Category Archives: Cladding

Building Science and Physics

I recently came across a few videos from America on the subject of building physics. They may help the self builder when trying to figure the wall system, roof design or insulation to use.

The videos are presented by Joseph Lstiburek who outlines the do’s and dont’s in a very direct manner.  He is the founder of Building Science Corporation.

In the videos he references the American method of describing heat loss which is the R value (resistance to heat loss per inch- a higher number is better) while in Europe we mainly use the U value (ease in which heat travels through an object-a lower number is better but it includes boundary air films). The R value is the thickness of the insulation divided by the K value or in the examples  presented by Joseph Lstiburek the R value of 2 of the Irish building is approximately equivalent to a U value of 0.5.

This video starts with the progress for insulating buildings in 1000 years (starting with an Irish Church) and covers the perfect wall, roof and slab and the importance of designing buildings for the climate they are situated in.

Video  (Below)

Commercial Thermal Bridging , LEED Building Problems, Water problems.

Physics Discussed (2nd law of thermodynamics)

  • Heat flow is from warm to cold
  • Moisture flow is from warm to cold
  • Moisture flow is from more to less
  • Air flow is from a higher pressure to a lower pressure
  • Gravity acts down

Quality Assurance-Figuring out what the right thing to do is

Quality Control– Executing it

The building layer order of importance for a wall, roof and slab and the importance of continuity between the layers as shown below in order of importance.

  • Water control layer
  • Air control layer
  • Vapour control
  • Thermal control

The 500 year wall-Keep the water out of it. Allow the vapour to get out from the inside or outside if it gets in. Keep the air out of the wall from the outside and inside. Put all the thermal layers on the outside and put the cladding on the outside.

He also analyses the LEED energy standard.

To Vent or not to Vent (Roofs-cold, warm and SIPS)

This video covers venting and airtightness of SIP roofs.

What happens when one uses a white roof membrane versus a black one.

Building Enclosures

Why increasing insulation is a game changer in the future . Moisture and durability issues that lie ahead because of extra insulation.

 

Passive House Exterior Cladding

Exterior Cladding Decision

There were a number of choices when it came to the exterior cladding.  Because the house is a timber frame I went for a ventilated facade with cement boards. The decision for selecting this was based on the following:

  • Because the house is highly insulated very little heat will travel through the wall structure. This entailed changing the wall build-up on the advice of our timber structural engineer. On a normal timber frame one installs a rigid OSB board on the outside and this then would typically be covered with a membrane and then battened for the cement board or tied to block work. When the timber structural engineer carried out a Wufi analysis (hygrothermal analysis-how the wall behaves with our humidity levels in Ireland) he advised that the structural strength of the OSB board would be affected if we placed it outside because of moisture build up. The OSB was then placed inside with only the vapor barrier on the outside and then the outside was battened /counter battened for the cement board.
  • My preference was to use block work on the outside (because of cost advantage) but this showed up in Wufi as needing large amount of ventilation and it would not perform as well as a cement board or a wood finish wall.  It seems that if one used block work the heat from the sun would take a long time to reach the ventilated space. The ventilated space it appears requires two pieces of physics to work correctly-Thermal Buoyancy (warm air rising and creating a drying out environment)  and Wind for the ventilated cavity. Relying on one of these I felt was a risk I did not want to take.
  • The other reason for selecting a ventilated cavity versus using an externally insulated (with a non breathable product) was the provision of a second level of protection to the wall structure if there was a fault in the external waterproofing . If for example water leaked in around a window detail it would dry out if the wall was ventilated but if water got in behind a wall that was externally insulated with Polystrene/EPS/PIR/PU  the opportunity to dry out was I felt limited.

By selecting a cement board (around 12mm thickness) if the sun shines on this  it would within a very short period of time let the heat transfer to the inside of the ventilated cavity and increase the drying out of the wall and also keep the insulation dry so that it can perform at its rated value.

Some Details

The membrane was glued around the external structure of the timber frame and wall to ensure that no wind would be blowing over the face of the insulation ie. Minimise thermal looping.

Orcon F membrane
Membrane glued with Orcon F to concrete and wood structure

 

 

 

 

 

 

 

 

 

 

 

The membrane was placed over the Insulation and glued to structure as seen above.

Metac Insulation, Passive House
220mm Metac Insulation placed between external structure before membrane applied.

 

 

 

 

 

 

 

 

 

 

 

External Membrane applied over Insulation

Solitex WA
Membrane covering Insulation

 

 

 

 

 

 

 

 

 

 

 

Battens for Cement Board Cladding

Preparation for Cement Board Cladding
Counter Battens On Window Walls

 

 

 

 

 

 

 

 

 

 

 

Aluminium ventilation vents for walls.

Wall ventilation for exterior Cladding
Aluminium Ventilation Grille

 

 

 

 

 

 

 

 

 

 

 

Cement Board sample

Cement Board

Cement Board Sample

The Cement Board is screwed onto the battens with stainless steel screws. A base coat and mesh is applied first and then a primer. The final finish is a Ral colour acrylic render to make the cladding water proof. If one has a walls facing south west and they are subjected to high rainfall it may be better to install a Silicone Silicate render or check that a chemical agent is included to reduce/eliminate algae growth that can appear as a green discoloration on the render over time.

Acrylic Render
Acrylic Render Finish

The overhang underside was also clad with an acrylic render.  The image below shows the overhangs shading the top windows. For example by around 9:30 AM in June the top windows are completely shaded for the sun and by 14:30 all the lower windows are in shade in order to protect the building from overheating.

Inter-connector between two buildings with 1.8m Overhangs.