Dynamic Insulation1

8/13/2019 Dynamic Insulation1

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Dynamic insulation


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DYNAMIC INSULATION MATERIAL

Material where the thermal conductivity canbe controlled within a desirable range.


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DYNAMIC INSULATION

Its a form of insulation where cool outside airflowing through the thermal insulation in theenvelope will pick up heat from the insulationfibers.

The overall heat transfer coefficient is nolonger constant.

It varies with the speed of air flowing throughthe insulation.


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Permeable walls

Dynamic insulation requires permeable wallssuch that when building is depressurized aircan flow from outside to inside through theinsulation.

This type of Dynamic insulation is mainly usedin cold climate where main energy use is forheating


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conventionally insulated air-tight wallDynamic insulated wall


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DYNAMIC INSULATION

As air flows inwards, it picks up the heat thatis being conducted outside by the insulationglass.

It serves two purposes:1. Reduces the heat loss.

2. Pre-warmed air is pumped indoor.


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Mechanical ventilation

Dynamic insulation requires a mechanicalventilation with heat recovery.

For example for air to be drawn continuouslythrough the walls a fan is needed to hold thebuilding at a pressure of 5 to 10 pascals belowthe ambient pressure.


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SCIENCE OF DYNAMIC INSULATION

consider an ideal case of one-dimensionalsteady state heat conduction and air flowthrough a uniform sample of air permeableinsulation.

distance x measured from the cold side of theinsulation.


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GOVERNING EQUATION

u =air speed through the insulation (m/s)Ca= specific heat of air (J/kg K)a =density of air (kg/m 3)

a =thermal conductivity of the insulation(W/m K)


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GOVERNING EQUATION

Boundary conditions1. T(x)=TO at x=0

2. T(x)=TL at x=L


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TEMPERATURE PROFILE

air flowing through a slab of celluloseinsulation 0.2 m thick in which one side is at atemperature of 20 C and the other is at 0 C
http://en.wikipedia.org/wiki/Cellulose_insulationhttp://en.wikipedia.org/wiki/Cellulose_insulationhttp://en.wikipedia.org/wiki/Cellulose_insulationhttp://en.wikipedia.org/wiki/Cellulose_insulation


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TEMPERATURE GRADIENT

Temperature gradient as a function of x:

Temperature gradient at x=0:

Temperature gradient at x=L:


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Dynamic u value:

Air speed u, (mm/s)Temp gradient at

x/L=0 (C/m)Conductive heat

loss (W/m 2)Udyn (W/m 2 K)

0 100 4 0.2

0.25 41.8 1.672 0.084

0.5 14.6 0.584 0.029

0.75 4.49 0.1796 0.009

1.0 1.26 0.0504 0.003


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Disadvantages On the warm side of the insulation the

temperature gradient gets steeper withincreasing air flow.

the insulation at 1mm/s the temperaturegradient on the warm side of the insulation x/L =1) is 621 C/m which compares with only 100C/m for the conventional insulation.

Specifically a space heating system six time largerthan that for a conventionally insulated housewould be needed


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Disadvantages

For a vertical internal surface this thermalresistance has a value of 0.13 m 2 K/W. [6] In adynamically insulated wall.

As the conduction heat flow into the wallincreases then so does the temperature dropacross this internal thermal resistanceincrease.

The wall surface temperature will becomeincreasingly colder.


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Temperature drop:


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Temperature drop

Air speed u,(mm/s) Temperature drop across the air film (C)

0 0.52

0.25 1.02

0.5 1.69

0.75 2.44

1.0 3.23


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Dynamic insulation materials

Insulations such as polyurethane (PUR)boards, which due to their micro-structure,are not air permeable are not suitable fordynamic insulation.

Insulations such as rock wool, glass wool,sheep's wool, cellulose are all air permeableand so can be used in a dynamically insulatedenvelope


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Air control layer

The maximum depressurization for adynamically insulated building is normallylimited to 10 Pa in order to avoid doors

slamming shut or difficulty in op The function of the air control layer in a

dynamically insulated wall or ceiling is providesufficient resistance to the air flow to achievethe required pressure drop at the design airflow rate.


