Royal Agricultural

College

Defects in existing buildings

Most buildings are oldMost buildings have

defects

Royal Agricultural

College

Topics covered

• Defects due to water damage• Defects due to biological agents• Defects due to mechanical damage

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Why is water a problem?

• Water is everywhere, you cannot keep it out

• “Universal solvent”• Water is required for biological

activity• Water expands when it freezes

Royal Agricultural

CollegeDamage to finishes

Black Spot Mould damage

Dissolving of materials

Frost damage

Dry rot fungal damage

Rust,corrosion

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College

• Rising damp (ground water)

• Penetrating damp (rain)

• Wet services (leaks)

• Condensation (humidity/dew point)

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Rising damp: remedial work• “Solutions” to rising

damp problems:– Internal “tanking”, placing

an impervious layer between the wet wall and the interior.

– Inject a chemical “DPC” but these can be ineffective in some situations

– Insert porous tubes, open to the outside to ventilate the interior of the wall

– Cut out a course of bricks and insert a full DPC. Expensive and disruptive. But it works

Royal Agricultural

College

Preventing penetrating damp

• Cavity construction: the cavity is there to drain away penetrating water.

• Impermeable “rain shield” wall covering, such as metal or plastic sheeting

• Complete DPC systems around all sides of all openings

• Good detailing• Good maintenance

“rain shield” often glass or sheet metal

Cavity wall: rain exclusion

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College

Remedial work

• Fix any damage to roofs and openings.

• Apply a “breathable” render, the “over coat” principle

• DON’T apply waterproof paints or coatings. This is will not work

Overcoat soaks up rain

Moisture dries out in dry periods

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College

Water damage from services

• Many problems caused by water are due to leaks from water and central heating pipes

• Check that what appears to be a leak in a cold pipe or tank is not in fact condensation on the outside of a perfectly sound fitting.

Cold water

Cold tank

Condensation drips

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Condensation• The air holds water vapour as a dry gas. • The quantity it can hold depends on the air

temperature.• When the maximum amount is held for the

prevailing temperature, the air is “saturated” (100% relative humidity).

• The temperature at which air becomes saturated by the amount of water it holds is called the “dew point”

• If further water vapour is added to saturated air, or if the temperature falls, the excess water vapour condenses out as liquid water.

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Surface condensation

• Surface condensation occurs when the temperature of an internal surface falls below the local dew point temperature. Liquid water condenses out on the surface.

• Risk of surface condensation is high when– internal temperatures drop– water vapour increases (in a shower room or

kitchen during cooking)

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“Boundary layer” of still air

Inside Outside

The few mm of air adjacent to the surface is held still by friction and acts as an insulating layer.

This is more pronounced on the internal surface than on the external, where wind blows the boundary layer away

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Temperature gradient through a wall

10° C on surface

5° C outside

Inside Outside

Air temp drops sharply in boundary layer on a cold wall

20° C inbody of room

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Low dew point scenario

10° C on surface

5° C outside

20° C inside air

Air is holding only a small amount of water (low relative humidity) and so the dew point is low, say 4° C

Inside Outside

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Low dew point scenario

10° C on surface

5° C outside

20° C inside air

Dew point 4°C

Low risk of surface condensation

Air is holding only a small amount of water (low relative humidity) and so the dew point is low, say 4° C

Inside Outside

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College

High dew point scenario

10° C on surface

5° C outside

Inside Outside

20° C inside air

Air is holding a large amount of water (high relative humidity) and so the dew point is high, say 12° C

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College

High dew point scenario

10° C on surface

5° C outside

Inside Outside

20° C inside air

Air is holding a large amount of water (high relative humidity) and so the dew point is high, say 12° C

Dew point 12°C

High risk of surface condensation

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College

Remedies for surface condensation

• Reducing water vapour content in the internal air– Ventilation– Cut down on water generating activities– Air conditioning

• Keeping surfaces warm (i.e above the dew point temperature), but there’s a problem…

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College

Internal insulation can cause interstitial condensation

5° C outside air temp

20° C inside air temp

Insidedew point 12°C

Outside dew point 4°C

Internal insulation

Thermal gradient through wall

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College

Internal insulation can cause interstitial condensation

5° C outside air temp

20° C inside air temp

Insidedew point 12°C

Outside dew point 4°C

Internal insulation

Thermal gradient through wall

Dew point gradient

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College

Internal insulation can cause interstitial condensation

5° C outside air temp

20° C inside air temp

Insidedew point 12°C

Outside dew point 4°C

Internal insulation

Interstitial condensation could occur in this zone

Thermal gradient through wall

Dew point gradient

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College

Solution: Vapour barrier on warm side of insulation

5° C outside air temp

20° C inside air temp

Dew point 4°CLayer impermeable to water vapour added on warm side: plastic or aluminium foil

Dew point 12°C

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Internal insulation can cause other unexpected problems

External air Temp -4°C

Wall surface temp 2°C

Temperature gradient without insulation

During freezing weather20° C inside air temp

0°

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Internal insulation can cause other unexpected problems

External air Temp -4°C

Wall surface temp 2°C

Temperature gradient without insulation

During freezing weather20° C inside air temp

0°

Temperature gradient with insulation

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College

Internal insulation can cause other unexpected problems

External air Temp -4°C

Wall surface temp 2°C

Temperature gradient without insulation

During freezing weather20° C inside air temp

0°

The wall is below freezing. If it is wet, it could break

Temperature gradient with insulation

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College

Summary

• Liquid water is the damaging agent• Determining the source of liquid

water is vital to avoid ineffective remedial action

• The most difficult problems usually result from uncontrolled condensation