All wrapped up: the building covered in wall wrap and some of the high cladding

Wrap and High Cladding

Andrew Kerrby author page

4 comments Construction

October

That’s a wrap

Once the roof was on it was time to wrap the building. This used to be done using ‘building paper’ – similar to the sisalation installed under the roof, building paper has a reflective side and a matte side. The reflective side either faces inside to keep warm air from escaping the building or faces outside to prevent hot air entering the building – this depends on the climate zone.

 

Light and airy: plenty of ventilation beneath the gable (before it gets covered up)

Light and airy: plenty of ventilation beneath the gable (before it gets covered up)

Breathe

Advances in mechanical heating (heat pumps), combined with buildings becoming better sealed (for energy efficiency), have led to an increase in condensation. In the past the temperature difference between inside and outside was not that much, there were more draughts in buildings, and people used to open windows and doors for ventilation to ‘air out’ buildings. Now many people sit inside, crank up the heat pump and don’t open windows for months at a time. The warm, moist air has nowhere to escape to. Condensation forms and sometimes results in mould. Yuk!

 

The large openings not wrapped

The large openings not wrapped

 

There have also been technological developments in building materials, including thermal blankets and vapour permeable membranes. Think of a really good rain jacket, the super expensive type – the type where it can pour with rain and you don’t get wet, from the outside or the inside. Even if it’s hot and you’re moving around the sweat magically escapes – the material ‘breathes’. I’ve never owned one of these jackets before (they are expensive) but know they exist and are very good. Technology such as this now also exists for buildings. The walls of 60k House are clad in a vapour permeable wall wrap – it protects it from outside water, is air tight trapping heat inside, and allows moisture to permeate from the inside to outside.

 

Overlap: ensure the wrap laps over other sheets, and always the top layer over the bottom layer so water can run off

Overlap: ensure the wrap laps over other sheets, and always the top layer over the bottom layer so water can run off

Blood, sweat and tears

Fixing wall wrap is quick and easy – so quick I hardly got any photos! The wrap can either be measured and cut, or kept on the roll and pulled tight as you move along the building. The wrap is then held in place with foil fixers – the sharpest objects known to man! These fixers come in strips and bend off once the end one is hammered in to hold the wrap. Plenty of blood, sweat and tears have gone into 60k House, particularly Greg’s blood…

 

A few days after wrapping the building there were some very high winds – typical for spring in Tasmania. This ripped a couple of patches of the wrap off the wall frame. It’s advisable to always cover the wrap as soon as possible because of this reason (and possibly UV exposure).

 

Wind assisted: high winds ripped part of the wall wrap off the timber frame

Wind assisted: high winds ripped part of the wall wrap off the timber frame

How high?

All of the cladding above 2130 (2.13m) is 4.5mm thick cement sheet. The height is not arbitrary – the height is determined by the size of the four large windows, which corresponds to slightly higher than a standard door, and is also roughly three apple crates high. This datum runs right around the building. Below this datum the cladding will be either 6.0mm thick cement sheet (higher chance of impact), timber weatherboards (the apple crate reference), or windows.

 

Batten down the hatches: vertical timber battens to provide an air cavity and to fix the cement sheet to

Batten down the hatches: vertical timber battens to provide an air cavity and to fix the cement sheet to

 

The cement sheet (and in fact all of the cladding) is battened out on 35mm thick timber battens. This creates an air cavity between the cladding and the wall wrap that improves thermal performance and provides a space for any water (from either inside or outside) to drain to.

 

On the level: evening out the timber battens prior to fixing the cement sheet cladding

On the level: evening out the timber battens prior to fixing the cement sheet cladding

 

It took roughly one day each to clad the high parts of the four sides of the building. Most of the time spent was setting up the battens to ensure they were level – pine framing sometimes moves, especially if it has been exposed to weather (which it was before the roof and wrap went on).

 

Trimming the excess roofing sisalation

Trimming the excess roofing sisalation

 

The cement sheet is easily cut with this hand tool

The cement sheet is easily cut with this hand tool

Detail

There is a slight step at this height caused by a change of depth in materials. This step means a flashing (folded piece of metal) is needed to shed the water outside. The flashing runs behind the cement sheet and over the front of the material below (window, cement sheet or weatherboard).

