Insulation

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The colder the climate, the more insulation is needed. The Inuit people of the Arctic get most of their insulation from the skin and fur clothes they wear that keep them warm even when living in houses made of ice.

In a similar climate, in the Antarctic, a Mantainer (container for people) was designed for Robert Swan's expedition, in which the wall of the capsule provides most of the necessary insulation. The structure (Figure 3.1) is made of a timber fibre honeycomb base panel (12 mm thick), compressible wool insulation to the exterior of the panel, all contained in a waterproof cotton skin. The double floor panels are of polyamide honeycomb. Similar pods are used in remote regions of the world, such as the Swiss Alps (Figure 3.2), and their success depends on choosing exactly the right insulation that is light and highly efficient, regardless, in this case, of cost. The specification of insulation in many of our own homes may be quite different. The trick with insulation is to choose the right insulation for the job.

But there is a wide range of insulation products on the market at a wide range of prices. How does one choose the right type for the job? The properties of each insulation product should be evaluated and, if necessary, checked with the manufacturer. A clear specification for the insulation should be drawn up. Does the insulation

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The Mantainer pod designed for Robert Swan's Antarctic Expedition. The pod uses photovoltaics to generate electricity and a gas microwave cooker. A multifuel cooker is included where waste can be burnt. Water comes from snow melt. The WC is compositing, and those wastes not compostible or combustible are removed at the end of the habitation (Ian Guilani, Mantainer, Ecotechnology, Henley, UK. Email: [email protected]).

n have to be waterproof? Is it best to use a reflective board, a high mass wall or a bulky insulation product (see below)? How much room in the construction is available for it? How toxic is the product and does it matter? Is it to be put on the exposed side of a house where there is driving rain or on the sheltered solar elevation? How fire-resistant does it have to be? For instance, if the insulation is to be used internally, then a fire-resistant type is essential; this subject is covered under the section 'Fire' in Chapter 6. Where the insulation is to be placed in well-sealed cavity walls this may not be such an issue. The usual basic rules for materials choice should apply: use materials that are as natural as possible and as local as possible. However, in some cases, such as the Mantainer pod, it is possible to justify the transport energy costs of moving insulation long distances because of the amount of energy saved when it is used on location. Who wants to lug a Calor Gas bottle up a Swiss Alp? Here, as always, common sense is essential.

Table 3.1. Some properties of insulation materials

Type of insulation

Expanded

Rock

Cellular

Cellulose

Phenolic

polystyrene

mineral

glass

foam

insulation

wool

Thermal performance Moisture resistance

Mechanical performance

Chemical properties

Fire behaviour

Toxicity

Embodied energy

NOT considered a vapour barrier. Low water vapour transmission, no capillary action, and high resistance to moisture absorption.

10% compression strength of 110-150 kPa.

Resistant to diluted acids and alkalis. Not resistant to organic solvents. Can chemically interact with polymeric single-ply membranes such as PVC.

Melts and shrinks away from small heat source. Ignites with severe flames and heavy smoke when exposed to a large heat source.

Thermal decomposition products are no more toxic than those of wood. 120.0 GJ t-1

0.034-0.036

Does not absorb moisture. At 95% RH, hygroscopic water content is only 0.02% by volume, 2% by weight.

No capillary action. 10% compression strength of 120-180 kPa.

May need isolating board under asphalt.

Non-combustible to 2000°C. Practical limit is 1000°C owing to the additives.

None.

0.2% by volume. No capillary action.

Not given (presumed low moisture resistance).

10% compression strength of 230-500 kPa. Average compressive strength is 600 kPa (87 p.s.i.). Pure glass without binders or fillers. Totally inorganic, impervious to common acids except hydrofluoric acid. May release hydrogen sulphide and CO in a fire.

Non-combustible.

None.

Not given (presumed poor mechanical strength).

Treated with inorganic salts for fire protection.

Withstands direct heat from a blowlamp.

None, fully biodegradable.

High moisture resistance, low vapour permeability, 90% closed cell structure.

'Good compressive strength'.

Low corrosion, pH approx. 6.5. Pre-1980s foam corrosive to metal deck surfaces and fasteners in dry conditions and aggressively corrosive when wet.

Class O fire rating.

Formaldehyde used in manufacture.

CFC emissions Does not use CFCs, HCFCs or CO2.

Effects of age

None reported. See Chapter 4.

Does not use CFCs, HCFCs.

None (batt). Settling (loose fill).

Does not use CFCs, HCFCs.

See Chapter 4.

Does not use CFCs, HCFCs or VOCs.

None reported.

Does not use CFCs. Uses chemical blowing agents instead of a physical one. See Chapter 4.

