Building Enclosure | Building Envelope

[Andy Parkinson]

I think Building Enclosure deserves its own thread; this will obviously need to be specific to the local codes, climate, situation, economics etc, but there will also be some general principles that apply. Durability as well as energy efficiency are key factors.  I can't claim to be an experienced building enclosure expert/professional, but its an area of interest to me and I have accumulated some knowledge.

I haven't seen much detail on proposed wikihouse building enclosures:  for the UK wikihouse I see liquid rubber proposed as cladding; the project costing spreadsheet also lists vapor barrier paint, jablite board (EPS - insulation), also foil tape (for air sealing?) and poly membrane (floor/damp-proofing?).  There is also some discussion of alternative materials to plywood, such as eco-sheet, OSB, fiber-cement board, MgO board.

The appropedia article also provides a good summary of the system: http://www.appropedia.org/WikiHouse . Relevant to the BE discussion is the point on roof ventilation that "excessive heat build-up below the roof material will cause warping of the plywood sheets. Roof-top ventilation along the ridge (with gable ventilation if appropriate) will avoid this". Also in reference to the integrated guttering that "When incorporating a hidden guttering system into the WikiHouse a gradient must be included to ensure that water does not pool at one end of the channel. The channel must extend at least 25mm beyond the roof-line in order to avoid water 'fly-over' i.e. flowing onto the fascia boards, soffits and down the wall of the WikiHouse through surface tension. Water damage can occur in the plywood below if this is not attended to...If Ecosheet is used for these wet details then no further waterproofing is necessary. A strip of duct tape will prevent leakages at the facing edges where the channel panels meet."

So thats a quick summary of what I have seen proposed regarding envelope/enclosure systems.

First off a couple of points:

1) I like the deep fins of the chassis, give plenty of room for insulation within the assembly (with a wide choice of insulation material options), and thermal bridging through the plywood fins will be fairly minimal.  Theres potential to increase the depth of the fins to increase insulation, or to add exterior insulation.

2) In terms of material properties plywood is preferable to OSB.  It is a 'smart vapor retarder' in that the higher its moisture content the higher its permeance (therefore helping the assembly to dry). OSB is much less forgiving: if it gets wet it turns into vertical mulch.

3) I think the integrated gutter idea is potentially a very risky feature, it would need to be perfectly sealed, and there's no forgiveness in the system in the case of leaks.  The above comment about using eco-sheet with duct taped seams to prevent leakages, sounds like a recipe for disaster?

4) The potential of warping of plywood (roofing) sheets in the case of excessive heat is another concern; and suggests a ventilated roof solution (which would also be more durable / less leak prone) i.e. furring strips applied over the outer sheathing of the chassis (or some other kind of stand-off system), then additional sheet of plywood with liquid rubber, shingles, bituminous membrane (aka tar paper / roofing felt) etc (other option would include corrugated metal roofing etc).

5) Building assemblies need to be designed to dry in case that they do get wet.  Either they need to be able to dry to the interior or the exterior, or preferably both.  Again this is a limitation of the liquid rubber cladding option as it will limit drying to the exterior, but risk will depend on climate and assembly details.

I appreciate the solutions will be location and climate specific, will require some understanding of basic building science, and that what has been proposed to date is just a starting point.  Some of the solutions may mean adding 'layers' which adds cost, but with some thought we should be able to minimize material layers and still build a durable system (for example plywood can be structural sheathing, with taped seams it can be the air barrier, and it is a vapor retarder, so several functions in one material).

So I hope the above comments are enough to get a discussion going?  Interested to hear other's ideas or questions on building enclosure. 

[Chau Huh]

I'm from the Pacific Northwest and my comments apply to this region where we have a decent swing in temperatures and a good amount of moisture.

Your comment concerning the plywood substitution over OSB is a must. I would take it so far as to also create the entire floor structure using plywood over OSB as well.

My choice of insulation and the skin would be as follows. 

• Slightly thicker walls so I can stuff as much straw in my walls as possible, plus I'm partial to wide window sills.
  
• Metal roof on purlins or furring to allow for some breathing room between the structure and the metal. A ridge vent would be great here I think.

So my wall would read like so from interior to exterior

• OSB
• Structure
• Straw
• Plywood
• Tyvek or (2) layers 30 minute building paper
• Vertical furring, could be designed into the structure but it would make applying the building paper more difficult
• Metal siding, the bottom of the exterior metal siding should be vented. This will allow air movement from the bottom to the top keeping things dry from any moisture build-up from the interior.

The roof would be similar

• OSB
• Structure
• Rigid insulation. higher values in less space
• Plywood
• Tyvek or (2) layers 30 minute building paper
• purlins, could be designed into the structure but it would make applying the building paper more difficult
• Metal siding with ridge vent

I would also add an impermeable membrane (peel and stick) under the building paper at the ridge, and where the 'rafters' connect with the walls.

Thoughts?

Thanks.