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Hello, Please see the responses which I received (providing for anonymity). My original query was: I am currently looking at a site where a decision was made that a gas protection membrane had to be installed into a build when the works were significantly progressed. At this point in time the first floor had been erected so a methodology was drawn together to retrofit the membrane into the build. In short this involved propping up the first floor and cutting out the relevant blocks in the external and internal walls. The developer assures me that the NHBC will confirm that they are satisfied with the installation. Is there any set guidance relating to this scenario? Has this been done elsewhere? Responses: 1) Not sure what your exact circumstances are but they may have been better looking at some of the gas resistance epoxy paints / resins which are available - that way they wouldn’t have to pull up slabs and they may have benefited from installer warranties. 2) If you can I would get in touch with your local NHBC counterpart for land contamination and make sure they are aware of the scheme. If you don’t know who they are I can ask my local counterpart for their details. I am about to have one of these retro-fitted schemes on a prior approval site and have insisted through planning that the installer has the CSkills NVQ Level 2 Gas Membrane Installation and have also requested independent validation of the installation. Off the record the people who are undertaking the building works installed a membrane so poorly on a previous site, an active system had to be installed post completion – hence why I am asking for the above as I have concerns about their ability to undertake the works. 3) We had a site a few years (a couple of townhouses on an old gasworks) where a vapour membrane was required and this had to be done retrospectively as the developers just cracked on. They did exactly as you said in your email in that they propped up the first floor and cut out the relevant blocks in the external and internal walls. It was tricky but was done to our satisfaction and the NHBC. As far as guidance for this scenario I am not sure if there is – I would though be interested to hear if you find out there is guidance on the subject. 4) This is pretty much a last option so doesn’t tick any of the good practice guides. That being said, validation and verification is key. We routinely do this where there has been a hydrocarbon spill adjacent to a building. Fairly straight forward if designed and verified. 5) In X we had a similar issue with a commercial building which was substantially completed without the promised/conditioned gas protection measures (CS2). The consultant, who was actually quite good, attempted to negotiate a retro fit with his client and provided additional ventilation calculations etc. which demonstrated that the installation of additional ground floor wall cavity vents and ventilation vents under the roof eaves of the building would be sufficient to generate air flow through the building to reduce gas flow/accumulation to an acceptable level. The ventilation calculations are set out in the CIEH "LA-Guide to Ground-gas (c.2008)" and relate primary to car subfloor car parks but they seem robust for our needs. We also required that the measures were documented I the Site's Risk Management Register so that future owners and tenants would be made aware of the risk measures in place. It was very much a bodge job in a managed warehouse type building and not an approach I would advocate for a private residential dwelling - which is a much higher risk prospect! I think attempting to retro fit a membrane will be fraught with difficulties with high risk of rips and poor seals. Whatever passes should be independently verified by suitably qualified person (CIRIA C735). 6) I am not aware of any guidance but it has been done before. It needs to be done very carefully but can be successful. There are other options available as well to seal the cavity without taking the membrane through the wall - depends on slab and wall details and level of gas risk 7) Retro fitting a gas membrane by jacking up the house sequentially taking out a course and fitting membrane across the cavity is possible but immensely expensive and fraught with difficulties. Depending on the risk profile I have seen sub floor monitoring, internal floor membranes and cavity testing and in the case of managed buildings active ventilation solutions all used to good effect. However if the developer has opted for this solution he has either tried these and failed or was not aware and has been led down a particular route. The key, as in all gas solution installation, is validation and so long as there is an appropriately qualified independent validator involved who is essentially signing off the work ( this must be an amber 2 site ) then you can be confident the installation will be in line with standards and work. Beware there is a proliferation of unqualified validators signing off associated companies work; for your own peace of mind I would check the independence of the installer / validator relationship. Thank you, Mr Alex Wall Technical Officer - Environmental Health Quadrant, The Silverlink North, Cobalt Business Park, North Tyneside, NE27 0BY Tel: 0191 643 6647 Email: [log in to unmask]