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I'm renovating the 4th and top floor of a house built in 1906 in Rotterdam, the Netherlands. The house has masonry load-bearing walls and wooden beams. The current situation is shown in the image below. enter image description here

The area in pink has the building envelope as shown in the B-B section cut below somewhere between grid lines 5 and 6. Note: This space currently is not insulated.

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The mason wall shown in the image below will be destroyed.

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The image below is taken from the 4th floor looking at the top of the 3rd floor ceiling after the floorboards were removed.

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A glass facade will be installed 1 meter behind the location of the existing masonry wall thus creating a balcony that is 1 meter longer than it is currently. This is shown in the proposed floor plan below.

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In an ideal scenario, this entire 3rd to 4th-floor building envelope would look like the following section. Note: Rockwool insulation and vapor barrier added

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However, this is not possible since the 3rd-floor ceiling cannot be removed.

Ideally, there is insulation in the floor cavity and therefore a vapor barrier is required. Is it a good idea to wrap the vapor barrier around the floor joists as shown in the section below?

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I’m worried this may cause condensation to occur around the floor joists which is the last thing I want. If that could happen I would prefer to leave the space un-insulated, unless someone has a different idea?

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    This is a tough one the balcony itself is where I would be focusing possibly a high density insulation panel covered with plywood then a membrane to seal it. I have seen many rolled roofing and hot tar with a deck over them fail. I found a membrane with an exterior carpet to be the best protection in a balcony that has traffic. Any furniture needs to be chosen with care the exterior carpet most commonly Astro terf helps to protect the membrane but a sharp chair leg can punch through. The balcony should have a tilt to make any water drain away. The membrane to glass is a challenge haven’t tried – Ed Beal Sep 15 at 15:47
  • @EdBeal thanks for the reply. So you're suggesting that above the 25mm thick wooden floorboards place a high-density insulation panel and then waterproofing above the insulation pannel?. I will place a wooden deck on rubber pads above the waterproofing to attempt and prevent the waterproofing from being punctured. And just to be certain you advice NOT placing Rockwool insulation in the ceiling cavity and also placing no vaper barrier. – Eric Sep 15 at 16:59
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    Yes the original decking is flat so water may pool adding some high density insulation on top maintains a air space (no wool below) reducing rot, then the decking sloped to drain.I have done this on a couple of homes. I live in the rainy part of the US the Pacific Northwest.The first ones we did the seal failed and the Sheetrock or plasterboard was ruined below a big mess, I started putting more slope on the deck but still if a chair was on the balcony it would punch through. I found membrane roofing and covering it with Astro turf has held up for over 19 years my daughter lives in that house. – Ed Beal Sep 15 at 21:02
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No, I do not think a vapor barrier should follow the profile of ceiling joists.

Condensation occurs when vapor reaches its dew point.

Placing a portion of a vapor barrier on one side of an insulation space and not on another side (where the wood joists occur), will stop the vapor from penetrating the insulation space but not the wood joist space.

Wood is porous and vapor will penetrate the wood. As it travels through the wood (vapor moves from warm to cool) it will hit its dew point and turn into condensation... causing the wood to turn into dryrot.

If you can’t remove the ceiling material, maybe you can paint it with a good finish paint material. (Paint is a vapor barrier too.)

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  • Thanks, @Lee Sam I've updated my question with an image that reflects my understanding of your suggestion. Can you confirm? In the spot where the ceiling, bottom of the floor joist, and vaper barrier meet how would you make that joint? Lay a bead of caulk? – Eric Sep 15 at 7:13
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I don’t think you need a vapor barrier in the bottom of the joist space. The finish paint coat should suffice. Also, caulking at joints, etc. is not required if the entire ceiling is painted (it may take several coats to achieve the perm rating you’re looking for). BTW, If you live in an environment where summer and winter change temperatures, then it gets complex.

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  • I do if you take a look at this graph of average monthly temperature you'll see there is about a 17C temperature swing from Winter to Summer. – Eric Sep 15 at 7:46
  • I wonder if it would be safer not to insulate, in the small part of the apartment it's been like that since the house was built and it seems there haven't been any issues. My assumption is that the floor cavity has enough airflow in it that even if the beams get a bit damp due to condensation then they are able to dry out. What do you think? – Eric Sep 15 at 7:47
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No, a vapor barrier should not follow the profile of the floor joist because that could cause condensation to form on the floor joist.

The system shown in the image below (as suggested by @edbeal), will be used to waterproof the structure and prevent water damage and fungis from forming on the floor joist. The floor joists were also painted to protect them from fungis as shown in this answer.

It is probably too risky to insulate between the floor joists at the moment. The heating costs saved do not outweigh the risk of condensation staying on the floor joist because air cannot travel in the ceiling cavity.

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