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Just getting my head around wall construction and the balance of vapour, heat and draught resistance and how they work together. I'm planning to renovate my walls (internal, as neighbours in the terrace aren't necessarily enthusiastic) house-wide - the solid double-brick victorian design (basically just bricks with plaster on it) has some significant room for improvement. But there are a dizzying array of options and configurations for internal wall insulation and it seems like information "out there" hasn't quite kept up with our developing understanding of water vapour and airtightness. Here's my current understanding, am hoping folks can correct me where I'm wrong (from outside to inside): 1. The air outside: The weather here in the UK is rarely on average over 21C, so the inside of the house will almost always be warmer. So the design needs to address water vapour that is *leaving* the house, getting cold and turning to condensation at some stage during its exit from the inside rooms. Not much reason to worry about water vapour coming in from outside. 2. The outer structural layer, that is in my case a solid double brick layer has a Vapor Resistivity, N s/(kg m) of 45 - 70 (got that from here: from here: https://www.engineeringtoolbox.com/vapour-resistance-d_1807.html). This means that it is "semi-permeable" 2b. Leaving on plaster (or not): Most of my walls have plaster (in some cases lime plaster, but haven't done an exhaustive survey), which I gather is straight-up "permeable". I can't see a consensus as to whether it's better to remove or not remove plaster, but I gather it may be necessary to remove (a) so that one can assess damp problems in case they aren't fully permeating the plaster to the inside layer of the wall (to show on paint etc.) and (b) so as to install some sort of membrane, or (c) obviously to address where there has been damage, and it seems to make more sense to leave it off if it's already off. But it seems like some folks like to save time and energy by leaving it on if there aren't mitigating situations. I'm about 70/30 leaning towards removing paint and plaster to expose the brick and check things out properly. 3. Air gap (or not): I've noticed some disagreement as to whether an air gap is now necessary. I gather an air gap is needed so that air can circulate vapour so that it can evaporate or vent out through the brick wall rather than condensing as it might in a tighter space without air. For the air gap, I can do battens over walls - and it seems that wood and stainless steel are the likely options here, nailed or screwed. Steel is more expensive, but will avoid rotting better than wooden battens (though damp proofing should ensure this isn't happenning!). Seems like Damp Proof Membranes are also an alternative, but a bit of overkill for my purposes here as it's not a cellar and all walls are above ground. But then the 4. PIR boards. I'll get these with foil backing, as this has an astronomical Vapor Resistivity value (4000), putting it on the very high end of "vapor impermeable". But it seems that some folks prefer the more labour intensive but easier-on-battens-long-term "warm battens" approach, where a insulation is split into two layers (a) a membrane is put down with half or more in layer of foil-backed insulation and then (b) an inner layer of battens with more insulation boards inbetween. This keeps warmth around the battens and ensures that moisture will condense further out in the wall. This seems reasonable to me - and I don't mind the more challenging job if there's an efficiency to be gained. What I'm not sure about is whether to use a membrane or foil-backed insulation boards for the vapour membrane at this stage. And if I use a membrane can I skip the air gap? 5. Plasterboard. "semi-permeable" here, skim on some plaster and ready to go. With regards to thermal transfer and insulation performance, here's what I've got: 1. Air = cold 2. Brick wall has u-value of 2 W/m²K or so 3. Air gap will reduce this slightly 4. PIR boards have a very low U value, which is why they'd be preferrable in walls to various wolls, cellulose, or other good options. I'm aiming for 70mm PIR or more (if possible) to try and get as close to or lower than my target U-value of 0.30. 5. Plasterboard adds a bit here too, but not much. Compliments of diydoctor, here's a diagram of what I'm doing: Option 1 (keep plaster, cold battens): Option 2 (warm battens - ignore blocks in picture): Option 3 (with service void): So questions are: (1) Is there a way of adding internal wall insulation on a solid double brick wall using new "tech" which will enable me to skip the air gap? (2) Should I strip plaster off the wall? (3) What's the best material for battens? (4) Any tips on how/whether to do a service void? I'm going to wire the whole house with ethernet and rewire electrical, so this wouldn't be out of place. (5) I've noticed comments elsewhere by @Jeremy Harris regarding a need to do graded vapour permeability, starting with (as he suggests): "the least vapour permeable and the outermost layer is most vapour permeable". This approach seems to be pretty difficult with internal wall insulation. Or am I allowed to ignore the plasterboard for the purposes of this kind of calculation? Would love to be enlightened. Note: this was quite helpful: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/658604/BEIS_-_SWI_Innovation_Final_Report_-_FINAL_Approved.pdf

I'm detailing the up-stand (kerb) construction for a couple of fixed glazing roof lights in flat warm roof. I've done a thorough internet search for design detail information - particularly for the correct positioning of the vapour control layer (VCL) around the roof light kerb. Roof light manufacturer websites are noticeably unhelpful, mostly ignoring the need for any sort of VCL. I'm constructing the kerb from resin bonded exterior grade plywood (WPB), and the insulation will be tissue faced PIR. I can't see any way of fitting the VCL on the warm side of the insulation without building the kerb in two parts. An inner plywood skin is clearly needed to allow the fitting of the VCL to its vertical outer face, as well as to the inner face of the insulation. Latest best practice advice is to avoid any timber between the insulation and the weather proof membrane, so the question that arises Is how to continue with a reasonably strong and rigid box to support for the significant weight of the glazing elements, but with minimal penetration of either membrane. Is preservation treated WPB appropriate? I'll possibly need to post a drawing here, so my current thinking can be appreciated - if anyone feels it would help understand what I'm on about. Does anyone have any experience of this tricky situation?

OK not the most exciting topic of discussion I admit! I've just about finished insulating the inside of the house and next step is the vapour barrier layer, which is also going to be the airtightness layer. Big range of stuff on offer from 150 to 1000 gauge, big range of prices too. Are any materials better than others when it comes to getting them stuck together? Anybody care to share what they've used? Don't want to spend hours looking this up if I can avoid it