Your Build – Summer 2019

Phase 2 Framing Your Build

F

abric first is a design philosophy that

looks holistically at every aspect of

the building envelope in terms of its

thermal performance and ability to

prevent energy wastage due to

draughts and air leakage. It

champions combinations of materials that will

maximise a property’s service life and enhance

comfort and wellbeing for the occupier.

The Importance of Air-tightness

While insulation is often specified based on

the material’s insulating properties, with a

particular focus on its Lambda value, often the

finished project fails to meet its designed U-value

due to gaps between pieces of insulation and

poor levels of airtightness. Even a small gap

between insulation panels of as little as a few

millimetres can dramatically alter the thermal

performance of the whole installation due to

unforeseen air leakages.

Thermal bridging at the junction between

insulated and non-insulated areas of the

building envelope - such as around windows

and doors – can also compromise the as-built

U-value of a building. This, in turn, leads to a

performance gap.

With a fabric first approach to considering the

design of the whole building envelope, however,

improved thermal performance can be achieved

using natural materials, such as Gutex woodfibre

and ThermoJute 100, which offer an enhanced

ability to deliver airtightness and reduced

thermal bridging. By selecting the right

combination of insulation and installing it in

combination with an airtight vapour control

layer to prevent the free movement of warm,

moist air out of the building, a fabric first

approach can reduce both heat loss and the

risk of condensation.

What is Windtightness?

Windtightness, is a widely overlooked aspect of

best practice building envelope design, which can

Fintan Wallace, an Architectual Technologist from Ecological Building Systems, discusses

the importance of ensuring the building envelope delivers its designed thermal performance

Fabric first approach to AIR AND

WINDTIGHTNESS

also result in a finished building failing to meet

its designed thermal efficiency.

The simplest way to explain windtightness is to

consider how a woolly jumper acts as insulation

on a cold day. The wind will quickly blow the

warmth out of the fibres, reducing its ability to

insulate, unless you wear a windproof jacket over

the top. With the windproof layer in place, the

jumper can do its job much more effectively,

preventing the heat from being blown out of the

insulating layer and trapping stale, moist air

inside the construction. The same principle

applies to insulated roof and wall build ups.

The problem with preventing cold air from

entering a building, however, is that this could

potentially also result in trapping moisture from

rain during construction within the building

envelope creating an increased risk of interstitial

condensation. Consequently, the windtight

membrane selected must be able to allow

moisture to exit.

Microporous Vs Monolithic Membranes

The conventional approach to achieving a

windtight barrier is to install a microporous

membrane that allows moisture to escape by

means of a passive air exchange, which works

efficiently when there is a relatively high

difference in vapour pressure between the inside

and outside. However, the more significant the

amounts of water vapour trying to escape, the

more quickly moisture particles move towards

the available escape route and the micro-pores

are susceptible to becoming saturated and

blocked, which may result in a film of moisture

being formed. This prevents vapour from