Is natural plaster airtight? Some low-tech, world-leading research in regional New Zealand

20 February 2023 by Jason Quinn

UPDATE Aug 2023 This project’s final blower door test showed a significant improvement in airtightness, which qualifies the initial commentary made below at the time of the preliminary test. The final result is 0.32 ACH. The project team attribute the improvement largely to the completion of the second layer of plaster. A repair to a tiny air leak in one window seal was also carried out between the two tests but it’s not possible to pinpoint the extent of the impact.

Of most interest to the natural building community is the discovery that a double layer of casein wash applied to the internal plaster surfaces also contributes to better airtightness performance. The wash is a primitive home-made paint, using very cheap and readily available materials. In the past, whey would have been strained from old milk; here, casein powder was purchased from a bodybuilding supply shop and mixed with water and a small quantity of boron. The transparent wash was applied to strengthen the surface of the plaster and make it more resistance to bumps and scratches. That it contributed to airtightness was a welcome surprise to Pat Mawson and his team, the specialist subcontractors responsible for the natural building materials and finishes.

The impact of the casein wash was measured using the same low-tech but effective test of the plaster used in the preliminary blower door test. Very lightweight plastic was taped to an external wall to which casein had been applied; and to the one remaining external wall which had not yet been painted. The infiltration of air could be observed happening at very different rates once the building was depressurised.

The casein wash is applied thickly and quickly. It dries to a smooth and transparent finish.

This result can give a lot of confidence to other owners and designers wanting to combine traditional building materials with Passive House level performance. The other notable benefits of the plaster finish, as reported by the owner after a month living in the house, is its remarkable effectiveness at buffering humidity and its acoustic insulation properties.

[Feb 23—original post] Is plaster airtight? Can natural materials be successfully used to reach Passive House targets? A recent blower door test yielded some initial answers: yes it’s possible but there’s not much wriggle room.

Sustainable Engineering is involved with an interesting rural home near Whanganui targeting Passive House certification. The homeowner’s decision to go with strawbale walls and natural plasters was strongly influenced by the Hiberna strawbale home in Wanaka. There was also a late change from conventional timber framing and wallboard to Light Earth Material (LEM) internal walls, also covered in earth plaster.

It’s worth watching because it’s only the second strawbale and plaster building to attempt certification in New Zealand and the first in the North Island. Like the Hiberna home, the internal layers of plaster are designed to function as the airtight layer. The early strawbale Passive House buildings in the northern hemisphere all lined the straw with high-tech membranes to ensure airtightness, an uneasy fit with the ethos of the natural materials.

The preliminary blower door test this month was successful but very close to the upper limit for certification. Ben Eyers, who built the Hiberna home (designed by his wife Jessica), had coincidentally visited the site the week before. Both the Eyers have consulted on this project—Guy Shaw of Energy Architecture is the Passive House designer and architect. Ben noted the two sections of external wall that had not received their final coat of plaster and suggested taping plastic onto the wall as a low-tech test of a single layer of plaster. It would provide some feedback as to how much difference a second coat of plaster made with regard to airtightness.

I used the same test to identify air leakage in a different project which used OSB, then widely accepted as an airtight barrier if taped correctly. But in that case, the OSB turned out to be permeable and an airtight membrane needed to be applied. But neither Ben or I knew of this check being used on a natural plaster. Ben learned about it long after his own home was completed.

After hunting down and remedying any other leaks (rodents like high tech tape?!) and still with the house depressurised, the unfinished wall was covered with plastic. It quickly inflated with air being drawn in through the wall. This was good news because more plaster would be applied … but on repeating the same test on an area of finished wall, it became obvious air was still getting in. The ingress was considerably slower but still reached the point of maximum inflation. The same tests were done in a pressurised house and the plastic sucked to the wall like cling film.

Test on finished plaster in a pressurised environment.

Test on unfinished plaster—note the much rougher surface—in a depressurised environment.

Two teams of builders have been working on this project, the local firm managing the contract overall and specialist earthbuilders from the Hawkes Bay. Both have done a great job. The test will be repeated now that the final plaster layers are complete and all involved expect the ACH number to improve.

Finished internal coats of plaster, here still drying.

This test indicates that natural plasters, applied by experts, can hit Passive House targets but there will be little room to spare on the airtightness front. All other detailing will need to be very, very good, to make up for the less effective plaster barrier. This house is a timber frame, modified to suit the depth of the straw bales, and uses Pro Clima tapes and membranes to airtight the floor-to-wall and wall-to-ceiling junctions and the roof space.

Ben Eyers was extremely interested in the result—he was phoning in from Wanaka as the test took place—as it shed light on his and Jessica’s build. They were relentless in hunting down potential leaks, even using thermal imaging technology on a cold winter’s night to help identify any remaining gaps. More effort could not have been applied to airtighting! That home is two stories and the midfloor detail was complicated and thought to be the culprit for the persistent 0.6 ACH result. But this test suggests it may be something fundamental to the plaster.

Comments 2

  1. “Not very airtight” doesnt have a scale. Many materials are “air permeable” including OSB and ‘intelligent membranes” and earthen plasters are expected to be. I’m glad there was a noticeable difference in the plastic inflation between the rough coat and the finish coat.
    Any idea how much less permeable ?
    The Hiberna house had an area of unfinished plaster behind some panelling and the same increase in permeability was evident.
    Nevertheless the Whanganui house does have some lovely finish plasterwork tho. 🙂

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