Browse Forums Heating, Cooling & Insulation 1 Nov 01, 2019 9:46 am To get air-tightness in a build, should both internal and external wraps be used, or is the external wrap enough? Is this is case of 80/20 rule, i.e. pay 20% extra (external wrap only) to get 80% of the benefit? I'm planning a new house and aiming for energy efficiency so am looking at applying passive house principles. But I'm also trying to minimise changes to standard construction techniques to keep complexity and costs down for my builder. Full-on certified passive houses generally have for the external walls both an external vapour permeable wrap and internal airtight vapour retarding/blocking wrap. The internal wrap is battened off with gyprock installed over the battens to create a services cavity. Plumbing and electricals are then installed in the services cavity so that penetrations through the airtight wrap can be kept to a mimimum. That additional internal wrap and services cavity is non-standard construction so is not cheap. I'm wondering whether a high level of air-tightness can be achieved using just the external wrap. Obviously this needs to be both air-tight and vapour permeable (e.g. Pro Clima Solitex Extasana or Enviroseal ProctorWrap) and everything needs to be taped up carefully. Presumably the wrap would then be run over the top of the walls and under the roof trusses to form a continuous barrier. Be very interested to hear from anyone who has experience with this and with performing blower door tests on buildings having only external wrap. Re: Air tightness: one or two building wraps? 3Jan 03, 2021 3:11 pm For various reasons I put the project on hold during 2020, but am hoping to push ahead early this year. I put the same question to the Australian Passive House discussion group https://www.facebook.com/groups/794257177640528/ In summary the response I got was, yes it is possible to use only a single membrane but the insulation needs to be placed on the warm side of the membrane. In a heating climate (e.g. Melbourne) that means insulation must go on the external side of the membrane and for a cooling climate (e.g Brisbane) visa versa. The reasoning here is that condensation must be prevented from forming on the inside of the membrane. Even though the membrane is vapour permeable it is not enough to ensure the wall will dry out, so better to reduce the amount of vapour getting into the wall or ceiling in the first place. For Sydney (where I live) it can be both heating and cooling so I came to the conclusion that it is best to have the two membranes, i.e. Pro Clima Intello on the interior and Pro Clima Extasana/Mento on the exterior. This means also allowing for wall and ceiling service cavities to minimise penetrations through the membranes. Re: Air tightness: one or two building wraps? 4Apr 28, 2022 3:23 pm Further update for anyone asking the same question. After more delays with my project I looked into this again and learnt that at least one Passive House builder in Sydney was using the external membrane only approach. To be absolutely sure I got a hygrothermal (WUFI) analysis performed for our wall and roof assemblies that did not have an internal Intello membrane. It was found that the condensation risk was very low in our climate (nearest BOM station is Terrey Hills). A different conclusion might be reached in other cooler climates or with other wall and roof assemblies - best to get your own analysis done. Re: Air tightness: one or two building wraps? 5Jun 13, 2022 6:35 pm I'm looking into this also (in Adelaide). I think we'll just be going with the plaster as the air barrier but will be minimising possible leakage points such as no ducted AC, no cavity sliders, minimal wall/ceiling penetrations etc. We'll also use pro clima wall membranes taped and sealed where required. I've been told it will be a struggle to get the house down below 3ach using this method. Our aim is for 3ach (anything below a bonus) and will be installing a centralised HRV system. We're yet to sit down with a consultant and our builder so this is not set in stone yet. If the consultant thinks it will be unlikely to get close to 3ach, we may just settle for good enough (5-7ach) and not worry about the HRV. FYI we're using a builder who hadn't even heard of HRV. They're open to discussing it, but I won't get my hopes up. Re: Air tightness: one or two building wraps? 6Jun 13, 2022 8:01 pm Honestly with the mould explosion going on around the country at the moment, i'd take any projections about weather and condensation with a grain of salt. If I were building, I'd be going with intello and extasana, roflexing any penetrations and taping the hell out of everything. Though if your builder isn't well versed on passivehaus or air-tightness (and you don't have a basically full time SS who can check what each trade is doing) it'll be tough to get decent results. Re: Air tightness: one or two building wraps? 7Sep 30, 2022 2:13 pm RustedOn Further update for anyone asking the same question. After more delays with my project I looked into this again and learnt that at least one Passive House builder in Sydney was using the external membrane only approach. To be absolutely sure I got a hygrothermal (WUFI) analysis performed for our wall and roof assemblies that did not have an internal Intello membrane. It was found that the condensation risk was very low in our climate (nearest BOM station is Terrey Hills). A different conclusion might be reached in other cooler climates or with other wall and roof assemblies - best to get your own analysis done. who did you use for your WUFI analysis? we've been advised to get one done for our roof assembly as well as we're also seeking to adopt passivhaus principles with air tightness and building wraps. did you have to make any changes or interventions to your roof detailing as a result of the WUFI analysis? Re: Air tightness: one or two building wraps? 10Oct 03, 2022 3:10 pm We built in Sydney with a single exterior wrap. We haven’t moved in yet so I can’t comment whether it will deliver what was promised. I will say though that the PH consultant thought that A/C would provide enough dehumidification to move moisture through the wall. The builder disagreed and made us put in a ducted dehumidifier. So we have A/C, HRV and dehumidifier. The insulation is on the inside of the wall because the greater need for our place was cooling. I think we have about $35000 of mechanical climate management equipment if you add the 3 pieces of equipment. Re: Air tightness: one or two building wraps? 11Oct 03, 2022 3:55 pm mockkie who did you use for your WUFI analysis? we've been advised to get one done for our roof assembly as well as we're also seeking to adopt passivhaus principles with air tightness and building wraps. did you have to make any changes or interventions to your roof detailing as a result of the WUFI analysis? I rang a number of companies listed here: https://www.wufi.com.au/wufi-professionals/ At the time I made enquiries, most were very busy and unable to start until after a month or more. In the end I chose Building Physics RP because they were available to start within the week. I was happy with the work they did. Only thing to note was that they would have used climate data based on NCC climate zones, not the climate data that would be used for Passive House modelling, i.e. based on nearest climate station adjusted for elevation. No changes were required to my wall or roof details. I was confident that would be the case as I had already tested the wall and roof assemblies with Ubakus. If your assemblies require an internal wrap, you should also look at creating a services cavity between the wrap and the wall/ceiling linining (i.e. gyprock). This is to reduce penetrations from services (electrical, plumbing) or even from hanging pictures. I still am nowhere near starting the build. The builder I engaged to act as a consultant has lost interest in the project. Re: Air tightness: one or two building wraps? 12Jan 11, 2023 7:44 am Sorry to hijack thread: If you use both an external wrap and an internal wrap plus services cavity (as described in the first post), what will be the total thickness of your external walls? Exempt development if the house is existing. If it’s a new build it’s likely not an issue either but always call council to confirm. They’re usually very helpful. 3 5057 We are getting the same kinds of figures. Have you committed to a design/builder yet? We are finding it hard to sign up with someone because you can’t see a design &… 3 5844 2 24169 |