Thursday, 27 January 2011

Airtightness around existing floor joists (and along the top of internal walls)

The last few weeks has seen tremendous progress on the build and one issue that I wanted to focus on in this post is how we managed airtightness around the ends of the existing floor joists.

The picture below shows how the floor joists looked before. Normally, the block work would only be plastered above and below the joists, where the finished wall would be visible. In a Passivhaus refurbishment, the plastering needs to run continuously around the joists.

We had originally intended to keep the plasterwork below, if it was sound enough, but mostly it just fell off with minimal if any encouragement. As discussed in a previous post, water penetration in the cavity, due to poor detailing at the original roof-wall junction, together with some poor choices with the original render and paintwork seemed to have been the cause.

The issue of how to make airtight joist ends has been covered in this AECB YouTube video. One of the key points they mention is that the approach needs to take account of whether the wall is going to be insulated internally or externally. If a wall is to be internally insulated, there will be a significant temperature drop at the end of the joist, which brings a risk of condensation because the joist end sits within the cold wall. In the film, to avoid this, they have mounted the joists on joist hangers on an internally insulated wall. Steps then need to be taken to achieve airtightness around the joist hangers.

In our project, the existing house is being insulated on the outside, so the inner leaf of the existing cavity wall will be the same temperature as the inside - i.e. no condensation risk. This has been our approach to achieving airtightness around the joist ends:

Stage One

Carlite Bonding parging around the joists onto the original concrete block wall or sometimes new built sections of Thermalite block wall:

The same principle has been applied to a steel girder fabricated with an additional L-shaped piece, welded with the bottom of the L welded to the web of the girder. This means that the profile of the end of the girder is a solid rectangle, like a joist end, rather than an "I" profile, simplifying making it airtight.

Stage Two
Priming around the joist ends with Pro Clima's Tescon Primer adhesive. This helps to make sure the Tescon tape sticks to the plaster.

Stage Three

Taping around the joist and girder ends using Pro Clima's Tescon Profil tape.


Stage Four

Dotting the corners with Pro Clima's Orcon F adhesive. This is a "belt-and-braces" step. We were not absolutely sure that our tape corner junctions would stay completely airtight. We applied dots of Orcon F to be sure! 

Stage Five

Apply Unibond to the Tescon tape to provide a key for the second parge layer (next stage).

Stage Six

Apply a second parge layer to seal in the Tescon tape. In the picture below, the joist on the left was originally a double joist. We cut short the thinner of the two rather than try to treat the double joist along its length. Obviously, if the double joint were needed for structural reasons, we would not have been able to cut one joist back.

Dealing with the edge joists
The joists run north-south in our build. There was no edge joist on the east side due to the run of the original roof rafters, however the edge joist on the west wall was mounted close up against the wall and there wasn't enough space around the joist ends to parge and tape around them. One option was to wrap the entire length of the joist in Intello Plus membrane, then tape it down above and below the joist. We decided it was more practical to cut out the existing joist, reduce its length a few cm and re-attach it with coach bolts along its length of the west wall. We used resin anchor gel to secure the bolts in the wall but also applied a generous ring of Orcon F around each a penetration to provide a flexible airtight seal (picture below).


  1. Fantastic workmanship - hope the airtightness testing reflects the efforts put in!

  2. Thanks. Yes, the proof will be in the blower door test pudding, as they say!

    One thing I didn't mention is that we had considered leaving a ring of incomplete floor boarding around the edge of the building, in case the airtightness test reveals a problem with one of the joist ends. Unfortunately, it wasn't practical as we needed to press on with the stud walls on the next floor. We will still be able to access the joists from below.

    I'll make sure I post an update when we do the blower door test.

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  4. If the wall is isolated on the inside and airtight, isn't the joist end safe from problems since there can be no hot air flowing through the wall and condensate on the cold end?

    1. The wall is airtight on the inside, not hermetically sealed like a submarine, but nevertheless much more airtight than in a conventional build. This is a crucial difference. Even in constructions with Passivhaus levels of airtightness, they are not hermetically sealed from the outside world and vapour pressure will push some water vapour from inside to outside during winter. In a retrofit with internal insulation, the joist ends will be signficantly colder than they were before the insulation was installed. This is topic which is discussed in more detail in The Passivhaus Handbook, published this week.

    2. Thanks for answering Adam. I will sure take a look at the handbook. About your remark that some water will get outside, I would think it will condensate on the wall surrounding the joist end since stone will be more cold than wood. Also, the same situation is with an insulated roof: some water will get through and condensate but it doesn't necessitate taking special measures to prevent the wood in the roof construction from rotting.