by Chris Magwood
Since the start of the straw bale resurgence in the mid 1990’s, builders have been advocating and using mesh products of various kinds to provide plaster reinforcement at junctions between the plastered bale walls and windows, doors, floors, ceilings and other intersections.
More recently, a focus on air-tightness (especially in cold climate buildings) has seen the use of mesh to cover air barrier materials at these junctions.
Cutting and applying strips of mesh is one of the slowest and most labour intensive elements of preparing a bale wall for plastering. It uses lots of staples and pins and takes time to get it done properly. And if it’s not done properly, the finished plaster can turn out to look less than desirable and the building may be leaky.
This year, we decided to experiment with a European tape product that is meant specifically for sealing junctions with plastered finishes. Siga is a manufacturer of air tightness products, and we have been using their tapes and sheet barriers for a couple of years. We noticed their perforated plastering tapes in a catalogue and gave them a try.
The tapes come in both exterior (Fentrim 2) and interior (Fentrim 20) versions, and in a variety of widths. All the tapes have a split adhesive strip, so that one side of the tape can be exposed and adhered to one side of a joint. Then, with one side adhered, the other side can be exposed and stuck down.
The tapes have a “fuzzy” surface, intended to help plaster adhere to the tape, as well as holes punched in the tape to allow plaster to push through the tape. While we were skeptical about how plaster would adhere to the tape, some experiments prior to installing the tape gave us surprisingly good results. Interestingly, we did experiments with two brands of tape and found adherence to the Siga was excellent and to the other brand it was very poor, so all these products are not necessarily equal.
We used the tapes to seal around pipe penetrations going through our bale wall, and to compare with the application of barriers and mesh around some of our windows and doors.
There was no doubt that the tape was much faster to apply. With one side stuck to the framing or to the pipe, we found the other side actually stuck reasonably well to the straw itself, enough so that the tape didn’t require any further help in the way of staples or pins to lie flat against the straw.
Plastering was equally easy over the tapes, though a square edge trowel can catch the holes in the tape. Plaster adhered whether applied by hand or by trowel.
The building was very air-tight (less than 1ACH/50) upon completion, so the taped seams worked as well as the mesh and barriers.
While it is still early to speak definitively, there was no more propensity for cracking where we used the tape than where we used mesh.
Siga is a very responsible company, and the adhesives on the tapes meet the most stringent European air quality standards. The total cost for the tape is slightly higher than the mesh and air barrier material it replaces, but easily makes up for the difference in labour time/costs.
This is definitely an approach that we will be using again.