Oxbow16 Posted March 17, 2023 Share Posted March 17, 2023 Sorry to be back so soon with another insulation question. I've taken a ceiling down to re-insulate from below. The rafters are 100mm, so I'm putting in 50mm PIR, and leaving a 50mm airgap behind for ventilation (soffits are ventilated). Cavity wall with no insulation. Access is a bit tight to the top of the wall and wall plate, but I'm hoping that with a bit of persusaion the PIR will go in there fine. My question is, should I be aiming to get the PIR pieces down to the wall plate (cut at angles so that they sit on top of the wall plate)? This would close/cover the wall cavity - is that a problem, or does the cavity need to remain open? A few pics to help visualise, Many thanks Link to comment Share on other sites More sharing options...
AliG Posted March 17, 2023 Share Posted March 17, 2023 I am not an expert on this. Your BC officer might be the best person to speak to. But I think you need to use non combustible cavity closers. They are usually rock wool socks. Something like these. https://snsbp.co.uk/product-category/brickwork-construction-products/cavity-closer/cavity-fire-barrier-stop-sock/ The reason is that a fire could get into the cavity and then move up there are set fire to the PIR and roof. I am assuming that you can get something down to the top of the cavity without blocking the ventilation at the soffits. It would also mean less cold air getting into your cavity from the roof ventilation which will slightly help with insulation of the walls. 1 Link to comment Share on other sites More sharing options...
ProDave Posted March 17, 2023 Share Posted March 17, 2023 I would be considering filling the cavity while you are at it. Any chance of a DIY pour something into the cavity (genuine question, is is possible, would it work?) 1 Link to comment Share on other sites More sharing options...
Radian Posted March 17, 2023 Share Posted March 17, 2023 I'm sure you can easily picture a 'continuous insulation envelope' and that's what you need to achieve. 1 hour ago, Oxbow16 said: The rafters are 100mm, so I'm putting in 50mm PIR, and leaving a 50mm airgap behind for ventilation (soffits are ventilated). Cavity wall with no insulation. ...So insulate the cavity by getting it injected with EPS after you have fitted your PIR. So long as you don't go mad with it, lay in some expanding foam on top of the wall plate before mating it up with your PIR. If it were me, I would start by cutting the PIR into short sections with the angled end to meet the wall plate - just to get you out of the cavity area and up to where you can see it all. Then you can use an inspection mirror to look at the bottom where it meets the wall plate to make sure it's closing off the cavity. If for some reason you're unable to get the cavity wall injected, at least it will be decoupled from the ventilated soffit area and improve it's insulating effect. 50mm PIR is very little insulation, I hope you're going to lay another 50mm across it, under the rafters. Now's the time! 1 Link to comment Share on other sites More sharing options...
Iceverge Posted March 17, 2023 Share Posted March 17, 2023 I would say it'd be prudent to block the top of the cavity to prevent any EPS beads escaping if you full fill it later. From a fire point of view it would preferable too. I would stay away from PIR between the rafters and use a 50mm mineral wool or woodfiber batt and then more insulation below the rafters. Something like Rockwool Flexi or steico flex. Woodfiber is particularly good for summer overheating protection. Ease of fitment, noise, heat protection, fire, cost, thermal drift, are areas that PIR doesn't cover itself in glory. 1 Link to comment Share on other sites More sharing options...
Dave Jones Posted March 17, 2023 Share Posted March 17, 2023 looks like a cold roof. dont block the airflow by insulating tight to the underside of the felt. 1 Link to comment Share on other sites More sharing options...
