl of the number of

logs

on the predicted

profile factor.

Selection of Number of Logs to Model

The variation 05 the wall profile factor was calculated for a wide

range

of

overlaps on logs of different sizes. This provided a reference point for the

profile factor for logs with insulated

and

uninsulated grooves. The heat flow through logs is more complex than at first

b

_{evident. The introduction of the}

groove makes the heat flow non-symmetrical. There is no plane of symmetry or a position where the heat flow lines are straight across the whole log. The calculations should ideally be done on a full wall as a result, which would be time consuming. The

size

of the error in calculating the heat flow through the single log (two-hafves

and

a joint) has been estimated by comparing the heat flow through the two half logs (Fig A f )

and

a

section

of two half logs in a

stack

of

4

logs.

The

results

for two-half logs were judged to be adequate for the calculation of profile factors; i.e. the adiabatic boundaries at the top and bottom of the two-half logs is a good approximation for the boundary

lmplementatfon of the Model and Assumptions Round Logs.

Table 1 a lists the material properties selected for the wood and insulation in the models. (The precise properties of the wood selected for this analysis are not critical

since

the thermal resistance is compared to the thermal

resistance of a full,

square

log

of

the same thickness; is., the profile

factor is

a ratio of two resistance terms that are each linearly dependent on the conductivity of the wood). Table I b lists the temperature, humidity, and air

film surface conductance used for the modeis. Table 2a lists the

characteristics of the round log walls that were modeled. The diameter of logs in

rnm

and

inches, range of overlap of logs, number of models

calculated,

groove design, number of slices per half, and the total number of calculation nodes for the finite difference calculations are

listed.

The overlap

of

the logs varied from 0.10 to 0.1 2 of the diameter. This produced a face width of 0.44 to 0.46 which is essentially the average of the narrow range

of

overlaps allowed in the standard for log walls.

Square

Cogs

Table 2b lists the corresponding characteristics

of

the rectangular / square

logs

that were modeled. The width of the logs in mm and inches, ratio

of

the width to height of the logs, number

of

models calculated, number of steps in the chamfers, and number

of

nodes for the finite difference calculations are listed. The calculations cover the majority of the rectangular log

Table 1

a

Properties of Materials Used in the Finite Difference Calculations

Table I b

Surface

Temperatures, Relative Humidity and Air Film Resistances

Location Temperature Relative Air Film

C Humidity Resistance

Report A31 07.1

Page A10

Table 2b Summary of

Results

for 25 Models of

203

and

305

mrn Width

SquareJRecZangular Logs

Example Resuf st Isotherms and Heat

Flux

Lines

Fig. A1 & A2 show example IS02 printouts of the simulations for round-scribed and rectangular-milled

logs

respectively. The heat flux lines (generally

horizontal) and isotherms (generally vertical) are shown in

these

figures. Two- dimensional effects

can

be

seen

near the joint in both cases. The two-

dimensional effects are broader in the round-scribed example as would be

expected. As well, those effects are more complex around the insulated

groove,

Resulting Profile

Factors: Round-scribed

Logs

The fallowing table summarizes the profile factors obtained

for

the

round-scribed

log walls evaluated with the 2-dimensional heat transfer analysis program. Round-scribed

Log

Groove Insulation Depth* Profile

Diameter (mm)

Factor

203

none 0.0 0.71

203

tangential

21.5

0.72 203 square: 0.94** 20.3 0.74 square: 0.94 35.4 0.78 square: 0.94 60.9 0.84 254 none 254 square: 0.94 square: 0.94 square: 0.94

I

"""

none

I

0.0

1

" Total cut into log is

10.2

and 12.7 mm more for 203.2 and

254.0

mrn fogs.

**

Insulation

width is 94% of interface between logs

The profile factors is also influenced by the following joint details:

Groove & Insulation Profile Factor

No insulation 0.7 1

Report A31 07.1 Page A12

Resulting Profile Factors: Rectangular-milled

The following table summarizes the profile factors obtained for the rectangular- milled log walls evaluated with the 2-dimensional heat transfer analysis program. Rectanqular-Milled

Thickness Height / RSI RSI profile

1

Thickness Solid Log Wall , Factor

Wood

1

Thermal resistivity

of

the wood at outdoor equilibrium moisture content (used

in

the calculations):

Pine 0.0081 (rn2.~/W) /mm

Cedar 0.0090 (rn2.c/W) /mm

Recommendations

The results would indicate that

for

profile-milled log walls, the National Energy Code for Houses should use a profile factor of 0.77, as this allows for the same

basic geometry according to the Log Building Standards and allows for a

proportionately similar amount of insulation in the larger sizes.

For rectangular-milled log wails, it is recommended that a profile factor of 0.97 be used.

Limited Comparison to Field

Data

Although field

monitored

performance

of

log walls

is

scarce, the thermal

performance of four round-scribed

log

walls (pine) and one squared log wall (cedar) was monitored in the field by a

cansuttant

using a portable hot box apparatus for CMHCA'. The application of the profile factor of 0.77 in equation (2) for these four walls compared favourably Zo the reported test results, as

milled wall for this comparison, so that a profile factor of 0.97 was assumed. The

calculated

and monitored

result

were

also

close,

as shown

in

the figure.

I

7

Fig Al. Cornparism of New Method and CMHC Tast Results

3.00

Pmposed Slmp!e Calwlalion M

2.50 2100

L

%

2

1.m X

a

f.00 am am 2U) 2W 280 aOP 820 3*0 880 3110 4 0 0 . A v o w s l o g Dlam- or T M k n r s s (mm) Reference to Appendix A

A l . Log Walls Field Tests. Contract report by Scanada Consultants Limited to Canada Mortgage and Housing Corporation, Septern ber 1986.