Removal of Moisture from Wet Roof Insulations by Venting and Deck Slope

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Publisher’s version / Version de l'éditeur:

Technical Note (National Research Council of Canada. Division of Building Research), 1975-08-01

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Removal of Moisture from Wet Roof Insulations by Venting and Deck

Slope

Hedlin, C. P.; Cole, D. G.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

'f

EClHI N ][CAlL

NOTlE

No.

593

PREPARED BY _{C.P. Hedlin and}

D.G. Cole

CHECKED BY _J.J. _{Hamil ton} APPROVED BY L.W. Gold

DATE August 1975 PREPARED FOR Information and Record Purposes

SUBJECT REMOVAL OF MOISTURE FROM WET ROOF INSULATIONS BY VENTING AND DECK SLOPE

The conventional flat roof system consists of a deck, in many cases a vapor barrier, insulation and an impermeable membrane. Despite this

protection, the insulation may be wetted by moisture entering through the top membrane, as vapor from below, or water may be present in the material of which the roof is constructed. This moisture will reduce the thermal resistance of insulation and may contribute to physical degradation of the insulations and roof covering. There are a variety of methods for allowing the moisture to escape from the insulation. One of these is the use of roof vents through which moisture can escape as vapor. A second is to give the roof sufficient slope so that the insulation may drain. Two studies were conducted at the Prairie Regional Station between 1968 and 1974 to obtain more information about these two drying techniques.

ROOF VENTS

Roof vents penetrate the top membrane, thus providing a path for moisture to reach the outside. Usually a number of such vents are placed in the roof in one or more rows. Reports on their effectiveness vary widely, some users claim good results while others have experienced little if any beneficial effect.

Two transport mechanisms may be in effect (Figure 1): (1) Convective movement of air carrying vapor. This may be due either to "breathing" or to "through flow" of air; and (2) Vapor diffusion. In both cases the outside air must be drier than the air in the roof if drying is to occur.

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•

ｾＮ

-

-4-somewhat wetter and the last strip was wetter than it had been at the outset. For perlite-fiber and wood fiber on the deck, much less redistribution

occurred. Conclusions

Both venting and deck slope appeared to cause moisture movement out of the insulation but the rates in both cases were so slow that permeation through the O.IS-mm polyethylene covering probably contributed substantially to the drying. This may have practical implications for avenues of moisture escape other than those that were the objects of this study. Practicality of the two methods depends, in considerable part, on the load of moisture with which they are expected to deal. In emergency cases, i.e., for removal of large amounts of moisture caused by roofing failure, they probably would not be adequate. For light loads, such as might be envisioned at the design stage, it is possible that they might meet a need. It must be recognized, particularly with regard to the vented panels, that these tests do not accurately represent a real roof situation, and the results must be interpreted with this in mind.

While it may have hastened drying, the use of vents appeared also to contribute to the decay of the wood fiber insulation by providing oxygen.

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-•

,--TABLE I

Moisture content of insulations at end of test and at the outset (in brackets) in per cent of gross insulation volume and by weight

••

I ,

Perli te-fiber Glass fiber Wood fiber I

I

Strip

Deck 6-mm space Deck* 6-mm space Deck 6-mm space

1 (13.5) 0 (17.5) (13.2) 0 (17.3) 19.3 (29.4) 4.8 (24.3) 2 (14.8) 0 (17.1) (18.7) 0 (18.0) 11.0 (24.1) 6.3 (31.4) 3 (14.9) 2.9 (18.5) (10.8) 2.7 (17.8) 20.3 (23.0) 10.9 (23.7) 4 (16.3) 50.1 (19.3) (17.4) 25.1 (17.4) 22.8 (17.8) 34.0 (23.5) Total moisture, (19,900) 20,100 (24,000) (23,200) 10,400 (23,300) 13,600 (30,000) 20,300 (34,400) grams Total loss, 3900 12,900 16,400 14,100 grams Total drainage, 1300+ 1900 1400 5100 2250 6730+ grams + early termination * leak

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•

I

ｾＮ

BREATHING

FIGURE 1

MOISTURE REMOVAL FROM THE INSULATION IN A VENTED ROOF BY DIFFUSION BREATHING AND CROSS-FLOW OF AIR

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Figure 2. Experimental roof panels with vents and

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•

15000 14000 CJ) ｾ

«

0::: ｾ 13000 0 ... ... ..._... ..._... ... ... ... _... ...

_...

o POLYSTYRENE o GLASS FIBER

°

WOOD FIBER x PERLITE FIBER .... ..._...

...

... 2000 o ....

..

... "'0 ..._...

..

...

..

...

..

'

..

..._... x ...

--INSULATION CONFIGURATION I. 2. 3. -ｾＺＺＺＺＺＭＭＭｯ

_

---0

0 _

--

-. ... · 0 _ - - - - ' . - '

---0

0,

ﾧｾＺｾ

..

0 x... .... ....

..

....

..

.... .. 8

...

1000

o

CJ) Z W ｾ u 12000 w ｾ 4000

z

W 0::: 3000 => ｾ CJ)

o

ｾ

o

10 20 30 40 MONTHS 50 60 70 FIGURE 3

TOTAL MOISTURE IN VENTED PANELS

ｾＮ

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•

• ..

STRINGERS INSIDE POLY ETHYLENE COVERING DRAINED WATER POLYETHYLENE COVERING CONCRETE SLAB .. / 70°F

ｾＮ

FIGURE 4

DIAGRAM OF SYSTEM USED TO STUDY DRAINAGE OF MOISTURE FROM WET INSULATIONS

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•

7 r - - - . - - - - , - - - . , - - - , - - - r - - - ,

---...

-

---••

6 ｾ 5 => o o W 4 Z ｾ a:: o 3 w a:: => ｾ (f) 2

o

ｾ a

.--.

.

/

I .---.

ｾｅｒｌｉｔｅ FIBER - ON DECK o·

. . - 6 mm. GAP A

/I

GLASS FIBER - ON DECK 0

_ - 6 mm. GAP.

WOOD FIBER - ON DECK 0 - 6 mm. GAP_ p ...O .... O .... O···O ,

"

a - ·

,'/ A/O' . - . - -·0

"

/ " 0" A - A ,.0""

0'---

" "

. ." " ' - 0 0 ' "

J/

/"

..

0" ｟ｾ｀ＧＺＺｾＺ

__ .,

0'" 1968...1.- 1969 ....L 1970...L1971 ....L 1972....L 1973 TIME. YEAR FIGURE 5

ACCUMULATED MOISTURE DRAINAGE FROM THREE INSULATIONS ON A CONCRETE SLAB KEPT AT 21°C AND WITH A 1 IN.48 SLOPE