Joists on cavity wall and airtightness

[stavros.london]

Hello all,

I'm renovating my property completely and want to make it as airtight as possible so I can install a MVHR. The house is two storey detached and has cavity wall in the ground floor but single solid wall on the first floor.

My questions are around the first floor joists, which they currently sit on the cavity wall as per the photos below.



- a side question first: you can see in the close up the difference between a clean top section of the wall and another one that has a lot of mortar (?) deposited on the block. Not sure why that is there but could I simply remove it so I get all top blocks "clean" and tidy?

- main question: how do I make the joist to wall junction airtight? I was thinking carlite bonding / parging on the exposed blocks and then airtightness tape around the joist end. Would that suffice? Also, would that create any condensation risk or damp issues or anything else?

Thanks in advance

 

[theprinceofdarkness]

That looks like a bungalow made with cavity brick walls filled with glass fibre, where some one has put a second storey on it using Celcon blocks. I would seriously do the maths on the upper wall and see how much its costing you in heat loss. Is the building rendered and painted? It looks nice and dry, but cavity walls are usually have to vent into the loft, albeit slightly, so sealing the top of your one would not be reccomended.
Frank

 

[stavros.london]

Frank thanks for the response. It was built like that, i.e. as a double storey with these walls, 30 years ago. I plan to insulate the walls externally next year as they're not rendered -just the original brick although the first floor has tile cladding on top of the brick.

What you say adds significant doubts to my airtightness approach. If I can't seal those junctions, I don't see much hope any suggestions on how to proceed from here?

Thanks

 

[^woody^]

but cavity walls are usually have to vent into the loft

No, cavities should never be open and always closed.

 

[John D v2.0]

What are your ground floors? If they're solid concrete, and you're rendering and insulation externally, would it work to insulate the double skin wall the same way and make the cavity sealed inside space?
I only say that because it gets rid of the damp and air problem.
If the ground floor is timber, you'd have to seal under the floor from the cavity which might not save any hassle

 

[stavros.london]

Ground floor is block and beam.

That was my thinking as well, to insulate the whole property externally, not just the first floor walls. This would create a nice thermal envelope with opportunities to eliminate most thermal bridges. But I'm not an expert and issues around damp, condensation, etc confuse me a bit (evidently).

 

[theprinceofdarkness]

Cavities used to be built open, no technology existed for sealing them. Unless some one can describe one here!! Their invention was to make walls water tight by allowing water that penetrated the outer skin dribble down the inside of the outer skin. In the day (after about 1890), heat insulation was not really considered plus the lime mortar could allow water penetration. So this kept the rain from the inner skin, so the wall seemed dry.
Now that we know how to make a very water tight outer skin and the price of fuel has increased, the heat loss seems to become the most important facit of a wall, hence all the wall insulation techniques. "New builds" using foam panels must have a gap (AKA cavity) in front of the insulation, just behind the outer skin. Still to allow water drips not to penetrate the insulation and inner skin. So the building trade has learn't a lot in 120 years.
A lot is made of retrofit foam beads and blown fibre, in that the water dribbles can evaporate up and out, but at a much reduced rare compared to a open cavity, which really amounts to a waterproof single skin wall. The pumped or chemically expanded foam that completely seals the cavity, should provide a total water seal, but also seals in any water so can be awful. I know of a foam filled walled that had to be replaced, because 4 wall ties had the wrong slope on them, so the trapped water ran into the room, so its very likely that even if their angle was correct that the wall would have been watertight.
Frank

 

[^woody^]

Cavities used to be built open, no technology existed for sealing them.

Wtf? Only the very bricks and mortar that was used to build the wall in the first place.

 

[btgriff]

I've found the Reading Draughtbusters site helpful. They have an article showing how they deal with this issue,
http://readinguk.org/draughtbusters/?page_id=268

 

[stuart45]

I've worked on loads of old houses that had the cavities closed with a course of headers. Have seen the plate laid in the centre over the cavity on one of them. I don't think the tech was that complex for closing them.

 

[Doggit]

An MVHR system doesn't require a house to be that airtight. It's designed to remove damp air, recover the heat from it, and transfer that to the incoming cold fresh air; it's not designed as a completely sealed system, and any leakage won't be that great round the joist ends.

 

[stavros.london]

Thanks to all about the replies. There are some conflicting opinions though so I don't feel any wiser

I'm leaning towards sealing the gaps to make them airtight and then leaving access or potentially installing wood moisture sensors to track the joists' health. Next year I'll insulate externally all walls which sounds like the best end state to be in.

Any strong objections or further suggestions?

Thanks

 

[Doggit]

Sorry, but why do you feel you need to seal the airgaps to be able to install an MVHR system; it's akin to a heating system, and that doesn't require an airtight house.

 

[stavros.london]

Because the system requires a certain level of airtightness in order to operate efficiently. If the house is leaking so much from that first floor void I may as well leave a window open -which would make the MVHR system pointless since you no longer have controlled ventilation.

 

[Doggit]

I can see where you're coming from, but I suspect you're worrying unnecessarily. If you had bare floorboards on the first floor, then you'd get fair amount of leakage through the joist ends, if like most people you have carpets, then you'd still get leakage, but at a reduced rate. The concept of an MVHR system isn't to heat the property, but to recover the excess heat and moisture being taken out of the damp areas, and then transfer that heat to the incoming fresh air. The warmed fresh air that moves through the property will get moved into rooms that do not normally then get transferred back to the recovery areas. What system are you intending to use.