New buildings

Existing buildings

Prices

Lifetime Recycling

On or below sub-floor slab, between timber floor joists. Partial or full fill wallboards. Flat or pitched roofs. Granular and bead forms of EPS can be injected into existing cavity. Range

£1.23-6.50 per m2. Board: 50 mm £4.95, 100 mm £4.64. Cavity fill: 65 mm £2.75 (including labour). None given.

Roof or ceiling, walls, floor or foundation.

Roof or ceiling, walls, floor or foundation.

60+ years.

None given.

Easily melted and reformed. Low density in place in the UK. precludes longdistance transport

Recycling programs Reclaimable on demolition.

Roof or ceiling, walls, floor or foundation.

None given.

None given.

100% recycled and recyclable.

Factory engineered panels. Particularly suited for HVAC pipework and ducting. Not mentioned.

'Long' - usually exceeds building life. Is possible. No programs in UK as reported.

Microporous silicia

Sheep's wool

Flexible melamine foam

Cork

Polyurethane foam

Polyisocyanurate foam

Moisture content 40% water of 1-3% by absorption weight. (by dry weight)

Undergoes significant Water repellent dimensional changes with zero capillarity.

with increased Relatively high rate moisture content of vapour owing to its open transmission. cell structure.

Closed cell structure Low water absorption forms monolithic, and vapour self-flashing surfaces. transmission.

Can be sprayed with elastomeric coating for further weather resistance.

Tensile strength Not given is low. 5% (presumed poor compression can mechanical be fully strength).

recovered.

Leachable chloride Not mentioned.

content is low, less than 50 ppm.

Leachable silicate content is high, greater than

1500 ppm.

'Foam does not have a particularly high strength'.

Compressive strength up to 20 kN m-2 without deformation. Bending strength 140 kN m-2.

Resists hydrolysis, Unaffected by water, alcohols, hydrocarbons, alkalis and organic most organic solvents solvents. and dilute acids and bases.

10% compression High strength to strength of 114 kPa. weight ratio. Tensile strength of 120 kPa. Shear strength 80 kPa.

Not mentioned. See Chapter 4.

Can be used with asphalt. See Chapter 4.

Non-combustible with zero flame spread.

Safe to handle. Avoid breathing dust from cutting or machining.

Not given.

Ignition point 560°C. Fire resistance B2.

None, fully biodegradable.

Can withstand up to 150"C with no reduction in performance. Rated as Class 1.

Non-carcinogenic.

30 kWh m energy Not given. consumption in manufacture.

No cyanides, chlorides or other toxic gases are produced. Rated as Class 1.

None mentioned.

Interior building applications may be covered with a fire resistive thermal barrier. Do not use welding and cutting torches on or near such foams. May cause irritation to skin, respiratory system, and eyes.

Not mentioned. None reported. CFC-free.

Can be harvested every Not given. 9-12 years during the tree's 160-200 year productive lifetime.

None reported. Uses HCFCs.

Can withstand high temperatures and when finally burnt forms a surface char, which helps to insulate the underlying foam from the fire.

None mentioned.

Low when made from recycled materials.

CFC-free.

None.

Mostly industrial applications.

None given.

Will degrade if left See Chapter 4 exposed to sunlight or water for extended periods Roof or ceiling, walls or floor.

Twice the cost of mineral wool products.

HVAC pipes, ducts, plant. Plant rooms, offices and conference suites, theatres, cinema auditoria, and recording studios.

£3.72-£29.07 per m2 depending on thickness.

None reported.

Single layer system or as part of a composite board.

None given.

Can be renewed with the application of the elastomeric top coating. Can be blown or sprayed into a cavity. Adheres to most surfaces. Conforms to irregular shapes and penetrations. Tanks, pipes, cold storage rooms, etc.

None given.

See Chapter 4.

Solid floor, suspended timber floors, partial fill of cavity walls, steel stud-framed walls, timber-framed walls, and roofs.

E6.CC per m2 with SC mm thickness.

None given. Not mentioned.

None given.

Can be fully recycled.

None given.

Cannot be melted and reused.

None given. Not mentioned.

None given.

Can be renewed with the reapplication of the elastomeric top coating. Not recyclable upon removal.

None given.

Can be manufactured from the waste stream of other polyester-derived materials such as PET bottles.

The next question is how much insulation to put into the roof, walls and floors of a building? This decision should be made early on in the design as it determines how thick the house envelope will be and, in turn, what is the most suitable construction detailing for the house. This choice is often made on the basis of cost.

