How to airseal a Brownstone to PH standards (party walls)

Brick party walls leak, so how do we airseal them?

Solid brick walls leak. Imagine that the mortar joints are a labyrinth of air-transfer straws - connecting one side of the party wall to the other, or to the outside, and thus causing much leakage during the blowerdoor test. After some trial & error, we have come up with a number of solutions that will help you get your party walls airtight.

Apply CONTEGA FC/SL around the beam, make 45 degree folds in corners to assure continuous tape Adhere the final overlap of CONTEGA FC with OR-F caulk adhesive

The historical/European approach - Plastering the walls

Plaster? Somehow plaster is a no-no in NYC/USA construction, except if you are a millionaire. The surprising thing is that most construction crews actually know pretty well how to apply plaster. Compared to the second-best option of fixing walls (pointing) and

CONTEGA FC taped to beam - plaster - airtight Party wall - rowhouse

applying Sto-guard or Henry, plastering can be a fairly cost-efficient way to seal your party walls. If your brownstone has plaster on big parts of the party walls already: please, please don't demo it - the plastered part of the wall is airtight already! Of course developers were as thrifty then as they are now, so unfortunately, in most cases, there is no plaster behind baseboards, between joists, etc.  At times you get lucky and there is actually a scratch coat of plaster behind the stair - count your blessings, this might save your day (but also might not....the blowerdoor will tell).

Next steps are:

CONTEGA FC plastered airtight to brick party wall (from above)

  • Cut back the floor 12"
  • Demo the crown molding or if you can't stomach demo-ing the molding, try to plaster the junction of wall and the crown's lath - the latter is not a 100% airtight fix of course
  • Tape the joist with CONTEGA FC / SL
  • Embed the CONTEGA FC / SL into the first coat of plaster, connect it to the existing plaster above.
  • Add a second coat of plaster, work it smooth and airtight and connect it to the plaster on the wall above and below.
  • Your airtight layer is complete when each square inch of the party wall is plastered and connected to neighboring airtight beams, membranes and floors.

OSB taped with TESCON Vana with service cavity up front (

The solid approach (plywood)

If the walls are bare or not in fantastic condition, you can opt to adhere airtight plywood (thickness and grade is up to consultant as airtightness varies from brand to brand). OSB could be used as well, but again the air-tightness of material should be verified. In this method you will have to cut around each and every joist. Since that is tedious, this is a spot at which you might consider using a membrane (if you have some INTELLO scraps left over that you can use them there), as this will be much quicker to size and staple membrane between joist than to cut the plywood.

Intello over party wall airitight taped with TESCON Profil to beams

After mounting plywood (or a membrane for that matter) that covers your entire party wall - tape all the flat seams with TESCON Vana. Then tape all the joist corners with TESCON Profil (50% on joist, 50% on airtight layer). If the wood is very rough or split, either sand it down or use CONTEGA HF  under the tape for a complete airseal to these uneven surfaces (or adhere the membrane with CONTEGA HF to the wood).

Also make sure wood is free of dust and solid before applying tape or adhesive. If the wood is split, use CONTEGA HF to fill the void, which otherwise could be a significant airleak.


See previous blogpost about sealing a row house roof with INTELLO Plus.

Floor (garden apartment)

Two ways to airseal the garden floor:  1) on top, connecting taped plywood subfloor to airtight wall with TESCON Profil or 2) with membrane at underside of floor joists (at cellar ceiling) taped to wall, with service cavity below.

Tape the (new) plywood subfloor with TESCON Vana. Connect this floor to the airtight layer applied to the party walls:

  • with CONTEGA FC /SL (if using plaster)
  • TESCON Profil (when using plywood, INTELLO Plus or PE-sheets etc.).

The advantage of using the garden floor plywood subfloor is that you reduce the number of beams to seal at the party wall by 40 to 80.

Don't forget to insulate and air-seal around the bulkhead and stair to the cellar.

In case it is not practical to airseal the top of the floor, you could opt to use INTELLO Plus and seal the underside of the beams air-tightly. This is not the preferred way to seal this floor but in some cases the only option.

Cellar (if included in thermal envelope)

Underslab vapor barrier airtight connected w CONTEGA FC to plastered concrete-, rubble foundation

Dig out cellar, add french drain, etc as per architect. Add insulation as per PHPP calculations - can be EPS, Roxul Comfortboard IS, CompaCFoam or other suitable insulation.

Run a membrane (Stegowrap or similar) over the insulation before pouring the concrete. This membrane can now be connected to your airtight layer with appropriate tapes.

Use CONTEGA FC (as shown in picture) to connect to the wall to be plastered. (Wall can be Thorosealed/Sto-ed or Henry-ed as well, but this can be very time consuming as noted above.)

In case plywood is used, connect the vapor barrier to the plywood with TESCON Vana. Make sure to tape all seams and joints, especially in the corners.

Check your work

Once the airtight layer is completed, perform a preliminary blower-door test and look for leaks. Take the time and check each and every floor joist in the house and fix any leaks that are found before applying floors, finishes or other elements that could impede the access to the airtight layer during tests.

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