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Polyurethane foam as a thermal insulation: a critical examination
Shirtliffe, C. J.
TSSN 070 1-5232
r q , 2 : - !-
POLYURETIWE FO.01 AS A THERMAL INSULATION :
A CRITTCAL EXAMINATTON by
C.
J. ShirtliffeGeneral Characteristics
Urethane o r polyurethane foam is the second most w i d e l y used
r i g i d p l a s t i c i n t h c c o n s t r u c t i o n industry. One of its unique properties -- its h i g h s t ~ e n g t h to w e i g h t r a t i o - - makes it t h e s t r o n g e s t o f a l l
the
common foam p l a s t i c s . It comes in a variety of forms: r i g i d o r flexible, low o r h i g h d e n s i t y , open o r closed ccllcd. It i s normally used in a r i g i d closed c e l l e d form f o r b u i l d i n gi n s u l a t i o n , Both factory-made board s t o c k and spray-on types are being used more extensively a s i n s u l a t i o n s i n b u i l d i n g c o n s t r u c t i o n .
Urethane foam is an e f f e c t i v e material b u t t h e r e a r e limi t a t i o n s t h a t
must bc understood i f i t i s to perform e f f e c t i v e l y . ' h i s Note has
been prepared in an a t t e m p t to a i d t h a t understanding.
Urethane is made from two components: a polyisocyanate and
a
polyhydroxyl. The components a l s o c o n t a i n c a t a l y s t s , a s u r f a c e - a c t i v e agent, and a blowing agent, which is normally a fluorocarbon refrigerant.
The r e f r i g e r a n t gases have l a r g e molecules that do n o t transfer h e a t r e a d i l y and a r e easier t o h o l d
i n
t h e plastic material. Normally r e f r i g e r a n t s R - 1 1 and R - 1 2 are used.The reaction of the two components produces a large amount of
heat which expands the g a s , t h u s producing the cells. The catalyst
c o n t r o l s t h e speed of r e a c t i o n . The surface-active agent a c t s in controlling the cell s t r u c t u r e and allows production of a foam with a fine cell structure, which i s ncccssary for a good thermal insulation.
There are two b a s i c t,ypcs of isocyanates used in t h e production sf commercial u r e t h a n e insulation. T h e s e are termed TD'I and MDT; chemical names and d e t a i l s can h e f o u n d in any rcfcrencc book on
urethane. Tn a d d i t i o n ,
a
w i d e v a r i c t y o f p o l y o l s a r e u s e d . These 3rcg e n e r a l l y selected according to c o s t o r t h e foam p r o p e r t i e s dcsired,
I n g e n e r a l , t h e more expensive rhc p o l y a l u s e d , t h e b e t t e r t h e p r o p e r t i e s , although t h i s can be i n f l u e n c e d t o some extent by t h e cqtiiprnent used in m i xing the components
.
Mixing of t h e components i s done e i t h e r i n s p r a y nozzEes o r by a
components must bc maintained constant to w i t h i n about 1 per c e n t to
m a i n t a i n zr uniform product. The temperature of t h e components g r e a t l y a f f e c t s t h e dispensing o f t h e m a t e r i a l . After mixing, t h e urethane expands 30 to 4 0 times in volume. This can be accomplished in one or t w o rises. When both R-ll and R-12 are used as the b l o w i n g agent in the foam, the material rises as t h e R-12 vaporizes and t h e n r i s e s a second time when t h e R-11 vaporizes. From 80 to 90 p e r c e n t o f t h e c e l l s are
c l o s e d i n a w e l l - c o n t r o l l e d , s i n g l e - r i s e factory application. This means t h a t t h e blowing agent does n o t escape immediately and the foam w i l l
m a i n t a i n i t s h i g h thermal resistance f o r a considerable time, t h a t i s ,
it will age slowly.
Normally t h e m i x i n g i s done in a f a c t o r y under carefully controllcd
c o n d i t i o n s but urethane can b e sprayed on at s i t e s u s i n g special
equipment. There is little information on the closed cell content for spray-on urethane that has been applied under various field conditions. It could be expected to be lower on t h e average than for factory-made foam. In most factories an upper p l a t e n compresses the foam and prevents it from r i s i n g freely. This causes the cells to be ellipsoid in shape. As a r e s u l t , t h e thermal resistance and vapour t r a n s f e r r e s i s t a n c e are increased and t h e m a t e r i a l w i l l age more slowly. Spray-on urethane has s p h e r i c a l c e l l s
o r
the c e l l s are e l o n g a t e d in t h e d i r e c t i o n of rise. This reduces t h e t h e m a l resistance and vapour transfer r e s i s t a n c e .In both factory-made and spray-on urethane the ope11 c e l l s can farm long chains i n the direction of rise. Such a material has a
appearance. These enlarged openings drastically reduce its effectiveness, It is more difficult to eliminate this type of c e l l structure i n spray-on material than i n factory-made m a t e r i a l .