In a recent article Peter Warm explained some calculations to evaluate the optimal thickness of insulation. He based the first equation on a semi-detached house in the UK, with space heating, over a 60-year lifetime with fuel costing £0.164 kWh-1 (Warm, 1997). Normal mineral wool batts at current costs are the cheapest air-based insulation available in the UK at present, and the results showed that actually the most economical thickness is around 200 mm. However this, and many other calculations are based on present fuel prices and insulation costs. We also know that over the next 20 years the price of energy must go up, though by how much is not known. What happens to the cost of insulation materials if the cost of oil and gas doubles or triples over the next 20 years? Warm also redid the calculations independently of prices. He looked at how much energy it takes to manufacture the insulation in comparison with how much energy the insulation saves over a 60-year life. It does suppose that the fuel types are equivalent but it gives a rough tool for answering the question. This changes the picture completely and the most economical thickness of insulation becomes around 650 mm! So the limitations on how much insulation we put in our homes are possibly the practical details of the mounting and fixing systems we use, rather than economic ones.

Already we have introduced a number of variables into the process of choosing the right insulation for a job, including type, fire resistance, time, local material availability, cost, and microlocation, i.e. is it an exposed or sheltered wall. Each of these factors will vary around the world because the amount of insulation required to ameliorate the climate inside a building depends also on the climate of the location (for instance maritime or continental), latitude and also altitude.

In his Doctoral study on energy-saving guidelines in South Africa, Piani (1998) calculated the amount of heating energy needed by each person to keep warm in the different geographical regions of South Africa. He recommended that wherever the heating demand was 2.20 kWh m-2 or less insulation in the roof for winter heating was not required, but in those areas where more heating was needed roof insulation would be necessary as standard. This, however, does not cover the need for insulation in the roof to keep the heat out in summer. Many readers will have suffered from the 'hot top-floor' problem caused by un-insulated roofs that act as

Proposed maximum required heating energy for different South African climatic regions in kWh m-2 per person (Piani, 1998, p. 123).

Proposed maximum required heating energy for different South African climatic regions in kWh m-2 per person (Piani, 1998, p. 123).

heat collectors during the day. It is late at night before the upstairs bedrooms are sufficiently cool to sleep in. This is especially true in countries such as India and Pakistan, where the high costs of electricity mean that only the very rich can afford air-conditioning.

Insulation is put around buildings to keep the heat in. Heat is a form of energy that is measured in the same units as any other type of energy, in Joules. Heat always flows from a hotter to a colder state, and it cannot be created or destroyed. In our homes we merely change the state of energy, or degrade it, when we burn wood or heat a kettle. The Celsius scale, with which we most commonly measure temperature, takes the freezing point of water, the beginning point, as 0°C and the boiling point at 100°C. When writing about temperature we describe a particular temperature as being X°C but the temperature difference between two temperatures we confusingly call Kelvin (K). This is named after the man who established the absolute temperature scale that starts at 'absolute zero', which is -273.15°C. Thus, the difference between 30°C and 25°C is called 5 K, not 5°C.

The specific heat capacity of a substance is a measure of how much energy it can store. For different materials, it can be described as how much heat energy is required to raise the temperature of a kilogram of the material by one degree Kelvin. Materials can store very different amounts of heat, have very different densities and be better or worse conductors of heat (Table 3.2).

Table 3.2. The relative density, conductivity and thermal capacity of a range of materials. Note the excellent thermal capacity of water. This makes it an excellent storage medium for heat

Type of material

Thermal capacity (J kg K1)

Density (kg m 3)

Conductivity (Wm-1 K-1)

Lead

126

11 300

37

Expanded polystyrene slab

340

25

0.035

Polyurethane

450

24

0.016

Steel

480

7800

47

Mineral fibre batts

920

35-150

0.035-0.044

Brick

800

1700

620-840

Glass

840

2500

1.100

Plasterboard

840

950

0.16

Marble stone

900

2500

2.0

Adobe

1000

2050

1.250

Concrete

840-1000

600-2300

0.190-1.630

Wood wool slab

1000

500

0.100

Dry air

1005

Strawboard

1050

250

0.037

Timber hardwood

1200

660

0.120

Chipboard

1300

660

0.120

Timber softwood

1420

610

0.130

Urea formaldehyde foam

1450

10

0.040

Phenolic foam

1400

30

0.040

Cork

1800

144

0.038

Water

4176

1000

There are three different ways in which a wall can be insulated:

• Resistive insulation. This is what most of us think of as insulation. These are the 'bulk' insulation products, which include mineral wools, strawboard, wood-wool slabs, glass fibre products, kapok, wool and cellulose fibre. They also include expanded and extruded polystyrene, polyurethane, urea-formaldehyde, vemiculite and perlite.

