Cavity wall

1. GENERAL
2. THE INSTALLATION OF WALL INSULATION
3. ADVANTAGES OF RIGID BOARDS COMPARED TO SOFT BOARDS
4. INSTALLATION
5. TAPING
6. ANCHORING INSULATION
7. INTERNAL CONDENSATION
8. CALCULATION OF THE U VALUE FOR CAVITY WALLS IN BELGIUM

1. GENERAL

The principle of every cavity wall (regional differences in wall thicknesses and cavity widths left out of consideration) is as follows:
1. Half bond masonry outer cavity leaf
2. Cavity
3. Half or header bond masonry inner cavity leaf
4. Internal plasterwork

The function of a cavity wall is to prevent any transfer of water to the inner cavity leaf.

In general, it can be stated that:
outer cavity leaf = rain barrier
cavity = capillary groove
inner cavity leaf + plaster = airtightness

Allowance must be made for the fact that, in downpours, sooner or later (according to the capillarity of the brick) more or less water will run off at the cavity side of the outer cavity leaf. This water must be drained outside at the cavity seals.
No definitive guarantee can be given with regard to rainproofing if:
- the inner cavity leaf is highly air permeable (e.g. clean masonry)
- the outer cavity leaf is constructed with non-capilliary bricks
- the cavity is (too) narrow

The cavity may or may not have extra ventilation. It is said to be extra ventilation when one or more joints above or below is left open per metre length.
During the construction, the inner and outer cavity leaves are connected to each other with cavity anchors.

Refer to the UBatc information leaflet “Insulated cavity walls with masonry walls” for an exhaustive description of the architectural design and implementation principles.

2. THE INSTALLATION OF WALL INSULATION


• Full cavity wall

The cavity is completely filled with insulation.

Advantage:
- position is less critical
- no chance of convection flows

Disadvantage:
- the insulation is in direct contact with the wet outer cavity leaf
- certain insulation products become damp slowly
- the negative influence on the insulating capacity
- the outer cavity leaf can only dry via its external side
- the outer cavity leaf cannot be painted, or constructed from damp-proof materials

Not recommended for:
- highly exposed façades (façades higher than 25 metres)
- facades of buildings in the coastal region higher than 8 metres
- all façades of buildings along the beach
- in strongly damp-proof façades, e.g. façades with waterproof paint or with the use of glazed bricks.

3. ADVANTAGES OF RIGID BOARDS COMPARED TO SOFT BOARDS

- no subsidence under the influence of gravity and the insulation boards becoming wet
- easily workable, perfect joints
- no irritating fibres
-  the same thickness of insulation everywhere (with soft boards indentations occur at the cavity anchor positions)

(1) From 2010, the minimum thicknesses depend on the insulation plugs and anchors used.
(2) Thickness to be determined from the PHPP calculation

4. INSTALLATION 

 

5. TAPING

To optimise windproofing, the seams can be taped with Rectitape. Taping the seams is not compulsory however but optional. When applying Rectitape, the whole strip is pressed down very well using a spatula.
When Eurowall boards are installed in two layers the seams can also be taped to optimise windproofing.

6. ANCHORING INSULATION

Eurowall is always mechanically anchored with a minimum of 5 fixations per m² or 3 fixations per Eurowall board.

The anchoring can be done in two ways:

1. Cavity anchors are cemented into the inner wall. Then the Eurowall boards are slid over them and secured with insulation clamps. Any openings are then sealed with PUR foam. This method is still used at a lot of sites.

2. Thermally improved cavity anchor (e.g. Combifix from Borgh)
The plastic insulation plugs and cavity anchors are only secured after installation of the Eurowall plate. The insulation plugs are made of plastic and the cavity anchors are fixed in them. In this way the cavity anchor is completely wrapped in plastic so that it insulates better than a traditional cavity anchor.

 For use with insulation thicknesses up to 200 mm.

Advantages:
1. Limited point cold bridge due to plastic plug
2. No openings around the cavity anchor
3. Can be used for insulation thicknesses up to 200 mm
4. Good positioning ensured
5. Very practical execution
6. Safe

FIXATION

7. INTERNAL CONDENSATION

With partial cavity filling, the internal condensation always occurs on the inner side or the outer cavity leaf and NEVER in the insulation.

Calculation results also show that no allowance has to be made in the buildings listed below for internal condensation with a cavity wall.
- Houses, social housing
- Appartments
- Care homes
- Offices with little air conditioning, i.e. R.H. ≤ 60%.

The cavity insulation has no effect on a relative humidity that is too high.

According to Prof. HENS of the Katholieke Universiteit Leuven the breathing of walls is no substitute for ventilation (extract from the report published by FECHIPLAST).

A moisture permeable insulation material is no better or worse than a vapour-proof material.
Good ventilation as a means of getting rid of moisture is much more efficient than the breathing of walls.

8. CALCULATION OF THE U VALUE FOR CAVITY WALLS IN BELGIUM

R_tot= R_si + R₁ + R₂ +…. + R_isol + …. + R_se

U_c = 1/R_t + ∆U_cor + ∆U_g + ∆U_f

Where:

∆U_cor   = term of correction for dimension and position

∆U_g      = term of correction for air gaps and holes
                 Because Eurowall has a tongue and groove system, this correction term is 0 W/m²K

∆U_f       = term of correction for mechanical fixation  

 
REQUIRED INSULATION THICKNESS (in mm)

The abovementioned thicknesses are guidelines, but may vary depending on the construction..