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Housing - Renovation: Cavity between existing house and new timber frame

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    This is my first post to the forum, I am building a lean-to extension on the back of a 1930s mass concrete house and would appreciate any advice or comments on the best way to deal with the 30-50mm cavity between the two buildings. The existing house is approximately 300mm thick mass concrete walls lined with 25mm horizontal battens and vee lining. I intend to strip back to the concrete and add 25mm solid insulation, then plasterboard. The new frame has breathable membrane, osb sheets, 138mm thick timber then plasterboard. Roof is I beams and overlaps the bottom of the existing house roof by 450mm approx. There will be a 50mm cavity and 100mm plastered blockwork all round the new frame. There is insulation below the new concrete slab and uninsulated timber floor in the old house. So what is the best solution for insulation of the frame along that wall? The cavity will be connected to the new cavity wall, so full of cold air… plus there is going to be a cold bridge up from the uninsulated ground below. It seems to me that the whole wall will need to be insulated, unless the cavity can be blocked, or filled, but wonder if there is then a condensation risk inside if it is no longer ventilated? Hope I have described the situation clearly, Will try to attach a picture as well…
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      CommentAuthordjh
    • CommentTimeJan 9th 2022
     
    Sorry, read very quickly so perhaps didn't understand but are you talking about how to insulate what will be an interior wall in the finished building? If so, then seal the ends so it really is an internal wall!
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    Yes, it will be an interior wall, with a door in the middle connecting the two spaces together.
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    Noticed that the new part L building regs in England are a bit scornful about practicality of sealing cavities properly airtight forever ... in a similar situation we insulated the inside face of the timber extension and left the cavity open to the top and sides to breathe.

    In a much earlier similar situation we naively treated it as an internal wall, but whenever we removed a socket or lightswitch a gale blew out! Probably an electrician had inadvertently drilled a connection between the cavity and the outside world.
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    What would be the best way to seal the ends, foam? I am a bit concerned about sealing it up in case that causes condensation, warm air from the new build gets into the cavity any meets the cold concrete wall opposite, but I don’t know how much of an issue that is likely to be. I’m in Scotland.
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      CommentAuthordjh
    • CommentTimeJan 9th 2022
     
    Posted By: WillInAberdeenNoticed that the new part L building regs in England are a bit scornful about practicality of sealing cavities properly airtight forever
    Do you happen to have a page number, to avoid reading 102 pages?

    whenever we removed a socket or lightswitch a gale blew out! Probably an electrician had inadvertently drilled a connection between the cavity and the outside world.
    I have difficulty visualising that situation. Electrician drilling holes into cavities yes,but if the internal cavity is sealed and insulated off from the external cavity it would seem to take some rare gymnastics to do it accidentally?
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    A) AD L 2021 page 16 s2.14 table 2.1, in the context of a cavity party wall (IE an internal cavity wall). If it is robustly sealed and insulated around all of its edges, it is assigned a U value that implies there's still going to be 40% as much external air flowing through it as there would be in a cavity completely unsealed round its edges. A fully-filled-cavity party wall is assigned U=0 as there's no possible external air movement through the cavity.

    B)Our earlier example (that turned out not to be airtight) had timber rails bolted on a layer of mastic onto the old wall as starters for the framing of the new walls and floor and ceiling. Internally there were studs/noggins running between those rails, with plasterboard over, creating a drafty cavity between old masonry wall and new plasterboard.

    The cables to/from sockets/switches ran through the cavity between old and new constructions. They would have to enter/leave that cavity somewhere unseen, so somebody probably made a hole and judging by the rest of his work it probably would be a big one (or many). Of course there are other ways the sealing of the cavity could have failed, I never ripped the new construction down to investigate, and we don't live there now.

    Chris, I don't think that foam would seal the edges airtight, you wouldn't be able to test it and find/fix any little gaps, you'd get mice burrowing through it (I know!) and it wouldn't cope with relative movements of the timber vs concrete structures. I would just fill your timber frame wall with something insulating. You could conceivably fully- fill the cavity between old and new walls by injecting polystyrene beads or something but could be awkward round edges.

    I prefer your construction with a deliberate separation between the old and new structures and not relying on the old wall for insulation of the new space.

    Edit for clarity.
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      CommentAuthordjh
    • CommentTimeJan 10th 2022
     
    Thanks for the pointer. I'd say 'scornful' was a bit strong, but I agree with the basic point. It sounds like your gales were due to poor construction; i.e. the space wasn't actually sealed.

    I agree it will be better to fill the cavity, it looks like it might be as simple as tipping EPS beads in from the top.But if it's possible to seal the edges then do that first. Don't use ordinary foam; you could use airtightness foam, which is low-expansion and which stays flexible permanently and/or you can use airtightness tapes, probably with primer (glue) to help it stick to masonry.
    • CommentAuthorphiledge
    • CommentTimeJan 10th 2022
     
    I dont really understand why the new TF wall is being fitted infront of the existing wall which was formerly external and now internal. Surely TF extension butts up to existing concrete wall and any thermal bridging is dealt with by internal insulation around the concrete wall so a cavity isnt formed??
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    <blockquote><cite>Posted By: philedge</cite>I dont really understand why the new TF wall is being fitted infront of the existing wall which was formerly external and now internal. Surely TF extension butts up to existing concrete wall and any thermal bridging is dealt with by internal insulation around the concrete wall so a cavity isnt formed??</blockquote>

    I think it has been done like this to make life simpler for the builder. The existing plastered wall is not perfectly straight, so the new frame is as close as possible but leaves a cavity of variable width. Anyway, it is there now and I am trying to decide what to do with it. I quite like the idea of filling it with something although thd roof is on now so it might be an awkward job….
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      CommentAuthordjh
    • CommentTimeJan 10th 2022
     
    Posted By: ChristopherI quite like the idea of filling it with something although thd roof is on now so it might be an awkward job….
    As long as the final finishes are not on it shouldn't be too difficult. Choose insulation that can be blown in, such as EPS beads or warmcel etc, then all you need is to cut some holes to blow the insulation in and maybe a few more to (a) let air escape and (b) see what's happening.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 10th 2022
     
    Here's what I would do, were you my client...