 

Working it out: resolving the detail of where the face of the cladding will sit

Working it out: resolving the detail of where the face of the cladding will sit

 

The first piece of cement sheet cladding being put up

The first piece of cement sheet cladding being put up

 

Neat detail: cement sheet cladding, flashing, window head

Neat detail: cement sheet cladding, flashing, window head

 

Light and shade: the corner of the building in the afternoon sun

Light and shade: the corner of the building in the afternoon sun

The cement sheet won’t get painted. That’s it. Actually it will get a clear seal finish to prevent it soaking up any water at the edges, but it will still look the same. And it will also get some timber cover strips for extra protection. But apart from that it’s finished. I like to use natural materials and leave them expressed – don’t try to hide or colour them.

 

Second wall cladding on: the remainder of the solid wall will be timber weatherboards

Second wall cladding on: the remainder of the solid wall will be timber weatherboards

 

Enclosure: inside the building feels very different with the wall wrap up

Enclosure: inside the building feels very different with the wall wrap up

 

Costs: framing timber – $594; flashings – $170; fastenings – $384; cement sheet- $474; builder – $3,770

 

Disclaimer: Any advice contained within this blog is of a general nature only and cannot be relied upon. Details provided are in good faith and relate specifically to this project. Any author will not be held responsible for advice or information presented.

 

4 Comments

  1. Hi Andrew,
    I have been reading through your blog with keen interest.
    I am glad to see you have sound passive solar and insulation experience but wonder about the roof insulation.
    Are you aware that merely using Sisalation in the roof may also cause condensation when using sheeting. It is technically a better solution to use a blanket type insulation product between the Sisalation and the sheeting. In the past we called it FBI (Foil backed insulation) and in WA it is generically know as Anticon. This transfers the dewpoint (when there is a temperature difference between the outside & inside of the sheeting)to the outside of the sheeting, where any condensation can run off, just like rain water. This also gives you a better R-value and is also good for heat loss in cooler areas like Tasi.
    I look forward to following your blog.
    Best regards,
    Shona Murdoch
    ND Architecture
    University of Johannesburg
    Perth
    WA Australia
    0420623918

    1. Andrew Kerr

      Hi Shona, thanks for your interest and comment. I am aware of thermal blankets such as Anticon and do specify its use for certain projects. I originally considered using Anticon on 60k House but decided it wouldn’t be necessary in this instance for the following reasons: the sufficient depth of the roof (Anticon is more needed in narrow roof spaces); the roof pitch (60k House is 20 degrees – it is advisable using Anticon for shallow roof pitches such as 5 degrees or below); adequate ceiling cavity ventilation will be provided; there will be no heat pump installed to produce warm, moist air; and I will regularly open doors and windows to allow the building to ‘breathe’. The dew point (where the temperature difference between warm and cold results in water forming – condensation) for 60k House should be on the top side of the sisalation, allowing any moisture to run down away from the ceiling cavity to outside the wall line. I will let you know how it goes.

  2. Hi Andrew,

    I like your website and am currently designing my own house, which is similar to yours.

    I would have also gone with the anticon blanket below the tin roof. How did it go with just sisilation? Any sign of dripping moisture?

    Also, externally it looks like you did a cavity wall but the cavity has a closed bottom. Thus, any water that gets into the cavity will not be able to drain out but will be absorbed into the lower wooden battens. Shouldn’t you do a closed-top and open-bottom cavity wall? The bottom strip can have perforations to allow moisture to drain out and at the same time keep out spiders etc.

    Nathan

    1. Andrew Kerr

      Hi Nathan, thanks for your comment. I’m glad to hear you are designing a similar house. I talked about my reasons for not choosing to use Anticon in the comment above (roof depth, roof pitch, ceiling ventilation, no active internal heating). As yet I haven’t experienced any condensation issues but will let everyone know if it becomes a problem. As for the cavity wall I was thinking about this today when looking at my photos and writing the latest post about the cladding. Yes, you are correct that having the bottom open would allow any moisture to escape (if there is any). However a ventilated wall cavity (open both top and bottom) will reduce the U value of the wall – not a major concern. However the bushfire construction requirements that stipulate there can be no gaps greater than 3mm in the external building envelope make it difficult to leave the bottom batten out while ‘sealing’ the building against ember attack. Metal mesh with apertures smaller than 3mm can be used and this might become the norm, or even the requirement, for ventilating wall cavities. At the time of construction I didn’t notice and relied on the builder’s experience. Will keep you posted if any problems arise. Cheers

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