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Air control layer The permeability of a material to air flow, , (m 2/hPa)

is defined as the volume of air that flows through acube of material 1m X 1m X 1m in one hour

A area of material through which air flows (m 2) L thickness of material through which air flows (m) V' volume flow rate of air (m 3/h) P pressure difference along the length L of material

(Pa)


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Measured Air Permeability of BuildingMaterials

MaterialDensity(kg/m 3)

Permeability(m 2/hPa)

ComponentPermeance(m 3/m 2hPa)

PressureDrop 1 (Pa)

Plasterboard

- 1.06x10-512 mmsheet

8.81x10-4 1140

Thermalblock

850 1.6x10-5100 mmblock

1.6x10-4 526

Fibreboard - 1.34x10-312 mmsheet

0.116 8.6

Pumalite 870 0.036100 mmblock

0.36 2.8

Cellulose /

wet blown47 0.283 200 mm 1.50 0.67

Cellulose /dry blown

65 0.25 150 mm 1.67 0.60

Sheep'swool

28 1.8 140 mm 13.0 0.08


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Design of a dynamically insulatedbuilding

Assume a house of 100 m 2 floor area with adynamically insulated ceiling.

air change rate for good air quality. 2.4m height,100 square meter area, lets say in

one hour the entire volume of air must bereplaced, we have air flow rate of 240m 3/hr.

Next the material for the air control layer ischosen to provide a suitable air flow rate at thechosen depressurization, taken as 10 Pa in thiscase


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For a 12mm thick sheet of fiberboard thisgives, for the maximum pressure difference of10 Pa, an air flow rate of 1.12 m 3/h per m 2 ofceiling.

This is equivalent to an air speed through theceiling of 1.12 m/h or 0.31 mm/s.


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Dynamic insulation works best with a goodthickness of insulation so taking 200 mm ofcellulose insulation (k = 0.04 W/m C).

The value of U= 0.066 W/m 2 C. Design of a mvhr with an air flow rate of

128m 3/h.


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Thermal Insulation Materials

used in eco-renovation


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Heat loss through building elements

Building feature Heat loss(%) Target U-value Possible solutions

Walls 35 0.15 Cavity, internal orexternal wallinsulation

Windows and doors 15 1.6 Double/triple/secondaryglazing/shuttersand curtains

Roof 25 0.10 Pitched, warm deckor cold deck roof

insulationFloor 15 0.15 Floor insulation

Gaps, cracks ,draughts

10 Draught proofing


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Depth of insulation required to reach aU-value of 0.15W/m2K

Material Depth

Expanded polyurethane 130 mm

Unfaced polyurethane 160 mm

Rockwool (60 100kg/m3)

195 mm

Glassfibre slab 205 mm

Expanded polystyrene 215 mm

Mineral wool 225 mm

Flax 230 mm

Cork board 240 mm

Glass fibre quilt 240 mm

Cork slab (160kg/m3) 250 mm

Woodwool board 250 mm

Cellular sheet glass 280 mm

Foam glass (140kg/m3) 305 mm

Cork slab (140kg/m3) 325 mm

Foam glass (130kg/m3) 330 mm


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Most climate friendly insulationmaterials

Material Embodied carbon (kgCO2e)

Cork slab (300kg/m3) -155

Cork slab (160kg/m3) -70

Cork board -65

Woodwool board -35

Flax -5

Recycled loose cellulose -1.9 Glassfibre quilt 3

Rockwool 7

Glassfibre slab 8

Cork board 9

Rockwool (60kg/m3) 13

Expanded polystyrene 15

Rockwool (100kg/m3) 20

Cellular sheet glass 28

Foam glass (140kg/m3) 30

Foam glass (130kg/m3) 31

Mineral wool 38

Expanded polyurethane 160

Unfaced polyurethane 175


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Approximate cost per cubic meterMaterial/product Price

Recycled loose cellulose (Warmcel) 11.67

Expanded Polystyrene Board (Jabfloor 70) 13.56

Woodwool (NOVOLIT) 33.33

Black Mountain sheeps wool 46.66

Rockwool quilt 53.81

Mineral wool slabs 56.91 Woodwool (HERAKLITH) 59.33

EPS Jablite Polystyrene Sheet 75.52

Cork sheet 80

Hemp Steico Canaflex 81.28

Woodfibre batts (Steico Flex) 106.89

PIR (Celotex XR4000) 117.93 Woodfibre batts (NaturePro) 127.74

PUR (Kingspan Thermawall TW50) 151.54

Woodfibre Board (Steico Therm) 176.66

Hemp batts (Black Mt) 317.98

Woodwool KOMBIVOL 330


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Material K-value(W/mK) Notes

Sheeps wool batts androlls

0.038-0.043 Can absorb some moisturewhilst remaining efficient

Wood fibre batts 0.038-0.043 Good for most walls,ceilings , roofs

Cotton-based batts androlls

0.038-0.043 Best for horizontal surfaces

Cellulose 0.038-0.040 Recyclable, renewable

Cork board 0.042-0.050 Commonly used asunderlay under hardwood

Wood fibre board 0.080 Good for wall and pitchedroof construction

Hemp batts 0.043 Relatively expensive

Properties of some Organic materialscommonly used in insulation


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Reference

http://www.superhomes.org.uk/resources/whats-best-insulation-material/
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