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Air sealing wooden joists to brick/masonry party walls

February 1, 2013

Joist air sealed to brick walls with CONTEGA FC and plaster.

Brick walls leak – even if they are a double, triple or quadruple wythe walls. The stacked masonry is basically a labyrinth of miniature air distribution ducts – making the assembly very porous.  A proven way to seal bricks or CMU, is good old plaster. Many old brick buildings, have original airtight 1″ thick plaster on their  party walls. However, behind baseboards and at floor joist this plaster is missing. Just plastering these two different materials – brick and wood – together won’t be durably airtight: the wood will expand and contract through the seasons,  by fluctuating moisture content and temperatures, which results in cracked plaster and leaky walls.
The solution to make these connections durably airtight, is to use a fleece adhesive tape that is adhered to the joist on one side, and embedded into a plaster (scratch/parge coat) at the other end. Connecting these airtight layers (wood and plaster), with a durable airtight fabric that can withstand small movement of the construction materials (See why this is important in blogpost: When Buildings Move – Airtightness Should Last)

This step by step guide is a method 475 has developed that builds on Pro Clima’s installation guide for masonry beams with CONTEGA FC [NOTE: Pro Clima no longer manufactures CONTEGA FC, but has upgraded to the new CONTEGA SOLIDO SL, which can be used in place of CONTEGA FC in the same way] that was also featured in this blogpost that shows several methods to airseal brick party walls. However this featured  method, makes corners thinner (no more folded rabbit ears) and installation quicker. Saving both time, tape and plaster.

Step 1
Clean off the dust. Inspect the joist for any splits/crakcs, and if any are found, fill these with CONTEGA HFFor rough cut, old beams: Smooth a one inch perimeter of the beamsat the brick wall by sanding it with a sanding block.


Step 2
Cut a pieces of CONTEGA FC that are two inches longer than the beam is high.

Note: pre-cutting tape for multiple beams with the same length in a convenient location saves time.


Step 3
Remove tape’s release paper and press it down firmly onto the joist.
If beam is very uneven (rough cut), instead of having to sand it completely smooth, you can apply a bead of CONTEGA HF to make sure the connection of the tape and rough wood is airtight.


Step 4
Cut the tape at a 45 degree angle, starting almost at the beam corner and cut the fleece/adhesive strip while pulling tape taut (use a very sharp blade in your utility knife).


Step 5
Fold the tape down around the corner of the joist. Repeat this step on the top of the beam, and repeat steps 2-5 on the other side of the beam as well


Step 6
Apply a generous blob of CONTEGA HF airtightness caulk/adhesive in the 45 degree corners


Step 7
Cut a piece of tape that is approximately 5″ wider than the beam itself. Adhere the fleece tape air-tightly to the wood short side of the joist. Again make sure the wood is even, not split and activate the adhesive by pressing the tape down firmly.


Step 8
Cut 45 degree corners where tape extends beyond the joist edges

Step 9
Wrap cut triangle of tape around the joist corner and press down to adhere to the 1st piece CONTEGA FC already adhered to the side of the beam. Now press down this long piece of FC that extends beyond the joist edges and adhere the it to top of the 1st piece of fleece. Press down firmly.


Step 10

The green CONTEGA HF applied in the corner should be visible between the two piece of tape. It should have been squeezed out a little bit when the ovelapping tape was applied, however you shouldn’t press the adhesive completely flat.

When in doubt if there was sufficient adhesive applied in a corner, you can add some additional CONTEGA HF to further seal this connection at this time.
Step 11
For the top of the joist, repeat steps 6-10 on top of the beam

Step 12
Apply plaster (or other airtight masonry product) to the brick wall and embed the tape into it for at least 1″. Apply a coat of plaster (or other airtight masonary product) over the fleece. When the plaster is cured, the fleece of the tape is now securely and airtightly embedded in the plaster that is covering the once leaky brick.

Multiple beams air-sealed with CONTEGA FC-plastered masonry at Prospect Architecture 4th street EnerPHit project

This application assures the long term air-tightness of the plaster – wood connection.
Please note: Due to the large variety of plasters (and other coatings) available on the market it is our recommendation to do adhesion test on to verify that the plaster bonds well to the fleece. Of course the plaster should be suitable to make an airtight connection (very sandy cement mixtures are not airtight).

Plasters with a high gypsum content in general adhere very well to the polyester fleece (for instance structolite). If finishing plasters contain chalk or cement, a first scratch/parge coat made with reinforced mortar that bonds well with the fleece, is required.

Thin plaster layers are more prone to cracking then thicker coats. >3/8″ or more is in general making them robust enough to resist cracking in the long term. Embedding CONTEGA HF in plaster of that thickness will also prevent unwanted cracking.

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