• Reflective insulation. This requires a highly reflective material, aluminium foil, to face a cavity across which high levels of radiant heat are being transmitted. The foil reflects the radiant energy back across the cavity, rather than absorbing it. This type of insulation will not work if the face of the foil is touching the opposite wall.

• Capacitive insulation. This is often described as 'thermal mass'

and is found in buildings in the form of 'heavy walls'. While resistive and reflective insulation work instantaneously, capacitive insulations affect the timing of the heat flows. The difference is best illustrated by the comparison of:

2 400 mm of dense concrete slab (also U-value = 2.17 W m-2 K-1).

Under steady-state conditions there will be no difference in heat flow through the two slabs (Zold and Szokolay, 1997). A significant difference will, however, occur if these slabs are exposed to a periodically changing set of conditions. Figure 3.4 shows the variation of the heat flow rate over 24 hours at the inside face of the two slabs, exposed to the same external temperature variation whilst the indoor temperature is kept constant. The daily mean heat flow is the same but the two sinusoidal curves differ in two ways:

1 The heavy slab's heat flow is delayed: the term 'time lag' is defined as the difference (in hours or days) between the peaks of the two curves.

2 The amplitude (mean to peak) of the heavy slab's curve is reduced well below that of the lightweight material. The ratio of the two amplitudes is called the decrement factor.

A simple rule of thumb to use when sizing mass in a very passive building, designed to minimize heating and cooling loads, is that the optimal depth of mass for diurnal use is 100 mm for each exposed surface. So, if rooms back onto each other the walls should be 150-200 mm thick. In more extreme climates the time lag can be increased, or the decrement factor decreased, by altering the width of the mass wall.

The periodic heat flow through a light and heavy wall of the same U value.

The periodic heat flow through a light and heavy wall of the same U value.

In a colder climate, for instance in Glasgow, a heavyweight building ensures more even and comfortable conditions than the lightweight building and uses slightly less energy to do so if the heating and occupation are regular on a daily basis. If a house is left unoccupied and then has to be reheated, the lightweight version uses less energy than the heavyweight building. During the heating OFF period the house will cool down considerably. However, with intermittent heating (e.g. night shut-down) but continuous occupancy, as in most residential buildings, the heavyweight version would maintain acceptable conditions during the OFF period (Szokolay, 1997). This is why so many of the older houses in Britain, and many other countries, had such thick walls: to keep the warmth in during winter when they were continuously occupied, and the heat out in summer. This does not work when they are used as weekend cottages.

Constructional section through the roof/wall junction of the Oxford Ecohouse (David Woods).

Constructional section through the A, internal ground floor wall; and B, first floor external wall of the Oxford Ecohouse showing the position of the insulation and damp proof membranes (David Woods).

A more flippant rule of thumb for the optimal thickness of insulation could be 'Think of a number and double it'. Insulation will pay dividends for many years when the price of heating energy rises and the price of insulation materials with it. It is not uncommon in some countries such as Switzerland to use up to 500 mm of bulk insulation in the roof and 300 mm in walls. For these thicknesses traditional rafters are difficult to detail and new materials, such as pre-formed masonite beams, are often used that can be specified to such depths (Vale and Vale, 1999). Nylon wall ties up to 300 mm wide are available for cavity walls from K. G. Kristiansen of Denmark (Fax: +45 755 08716).

It is very important to get the construction details correct, and to build correctly on site. Very close site supervision is needed to ensure that the insulated cavities are not filled with dabs of mortar from the walls that will then form cold bridges across the cavity. When sprayed insulation is used great care must be taken that corners are properly filled. Wall batts must be fixed so they do not sag in cavities or in the roof space and insulation must be properly cut to fit snugly against surfaces. In cold climates the water tanks must be included within the insulation envelope to prevent them freezing.

Corners are particularly difficult to get right and their filling should be supervised to ensure that the most vulnerable elements in the building are not exposed by poorly fitting insulation. In areas subject to driving rain care should be taken to place a cavity between the insulation and the outer skin of the building down which water can run without soaking the insulation. If only one side of a cavity has insulation then proper wall ties must be used to hold the insulation in place to maintain the cavity in its right location in the wall.

It is no use putting in very high levels of insulation when the windows have a much lower thermal performance, so very good walls should have very good windows. Check with local window manufacturers about the performance data for available ranges of windows and choose the best windows you can afford, because these will pay dividends over the years.

For a more detailed account of how to calculate insulation thicknesses see Zold and Szokolay (1997). For an excellent guide to detailing insulation into different building types see BRE (1994).

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