Some Notes on Reflective and Mineral Wool Insulation

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Technical Note (National Research Council of Canada. Division of Building Research), 1955-05-01

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Some Notes on Reflective and Mineral Wool Insulation

Dickens, H. B.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

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197

NOT FOR PUBLICATION

PREPARED ay ｈｯｂｾ ｄｩ｣ｫｾｮｳ QHECKED BY WoHoBo

FOR INTERNAL USE

APPROVED BY NoBoHo PREPARED FOR General ",Information DATE May

1955

SUBJECT Some Notes on Reflective and Mineral Wool Insulation

In selecting ｭ｡ｴ･ｲｾ｡ｬｳ for use as insulation in houses it

is wise to consider only those that are permanent, odourless and

｣ｾ･｡ｮ and which have .satisfactory resistance to fire" moisture, "

vermin, and bacterial growth:! as werl as t"6 settlement0 Both mineral

wool and reflective insulation satisfy these requirements" The basis

for differentiating between them must therefore depend ｯｾ such

factors as resistance of the material to the ｰｾｳｳ｡ｧ･ of heat, to its

performance in an actual wall, and of course, the, cost of the material

and its installation o Before these properties can be properly assessed

it is necessary to understand the way in 'which the, materials, function

as insulation and to appreciate the inherent ､ｩｦｦ･ｲ･ｮ｣ｾｳ between themo

A reflective material is useful as insulation because of

its ability to reflect up to

95

heat transfer by radiation can only occur across an air space and therefore reflective material is only effective when it faces an

air space. If placed on the warm side of the air space the reflective

material reduces the emission 'bf radiant heat energy and if applied

to the cold side it reduces the absorption of such energy. It is

equally effective in either position. It should be noted that

radiant heat flow is only one form o£ heat transfer across an air

space in a wall and comprises about

40

50

heat flow across the spaceo The remainder is by conductance apd

convections both of which are unaffected by a foil facingo Generally

the degree of insulation afforded by reflective material depends upon the number of reflective surfaces and adjacent air spaces provided.

In addition studies show that ｴｨｾ value of 'reflective insulation

depends on the direction of heat flow whether upwards downward, or

horizontal0 It is more efficient with downward ｨ･ｾｴ flow and less

efficient with upward heat flow than with horizontal heat flow. Thus

a given amount of reflective insulation in the ceiling or roof of a building does a better job of keeping heat from entering the building in the summer than it does of reducing heat flow through the ceiling

of a building during the winter months. The insulating value of the

same arrangement when placed vertically, as in the walls

ot

,

= 2 =

In ｣ｯｮｴｲ｡ｳｴｾ a fibrous material such as mineral wool gains its insulating value from the 'numerous tiny air spaces created

within the material itselfg which9 because of their number and the

small temperature differenoeacross eachg not ｯｮｾ reduce the rate

of radiant heat transfer but also limit heat transfer by convectiono Mineral wool has the same insulating value regardless of the

direction of heat flow through it and is therefore equally effective

no matter in what part of the structure it is installed. Its

insulating value is chiefly affected by its densitY9 fibre ｡ｲｲ｡ｾ･ｭ･ｮｴﾻ

and moisture content as well as by the thickness to which it is placed. Theoreticall! it is possible to provide an equally

well-ｩｮｳｾｬ｡ｴ･､ wall using ether mineral wool or 'reflective materials,

if sufficient insulating material ｾｳ used and is properly installed.

For purposes of comparison it can be shown by calculation thatg

(a) One sheet of double=faced foi19 installed ip a frame wall

so as to provide two air spaces each faced one side with

reflec,tive materiall) provides ｳｬｩｧｨｴｾｹ ｾｯｲ･ insulation

value than the same wall with one inch of mineral wool installed between the studs.

(b) Two sheets or double-raced foil providing three air spaces,

two faced one ,side and one faced both sides with reflective material)) installed in a frame wall provide slightly mo,:re

insulation value than that same wall with two inches of mineral woo:l installed between the studs ..

(c) In a ceiling where heat flow is upward during the heating

season four air spaces would be requiredp each faced one

side with a reflective material to provide slightly more

insulating value than two ｩｮ｣ｨｾｳ of mineral woolo

(d') With downward heat flow as would occur through a ceiling

during the summerp one sheet of double=faced foil installed

so as to give two air spaces each faced one side with a reflective sur1ace will provide insulating value roughly equivalent to two inches 'of mineral wool in the ceiling if

ｾｨ･ air spaces between the foils are each 314-inch wide

and to f'iv'e inches of mineral wool if the air spaces are each ｦＶｵｲｾｩｮ｣ｨ･ｳ wide..