Oxbow16 Posted March 17, 2023 Author Share Posted March 17, 2023 Great responses – thank you very much. I’m going to close the cavities with the rock wool socks as kindly suggested A couple of questions please: - How compressible are the socks? The cavity seems to range from 70-80mm. Obviously I want to make sure they will stay in place and not drop into the cavity, and I presume it is just the compression that achieves that. OTOH, I don’t want something so thick it won’t compress enough to fit. So what would be the ideal size for me? - Are they bendy and manoeuvrable enough to allow me to thread them under the rafters? If not, what’s the best way to fit them? In answer to some of the other points... 2 hours ago, AliG said: I am assuming that you can get something down to the top of the cavity without blocking the ventilation at the soffits. Yes, I think so, so long as they are flexible enough to be manoeuvred. Whatever the case, I'll be sure to not block the soffit vents. In fact, that's half the reason for doing this as they were previously blocked with vermiculite. 2 hours ago, ProDave said: I would be considering filling the cavity while you are at it. We live in an area where cavity wall insulation is not recommended (driven rain/very wet). 2 hours ago, Radian said: If for some reason you're unable to get the cavity wall injected, at least it will be decoupled from the ventilated soffit area and improve it's insulating effect. With no cavity wall insulation, will the rock wool sock be enough to achieve this decoupling? I've also got some Gapotape which I can put onto the end of the PIR so that the Gapotape and sock butt up against each other. Would that be enough? 2 hours ago, Radian said: 50mm PIR is very little insulation, I hope you're going to lay another 50mm across it, under the rafters. Now's the time! Yes, that's exactly what we're doing 49 minutes ago, Iceverge said: I would stay away from PIR between the rafters Space is very limited unfortunately, so using PIR to make the most of the space we do have. Thanks a million Link to comment Share on other sites More sharing options...
Radian Posted March 17, 2023 Share Posted March 17, 2023 1 hour ago, Oxbow16 said: - How compressible are the socks? The cavity seems to range from 70-80mm. Obviously I want to make sure they will stay in place and not drop into the cavity, and I presume it is just the compression that achieves that. This is why I was trying to avoid using them. Many of the ones used in my house had slipped down over time and I just don't trust them to stay put. By sealing the PIR to the wall plate and filling the cavity, you would have a continuous insulation envelope that followed the contours of uneven wall, rafters etc. 1 hour ago, Oxbow16 said: We live in an area where cavity wall insulation is not recommended (driven rain/very wet). Sure, but there are different types of insulation and often installers only fit one type. If you talk to a company that only fills with blown fibres, they'll tell you its not advisable to fill - full stop. EPS beads are a different matter altogether as they don't saturate and allow water on the back of the outer leaf to drain down rather than track across. 70mm is a long distance for moisture to transfer from bead to bead and make a damp bridge. Also, the beads don't choke-off airflow entirely. I suggest you contact an EPS installer, check that they use bonded graphite beads, and ask if your location is suitable - and what guarantees they give. The bottom line is that beads are the easiest to remove in the extremely unlikely event of trouble. But I've yet to hear of anyone having trouble. We are in the highest risk zone 1, about a mile from the sea and I researched this extensively before signing-up. But if anyone can point to a case of EPS beads being a problem with driven rain, I'd be extremely interested in hearing about it! 1 2 Link to comment Share on other sites More sharing options...
saveasteading Posted March 17, 2023 Share Posted March 17, 2023 You may not need special 'socks'. Rockwool can be cut and then folded or rolled tightly and squeezed into gaps. Thus it is much cheaper and always fits. If you use the cavity type it is also dampproof. 1 1 Link to comment Share on other sites More sharing options...
AliG Posted March 17, 2023 Share Posted March 17, 2023 4 hours ago, Oxbow16 said: - How compressible are the socks? The cavity seems to range from 70-80mm. Obviously I want to make sure they will stay in place and not drop into the cavity, and I presume it is just the compression that achieves that. OTOH, I don’t want something so thick it won’t compress enough to fit. So what would be the ideal size for me? - Are they bendy and manoeuvrable enough to allow me to thread them under the rafters? If not, what’s the best way to fit them? I have not worked with them myself, I need to buy some for a small area where cavity closers are missing so have been looking into them. Looking at the info online, it seems like the socks are 20-30mm wider than the gap you need to fill. You'd probably be looking at a 100-110mm sock. As rock wool is very compressible, I would assume there is quite a bit of leeway. You could always buy one and give it a go. I am not sure you could just push in insulation as without the plastic cover it would be hard to manoeuvre. I had assumed you couldn't insulate the cavity or had some other plan for the walls. I am not sure it will make a big difference not having cold air coming in at the top of the cavity, but it is better than nothing. 1 Link to comment Share on other sites More sharing options...