The d e n s i t y o f urethanes can vary from 1.5 to 30 l b / f t 3
( 2 4 - 480 kg/rn3). Most spray-on material is made in densities of
2 to 3 l b J f t (32 - 48 kg/m3)
.
Lower d e n s i t y foam can be produced fromimproved formulations of p o l y u r e t h a n e provided t h a t well designed equipment i s used; however, the condition of the surface and t h e environment become more critical. As material c o s t is d i r e c t l y proportional to d e n s i t y , low density r n a t e ~ i a l s arc most often selected. The structural and thermal properties are generally more suitable f o r building purposes at densities between 2 and 4 pcf (32 - 64 kg/m3].
Urethane foam has good chemical resistance, which helps in a v o i d i n g problems in selecting adhesives and coatings. It a l s o increases i t s
usefulness in industry where chemicals i n l i q u i d o r vapour form are often
p r e s e n t . Urethane is n o t resistant to a11 chemicals, however, so some caution is necessary. It i s a l s o not resistant t o u l t r a - v i o l e t r a d i a t i o n .
The dimensional stability sf urethane is affected by temperature and relative humidity. Although the m a t e r i a l i s more or less dimensionally '"table" (under 5 p e r cent change in each d i r e c t i o n and under 15 p e r cent change by volume) up to 2 2 0 ° F (EOS°C) if dry and a t a low r e l a t i v e
h l u n i d i t y . A t temperatures above about 1 2 0 " ~ (49°C) and w i th r c l a t i v c humidity over 90 per cent, it becomes dimensionally unstable - changes of 5 t o 20 per cent
in
each d i r e c t i o n and 15 to 60 per cent by volume can occur. T h e r e i s relatively l i t t l e information an this important characteristic, making it difficult for designers to compensate f o r this shortcoming.A l l urethanes have quite a high resistance to water vapour t r a n s f e r . They must be covered, however, with coatings and vapour
barriers t o protect them because of problems with water absorption and
dimensional instability. Although spray-on material has a skin which increases the r e s i s t a n c e to water vapour transfer, t h e quality of the skin is not controllable. As well, the p r o p e r t i e s of the skin have n o t been established well enough to allow designers to make use
of
it insystem d e s i g n .
Urethanes can have a h i g h flame-spread r a t i n g relative to o t h e r building materials. When burning they can g i v e off large m o u n t s of smoke and toxic fumes. The flame-spread rating can be reduced by a d d i n g
phosphorous, c h l o r i n e , b ~ o m i n e or a combination of these to the b a s i c components. In addition, t h e basic chemistry can be changed t o produce an isocyanurate structure t h a t i s i n h e r e n t l y more f i r e resistant. This
increases t h e cost of the material and i n t r o d u c e s undesirable
characteristics such as friability and brittleness, and i n c e r t a i n cases reduced chemical resistance. Materials intermediate between urethanes and isocyanurates a r e being produced with a more acceptable mix of properties.
Other characteristics
of
urethane insulationPolyurethane i s one o f the more expensive residential building insulations. On the b a s i s o f c e n t s per unit resistance for a square f o o t area, the c u r r e n t c o s t of fully aged urethane runs from f i v e t o
seven t i m e s t h a t of glass f i b r e insulation and two t o three t i m e s the cost of polystyrene i n s u l a t i o n . Aluminum-foil covered material can be
more cost competitive.
Spray-on u ~ e t h a n e does have some advantages over o t h e r insulations. The cost of labour fox applying t h e materfal i s low, o f f s e t t i n g to some
extent t h e high cost of The material. T h i s i s of particular advantage in structures t h a t are d i f f i c u l t to insulate. There is l i t t l e
possibility f o r saving, however, in a standard frame house where t h c installation of glass f i b r e blankets or board-type insulation is
relatively inexpensive.
