low_and_there Posted February 8, 2022 Posted February 8, 2022 Looking for wisdom from this forum on how to maintain an uninterrupted airtight layer between the ground floor and the first floor in our retrofit (solid brick mid terrace). The joists run front to back and on the front we have to do IWI (On the rear we’ll be doing EWI so my assumption is that we have a thermal bridge issue to solve on the front; but the rear won’t be such a challenge). I have seen a few different solutions proposed for this: i) rehang joists on joist hangers ii) create a new wall plate on which to rest the joists iii) support from below, leaving a complete gap between floor and wall iv) employ the “Tony tray” method Options (ii) and (iii) aren’t really suitable for our small footprint, bay window shape and structure. I don’t much fancy option (i) with a brick wall… so am extremely interested in the Tony tray option but haven’t been able to find an example of how to use it in a retrofit… is it possible? Can the airtight (breathable) membrane be wrapped around a joist in situ? And if so how…? Any pointers to diagrams, photos or videos extra appreciated! I can accept maybe not being able to achieve an airtight layer in it’s entirety, but what worries me is leaving the end of the joist getting damp in the brick area, which is beyond the insulation and airtight layer. I plan to use only natural breathable insulation, tho, so maybe my concern is unwarranted? thoughts extremely welcome!
markc Posted February 8, 2022 Posted February 8, 2022 Easiest was to make airtight between ground a first floor would probably be to install a membrane on the walls and ceilings as a continuous layer. removing cold bridges in an old property is more difficult due to the construction methods and joint details. why between ground and 1st floor? A flat I’m assuming?
low_and_there Posted February 8, 2022 Author Posted February 8, 2022 Hi @markc, it's a whole house retrofit we're doing. The ground floor airtight layer is starting above the Joists so nothing to solve there; the connection between the GF and the FF is what I'm looking at in this thread, and my question is how to make it airtight when the Joists are already placed on the masonry (bricks) and can't be easily moved to put the airtight membrane between them...
markc Posted February 8, 2022 Posted February 8, 2022 26 minutes ago, low_and_there said: Hi @markc, it's a whole house retrofit we're doing. The ground floor airtight layer is starting above the Joists so nothing to solve there; the connection between the GF and the FF is what I'm looking at in this thread, and my question is how to make it airtight when the Joists are already placed on the masonry (bricks) and can't be easily moved to put the airtight membrane between them... ahh, yes so it’s not that you want to seal the downstairs from the upstairs but rather looking to stop cold air coming into the floor void. Tony tray is out of the question so I think you are down to spray foam around the joists/walls or tightly packed insulation. the danger here is trapping moisture against the joist causing rot to set in.
low_and_there Posted February 8, 2022 Author Posted February 8, 2022 Yes, that’s precisely it. How can the risk of rot and decay be mitigated or avoided entirely in a retrofit context like this? Someone must have solved this…
Russell griffiths Posted February 8, 2022 Posted February 8, 2022 You want a liquid. There are two on the market I know of. BLOWER PROOF MEMBRANE and PASSIVE PURPLE. I have used both either spray or paint brush. The blower proof is thicker and has some fibres , it’s easy to paint on , but a bit expensive. 1
Marko Posted January 24 Posted January 24 @low_and_there which method did you eventually go for? I've got an almost similar scenario to address.
Redbeard Posted January 26 Posted January 26 I've done lime parge coat on GF and FF walls and in the ceiling/floor space, tight around the joist ends. Leave plaster to dry thoroughly, prime plaster and joist with air-tightness tape primer, then tape. Usual tricky spots are at joists parallel with flank or external walls, where you cannot get in to plaster, prime and tape. Probably one of the 'gunges' or FM330 in those spots. 1
Mike Posted January 26 Posted January 26 (edited) 7 hours ago, Redbeard said: I've done lime parge coat on GF and FF walls and in the ceiling/floor space, tight around the joist ends. Leave plaster to dry thoroughly, prime plaster and joist with air-tightness tape primer, then tape. Where I have internal wall insulation I've done similar with the ceiling joists on my current project - though plaster rather than just parge - where they meet brick / stonework. In the unheated & uninsulated entrance lobby the gaps between are filled with FM330 for airtightness. I also have a mansard wall where the joists sit between timber beams above and below. Those have also been foamed with FM330. Both solutions are perfectly airtight. Edited January 26 by Mike
Redbeard Posted January 27 Posted January 27 8 hours ago, Mike said: Where I have internal wall insulation I've done similar with the ceiling joists on my current project - though plaster rather than just parge Just for clarification, 'my' parge coats are approx 6mm trowelled lime plaster. I know others here use a brushed-on weak(?) sand/cement mix, but mine is not that. Where old lime plaster in good condition with good adhesion still exists, and there is no impermeable coating, I leave that (probably c 18mm) and simply augment where it never was.
Mike Posted January 27 Posted January 27 13 hours ago, Redbeard said: Just for clarification, 'my' parge coats are approx 6mm trowelled lime plaster. I know others here use a brushed-on weak(?) sand/cement mix, but mine is not that. Where old lime plaster in good condition with good adhesion still exists, and there is no impermeable coating, I leave that (probably c 18mm) and simply augment where it never was. All sounds good. The only reason I chose 'full' plaster was to even-out the undulations in the old brick / stone walls, so that the IWI had a snug fit.
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now