    - use a multitool (AKA Feintool, AKA zip saw) and cut out the OSB sheathing from the stud wall, that backs onto the concrete.
    - foam the perimeter, (along floor, walls, eaves) so no air can blow in. I suspect there are all sorts of naughty gaps around that conc to stud interface.
    - glue (pink foam) rigid insulation to the concrete (let's say 50mm thick, or less if cavity is much thinner). Stop cold rising through concrete wall from foundation, and possibly sideways from where it passes to the outside, beyond the extension. Reduce risk of condensation at the conc wall if it is cold.
    - fill the stud wall, up to the new rigid insulation, with wool type insulation. I'd use Knauf frametherm32 or 35.
    - not essential, but I'd fix a VCL at this stage, as a check/retardant to vapour, it won't stop it. Low cost/effort.
    - if you do, join this up with any VCL in the new roof, and side walls. Tape and clamp with service battens to give mechanical fix.
    - if you feel the wall has any racking function (unlikely), then nail a new sheet of OSB over the internal face.
    - if you want to run services, then add 25x50 or 25x100 battens.
    - then plasterboard.

    A bit of effort, but low tech/skill. When you've done it, you'll know there isn't really anything more you could have done, so rest easy after. That "I wonder if I'd...." is not a nice musing.
    • CommentAuthorphiledge
    • CommentTimeJan 10th 2022
     
    If the internal cavity joins up with the external extension cavity, any loose fill insulation is going to part fill the external cavity and bridge between exterior of TF and external block. Maybe possible to drill through the corner of the TF sheathing and squirt a sucession of squirty foam blobs to close the end of the internal cavity?

    What has the designer/architect shown on the drawings??
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    Philedge I could seal the cavity ends because the new block wall is not built yet, I am just not sure whether I should. The drawings show an open cavity around the timber frame, (which continues between the frame and the existing house) no insulation inside thd frame but 25mm kingspan or equivalent on thd front of the frame behind the plasterboard. I am not sure that this is the best way to proceed, but perhaps it is? It seems to me that leaving the cavity open & full of cold air will make both sides of the building colder, despite some insulation, although closing it off may lead to condensation and damp in there which I guess is why it is drawn that way?
  16.  
    IMO fill the cavity with EPS beads warmcel or some other blown in insulation. If the ends are still open then the job should not be too onerous.
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      CommentAuthordjh
    • CommentTimeJan 10th 2022
     
    Posted By: ChristopherI could seal the cavity ends because the new block wall is not built yet, I am just not sure whether I should. The drawings show an open cavity around the timber frame, (which continues between the frame and the existing house) no insulation inside thd frame but 25mm kingspan or equivalent on thd front of the frame behind the plasterboard. I am not sure that this is the best way to proceed, but perhaps it is? It seems to me that leaving the cavity open & full of cold air will make both sides of the building colder, despite some insulation, although closing it off may lead to condensation and damp in there which I guess is why it is drawn that way?
    If the cavity is still accessible from the sides then it should be possible to fill it easily without removing the OSB. I would ask the designer to explain why the cavity between the old and new is there and especially why it is connected to the ventilated cavity around the extension. You could also ask their opinion of filling the 'internal' cavity with insulation. I don't think there's any risk of condensation etc. Where would it come from?
  18.  
    The question in the OP related to the existing cold bridges where the top, bottom and sides of the existing masonry wall are unavoidably in contact with outside air. It mentioned that the new construction is vapour permeable, presumably accepted because moisture vapour can escape through the ventilated cavity. The OP is looking for some confidence that blocking up the ventilation wouldn't cause condensation on the cold masonry, which has no obvious route to dry. Maybe I misunderstood.... I'm going off the idea of filling the cavity with permeable insulation! Though GP's idea is clever as it would warm the original building too.

    How are the side walls of the extension to be insulated - can you make the insulation layer run continuous round/over/under the extension? Likewise the air/vapour tight layers?
  19.  
    A good summary, Will, you are not misunderstanding anything. I do not like the idea of leaving the cavity open, but am hesitant to block it up. Filling it seems like a good idea, but maybe another option is to leave it open and add some extra insulation inside the frame. I’ll speak to the designer and update with his opinion, in the meantime thanks to everyone who has replied.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 11th 2022 edited
     
    DJH question above..." I don't think there's any risk of condensation etc. Where would it come from?"

    ...from the moisture liberated within the household.

    If you mean, why would the vapour condense...well the conc wall will be cold from being in contact with the ground, and also the outside air temp at the extension wall and roof junctions. Surface around 12oC will likely start to suffer condensation (room air say 20oC/60%RH)

    If you mean how could the moisture get there...unless it's extremely well sealed at the entirety of the TF wall periphery and over its surface, then vapour will find its way there.

    I don't see a need for the OP to prove it's a problem leaving a cavity. It's a simple piece of work to ensure there is no potential heat leakage in the centre of his home. Just do it.
    • CommentAuthorphiledge
    • CommentTimeJan 11th 2022
     
    If the cavity is left open, its just adding to the area of external wall that the house has with limited options to insulate the section of existing concrete wall forming one side of the cavity.

    With the outer leaf of the extension still to be built, now seems the ideal opportunity to extend the exterior TF wall a couple of inches to butt up against the existing concrete wall and remove the section of TF forming the cavity.

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