Spray-on polyurethane can provide an excellent air barrier a t the same time as i t p r o v i d e s t h e r m a l i n s u l a t T o n a l t h o u g h its e f f e c t i v e n e s s
as an a i r b a r r i e r has not been proven over long-term periods. Almost all cracks and openings can be sealed by the foam. In a frame house
s e a l will probably remain i n t a c t f o r some t i m c , Many older houses and
some n e w e r ones may, however, have movements so large t h a t urethane w i l l n o t maintain a s e a l . A lack of d e s i g n d a t a makes it difficult t o judge when t h e s e a l will be maintained.
The ability of urethane foam t o absorb acoustical energy is much
lower than glass fibre insulation. The advantage provided by t h e a i r s e a l i n reducing noise transmission is t h u s o f f s e t to some e x t e n t by this inability to a b s o r b acoustical energy.
PoPyu~ethane foam is basically 3 dimensionally unstable material.
This is because t h e g a s p r e s s u r e i n the c e l l s increases to as much as
1 2 p s i ( 8 3 kPa) above atmospheric pressure a s air permeates i n t o the
cells. Thus There i s a potential f o r a release of large forces when restrained o r a l a r g e expansion if unrestrained,
As explained previously, at temperatures above 120°F (4g°C) ar a t
relative humidities approaching 90 o r 95 per cent and temperatures above
normal ambient, the marerial begins to ma grow" if unrestrained. The c e l l
walls stretch and the material expands.
Iligh humidity has a detrimental effect:
on
the material at hightemperatures. Polyurethane is p l a s t i c i z e d by t h e water vapour causing
i t t o l o s e its s t r e n g t h and allowing the closed c e l l s to return to a spherical form and then to expand. I f expansion i s r e s t r a i n e d i n one o r t w o directions, larger expansions occur in t h e unrestrained d i r e c t i o n s .
I f it i s completely confined, t h e material applies a force t o the
sur~ounding structure. This can be large enough to cause damage even to
such s t r a n g structures as prefabricated concrete p a n e l s .
Polyurethane is b e s t used in a cool, dry environment, A good d e s i g n will e n s u r e i t s protection from extremes i n temperature and from becoming w e t . It will also keep t h e foam d r y by providing a vapour barrier that w i l l keep excessive r n a i s t u r c from e n t e r i n g t h e material and condensing o r f r e e z i n g in the colder p a r t s of the foam. Polyurethane does n o t have
good freeze-thaw resistance. If t h e material is w e t and i s s u b j e c t e d to freeze-thaw cycles it may disintegrate after o n l y a few dozen cycles.
A i r p e n e t r a t i o n and r e f r i g e r a n t gas l o s s lead to a seduction i n the thermal resistance o f the foam, termed "ageing." Only metal s k i n s have
proved effective in p r e v e n t i n g this ageing. The r a t e o f ageing for
non-metal clad foams depends on the fornulatian and structure of t h e faam.
Thc cffcct of a g e i n g i s to reduce t h e time-averaged thermal resistance
of the foam. blanufacturers provide little information on t h e ageing of t h e i r materials. Tfic energy standard for France specifies a time-averaged value of thermal resistivity of 5 (~tu/hr/ft~/"~-in.) ( 3 5 ma°C/W)- This value is applicable f o r thicknesses of 2 or 3 inches (50-75 mm).
A resistivity berween 5 and 6 i s probably mare p c r t i n c n t for thicknesscs
over 3 i n c h e s (75 mm). Isocyanurates m a y "age" more r a p i d l y t h a n urethane
A t temperatures below + 5 0 " ~ (10°C) t h e r e f r i g e r a n t gas i n t h e c e l l s ,
normally R-11, will condense causing some small shrinkage o f t h e m a t e r i a l and reducing i t s resistance to h e a t flow. This becomes progressively worse as the temperature i s reduced to -50°F [-45OC), where t h e gas i s
~ l m o s t a l l condensed. The effect of t h i s decrease in thermal r e s j s t a n c c
i s to reduce the economic attractiveness of urethane as an i n s u l a t i o n f o r cold s t o r a g e buildings. The cxact extent o f t h e effect on aged u r e t h a n e foam is unclear. Air-filled foam plastics can be much more economical f o r these applications and can take little or no cxtra space.
Special
With Spray-On PolyurethaneAs spray-on urethane is a material t h a t i s manufactured on s i t e , thc applicator mst take the responsibility for the vcry i m p o r t a n t j o b of quality control. This u s u a l l y involves inspection by a t r a i n e d
i n d i v i d u a l who understands the process thoroughly and can adjust it to maintain a good quality product.