ETC Posted March 17, 2023 Share Posted March 17, 2023 Is the wall plate on the inner or outer leaf? 1 Link to comment Share on other sites More sharing options...
ETC Posted March 17, 2023 Share Posted March 17, 2023 Fit 50mm battens to each rafter. Fit 100mm PIR between rafters maintaining a 50mm gap above the insulation. Fit 62.5mm insulated plasterboard across the face of the rafters. Fit 400mm mineral wool insulation above the flat ceiling. Staple the TCB cavity barrier to the back of the wall plate. 1 Link to comment Share on other sites More sharing options...
Oxbow16 Posted March 17, 2023 Author Share Posted March 17, 2023 1 hour ago, ETC said: Fit 50mm battens to each rafter. Fit 100mm PIR between rafters maintaining a 50mm gap above the insulation. Fit 62.5mm insulated plasterboard across the face of the rafters. Nice idea but there's not enough headroom in the bathroom unfortunately. It's going to be cramped enough as it is with the 100mm insulation (50 in between/50 perpendicular). 1 hour ago, ETC said: Is the wall plate on the inner or outer leaf? It's on the outer leaf. 2 hours ago, AliG said: I have not worked with them myself.....You could always buy one and give it a go. I'm going to see if I can pick some up tomorrow and will report back on how well it fits etc. 3 hours ago, saveasteading said: You may not need special 'socks'. Thanks for the suggestion. I'm all for saving money etc., but in this instance I'm going to use the right stuff. Thick it will be a lot easier to work with as well, given my situation. Thanks @Radian for the info and suggestions. I'll keep it in mind and am also intrigued to look into it, having until now always just assumed that CWI is a no no in our location. Around here all you tend to hear about is folk having it removed, not installed! That being said, for this particular wall only 1.5m in height is external. There's a single storey extension against the front of the house, so most of what used to be external wall is now internal, leaving just that 1.5m section as external. Many thanks to everyone fot he replies Link to comment Share on other sites More sharing options...
ETC Posted March 17, 2023 Share Posted March 17, 2023 You could always insulate the inside of the wall! Link to comment Share on other sites More sharing options...
Oxbow16 Posted March 17, 2023 Author Share Posted March 17, 2023 39 minutes ago, ETC said: You could always insulate the inside of the wall! Nice idea. But the room is only 1.5m wide. Any smaller than that and I'll have to bath in a bidet! Link to comment Share on other sites More sharing options...
ETC Posted March 17, 2023 Share Posted March 17, 2023 Insulated plasterboard? Link to comment Share on other sites More sharing options...
Allex Posted January 15, 2024 Share Posted January 15, 2024 On 17/03/2023 at 09:52, Oxbow16 said: I've taken a ceiling down to re-insulate from below. The rafters are 100mm, so I'm putting in 50mm PIR, and leaving a 50mm airgap behind for ventilation (soffits are ventilated). Cavity wall with no insulation. Access is a bit tight to the top of the wall and wall plate, but I'm hoping that with a bit of persusaion the PIR will go in there fine. Many thanks Isn't this an unusual situation to have the wall plate on the outer leaf? Link to comment Share on other sites More sharing options...
Roger440 Posted January 15, 2024 Share Posted January 15, 2024 5 hours ago, Allex said: Isn't this an unusual situation to have the wall plate on the outer leaf? I thought so too. Initally, the thread wasnt making sense. Mine is on the inner. So im unclear why or if i would need the shocks to close off the cavity? Link to comment Share on other sites More sharing options...
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