*

*

ｾ･ｩｬｩｮｧＧ､ｵｲｩｮｧ summer or a floor during winter9 that the width of

air space is of particular significance.. In all other cases 'the

optimum insulation value of an air space in combination ｾｩｴｨ a

reflective material is obtained by providing a width of air space 3/4 inch or greater.

- - -

-=

3

-In ｰｲ｡｣ｴｩ｣･ｾ however, each insulation arrangement in place

may exhibit particular characteristics ｷｾｩ｣ｨ cause variation in the

insulating value from point to point over the wall and tend to change

the overall pattern of heat flow through the wallo This has been

borne out in recent investigations carried out by the Division on full=scale wall panels which indicate that convection currents set

up within air spaces in walls cause the air ｳｰｾ｣･ to be warmer at

the top of tbe wall than at the bottom, and generally have a much

greater influence on the ｴ･ｭｾ･ｲ｡ｴｵｲ･ distribution and, heat flow

pattern of an insulated wall than is usually assumedo

The resulting variations in heat flqw and temperature

become even more serious when the ｩｮｳｵｬｾｴｩｯｮ is not tightly sealed

against all adjacent framing members. When irisulation of the

blanket or curtain type is used even small gaps at the top and bottom

of the stud ｾｰ｡｣･ permit convection air' currents, already in motion

in the air ｳｰ｡｣･ｾ to bypass the insulation completely, thus carrying

heat directly from the warm side to the cold side of the insulation

｡ｾ､ greatly reducing its efficiency. Tpe riet result of this convective

action when gaps are present is to increase the overal+ rate of heat loss through the wall and to accentuate the temperature

difference between the top and bottom of the air space. This in turn

produces lower temperatures over the base of the wall and increases

the possibility of, surface, condensationo This can be serious in

extreme cases of high humidity and/or low outside temperature. Convective action within any air space in walls occurs regardless of whether the wall is insulated with mineral wool or a

reflective materla19 but leakage due to loosely fitting insulation

has been shown to be more pronounced in walls containing air spaces

on both sides of the Tnsulatlng blanketo These adverse effects in

walls can therefore be reduced by installing the insulating material

so that only one air space is presento Further benefits can be had

by arranging this air space on the cold side of the insulationo

Unfortunately this remedy cannot be applied with reflective insula-tion since 'the air spaces which it proVides are an essential part of the insulation arrangement and more or less uniformly spaced from the hot to the cold sideo

These several considerations empnasJze the importance of proper installation in the overall performance of both types of

insulating ｭ｡ｴ･ｲｩｾｬｳ to obtain a tight seal and prevent air leakage.

In this connection it is known that some blanket-type ｲ･ｦｾ･｣ｴｩｶ･

insulations are ｾｯ constructed that their prpper installation is

made most difficult, if not impossible9 when placed between studs

which vary even slightly in spacing from the accepted 16 inches. When the stud spacing is greater than 16 inches they cannot be

sealed tightly against the framing memberso When the ｳｴｵ､ｾｰ｡｣ｩｮｧ

is less than 16 inche,s the reflective blanket may not be opened

to its fullest extentq This latter condition may, depending on the

4

-the foils and in some cases may result in one or more fo'ils touching,

ｴｨｾｳ rendering the associated air spaces ineffective at these pointso

The problem is particularly significant in view of the studies made by this Division on houses in various parts of Canada which show that a large percentage of the exterior wall area of ,frame houses is likely to have studs spaced at distances that WQuld cause

difficulty in the placing of reflective insulation o

* '

There are other.reflective blankets so constructeq that the

specified ｮｾ｢･ｲ of air spaces cannot be maintained ｣ｯｭｰｬ･ｴｾｬｹ to

the edge of the studs o This results in reduced wall surface

temperatures over the studs as compared with a similar wall ipsulated with ｭｩｮｾｲ｡ｬ wool and in turn can lead to higher ｯｶ･ｲ｡ｾｬ heat' loss, greater likelihood of copdepsation on that portion of the wall

surface directly over the studs and on the heads of nails driven

into the ｳｴｵ､ｳｾ and finally a greater tendency to dust markingo

A review of the factors described indicates the need for

ｾ｡ｲ･ｦｵｬ appraisal of any insulating material of the "reflective or

mineral-wool type before final selection is made o The need for

'careful installation is of prime importance, ｰ｡ｲｴｩ｣ｵｬＧ｡ｲｾｹ with

reflective materials o Generally the reflective materials in the

form usually available are likely to require greater care in

obtaining a satisfactory ｾｨｳｴ｡ｬｬ｡ｴｩｯｮ job than the mineral wool

insulations in blanket or batt ｦｯｲｭｾ

*

3073,

DoHo Rutherford0 Price 10 cents o Available from