The quality o f spray-on urethane depends on many f a c t o r s including: the ratio of the two components used, the temperature of the components in the tank, the temperature and relative humidity o f t h e s u r r o u n d i n g a i r , t h e type of equipment used, the operator's skill, contaminants and
d u s t on the surface to which the material is applied (substrate), and
even thermal conductivity o f the surface material and temperature on the o t h e r side of t h e substrate.
The quality o f t h e bond between the foam and the substrate depends
on
many of these factors as well. I f the f i r s t l a y e r does n o t r i s e p r o p e s f y it will probably not adhere very well. This l a y e r can a l s orob some of the constituents from the next layer of faam leaving it \ ~ ~ c n k
and f r i a b l e . A failure in adhesion may occur at this next layer r a t h e r
than right a t the surface.
Polyurethane foam can also be sprayed on surfaces i n t h e form of a froth, which is partially expanded foam. A f ~ o t h i s normally psoduccd by using bath R-12 and R - I 1 r e f r i g e ~ a n t i n the material. R-12 b o i l s at
a lower temperature, partially expanding the l i q u i d . This i s t h e n sprayed a g a i n s t t h e surface, where t h e heat o f reaction of the two components h e a t s the froth causing the R-11 to b o i l and finish t h e
foaming. It is claimed t h a t the s u r f a c e and atmospheric conditions h a v e less e f f e c t on urethane applied i n t h i s way. The type of equipment necessary to produce a f r o t h is n o t y e t commonly a v a i l a b l e n o r is much known about the problems of this kind of operation.
New spray equipment will probably r e d u c e t h e d i f f i c u l t y i n producing
good quality spray-on u r e t h a n e
in
f u t u r e . For example, i n t h c p a s t , mnny mnchjnes had adjustments t o c o n t r o l t h e mixing r a t i o and t h e macllincs tended t o go o u t of adjustment. The newer rnachincs, however, n r c s c t to a fixed r a t i o which s u i t s the components selected; t h e y cannot be :~cljustcrl nor go o u t of a d j u s t m e n t . This had reduced t h e amount o f o n - s i t esupervision necessary to assure a good quality foam. Other factors may
s t i l l cause t h e mixing rario to vary.
There are a large number of manufacturers producing one o r both o f the components used in making urethane foam. The quality of these components i s continually being improved. In a d d i t i o n , s p e c i a l
formulations are being developed to allow spraying under less f a v o u r a l ~ l e c o n d i t i o n s .
In
particular, cold weather formulations are being developed.Wind is a problem when spraying on t h e outside of a structure.
Overspray
can
cause a mess, which may result in expensive clean-up chargesand a h e a l t h h a z a r d ; it a l s o g r e a t l y reduces or eliminates the profit of the applicator, A l s o , t h e overspray and fumes are dangerous to t h e operators and persons in t h e immediate area during spraying o p e r a t i o n s .
It is recommended t h a t , t h e operators use fresh-air masks. Surrounding areas should be well ventilated and cleared of a l l personnel. Nearby
furniture and f i n i s h e d surfaces may r e q u i r e protection.
A number of l a r g e companies use spray-on urethane to insulate their s t r u c t u r e s and equipment. Some of these control the quality of the praducr. they receive by constant inspection, sampling, and t e s t i n g .
I f t h e material is substandard it must be removed and replaced. This i s n o t f e a s i b l e f o r an i n d i v i d u a l or small c o n t r a c t o r . The reputation of t h e applicator must, t h e r e f o r e , be relied upon. It is n o t easy to determine t h i s reputation without an extensive investigation. The i n d i v i d u a l or small contractor is therefore gambling when he uses a spray-on urethane. A long-term guarantee from t h e applicator would not h e l p as t h e cast to replace spray-on urethane, once it is built
i n t o
astructure, is so great t h a t it may not be practical.
Applicatorsf associations are now being formed which encourage proper t r a i n i n g , proper supervision and a certain m o u n t of financial banding. This may alleviate the r i s k .
Although this Note has pointed out t h e negative aspects of
polyurethane i n s u l a t i o n , t h e material has many positive features t h a t
have not been emphasized here. The m a t e r i a l i ~ s e l f is quite acceptable for usc i n b u i l d i n g s as l o n g as it is used for t h e correct application
and applied properly. An u n d e r s t a n d i n g o f the negative aspects is essential in defining the correct application.
