Super Insulated Walls on a BUDGET?!? | Double Stud Walls Explained

what if I told you that you could get R40 or even r60 in your walls for just a few dollars more per square foot that's right we're talking about double stud walls and how we can get passive house levels of performance and comfort using this strategy this is one of the best ways that you can get a super insulated home on a budget without breaking the bank in this video we're talking about how to design and build double stud walls some of the planning considerations that you need to be thinking about and how they compare in cost to some of the other installation options that we have on the market let's get into it so double stud walls are often used as a strategy in passive house projects to achieve higher levels of Energy Efficiency without adding too much additional cost by simply adding an additional framed wall to the assembly we can create a cavity space that can accommodate really as much insulation as we want depending on how far away we space that secondary framed wall from the primary framed wall if we plan for it we can also use 2x4s as opposed to 2x sixs to frame the assembly which is beneficial to reducing material costs as 2x4s are cheaper and tend to be more widely available we're also able to insulate the cavity with dense packed blown in insulation of our choosing without having to resort to using other types of insulation like spray foam or exterior rigid insulation blown in cellulose tends to be the most common but you could also use blown in Wood fiber blown in Mineral wool or even fiberglass which opens up a lot of possibilities for different double stud wall designs depending on both the availability and cost of the insulation products in your market now when it comes to double stud construction there's a few things you need to be aware of when planning out the design and specs of the wall assembly first of all because these walls are super insulated they're more susceptible to potential moisture problems because of the lack of heat flow in the assembly in order for drying to occur we need heat flow and so in a double wall system we get substantially less heat flow than in a typical insulated wall built to code this means that we need to be doing everything that we can to prevent and control bulk water from getting inside that means providing some nice steep overhangs to shelter the walls from rainwater penetration this also means that we need to specify a self-adhered or fluid applied weather resistive barrier system to improve the water resistance of the assem since self-adhered and fluid applied systems are bonded to the sheathing unlike building wraps or house wraps which are just tacked on and taped as these tend to be more risky we need to be providing a ventilated rain screen behind the cladding not just a drainage Gap because we want that convective air flow to help dry out the sheathing if it gets wet since we don't get the benefits of interior heat flow and we want to make sure that we're following best practices with regards to flashing that means we want fully drained s pans around windows and doors we want metal head flashings with drip edges around rough openings to kick water away from the walls we want base flashings at the bottom of the walls and so essentially we need our wall assembly to be bulletproof in terms of its ability to resist and shed water we also need to worry about air leakage in these super insulated assemblies air leaks and well insulated assemblies are the second leading cause of moisture related failures after bulk water air carries and transports moisture in the form of vapor and if we have warm moist air from the interior condition space leaking into the super insulated wall cavity and condensing on the back side of the sheathing that wall has basically no ability to dry out out we can get things like mold growth and rot over time if these air leaks are not addressed we also have to worry about air leakage from the exterior of the assembly that could cause interstitial condensation to form within the wall assembly especially in those more humid climates like on the east coast of the United States where Summers tend to be hot and humid we need both an interior air seal and an exterior air seal to ensure long-term durability and performance so let's go over the details and specs of what I would consider to be the most affordable and least risky method of constructing a double wall assembly and how we can achieve any between an R40 and an r60 wall so starting off with the framing we're using 2x4s for the double wall framing since they're substantially cheaper than 2x6s and more readily available and we want to offset the walls in a way that allows for the studs to be staggered so that we have less heat loss at the stud locations we don't want the studs on the primary exterior framed walls to be aligned with the studs on the interior framed walls since it will reduce the effective thermal resistance of the assembly at those locations now the distance that we space the walls apart will ultimately determine what kind of R values that we can get out of the wall assembly for example if we want to achieve an R40 double wall assembly with blown in insulation which is R4 per in we need roughly a 10-in cavity which means that we need to space the 2x4 walls apart by about 3 in now if we wanted an r60 wall assembly with cellulose we would need a 15-in cavity which would space our 2x4 walls apart by 8 in doesn't really add too much additional cost to go from R40 to r60 since you're more or less paying for the installers time when it comes to blown in cellulose however the added thickness of the walls can impact the appearance of the building and the interior square footage if you haven't already planned for that now in terms of exterior sheathing it's really important that we specify plywood and not OSB remember because these walls have less heat flowing through them we need to maximize the drying potential of the assembly and we need to make sure that we're using materials that are more resistant to moisture when it gets wet OSB has a vapor permeance of about 2 to 3 perms which isn't that much in terms of its ability to dry out meaning that stays wetter for longer there's also less real wood in OSB as it's made from small wood chips or strands that are glued together which means that it's more susceptible to mold and rot fungi whereas plywood becomes a lot more Vapor open when it gets wet and it's composed of a lot more real wood and will resist rot fungi more effectively than most OSB products now if we're insulating our walls to r60 or higher we actually need to start using a gypsum sheathing like dense glass gold rather than plywood because we need the enhanced drying potential and mold resistance of the fiberglass matte face gypsum product and that will add some additional cost to the project then for our self-adhered weather resistive barrier we're calling out blue skin VP 100 this is a pretty affordable self-adhered membrane that's Vapor open at around 30 perms it'll act as our exterior air seal and it'll be suitable for most double wall assemblies up to r60 for double walls that are insulated to r60 or higher we need to switch to a more Vapor permeable weather resistive barrier that's closer to 50 perms or higher in cold climate applications we need to use a primer to ensure that the blue skin is able to adhere to the sheathing and fully Bond we don't want that weather barrier flapping in the wind since this would violate the Integrity of both the water and air barrier then we can fasten 1x3 furring strips to the studs through the weather resistant barrier to provide a ventilated drainage gap for the cladding and then we're calling out a fiber cement cladding which will provide long-term durability to the wall assembly and it's worth that small increase in cost then as I mentioned before we're insulating with dense packed blown in cellulose which is essentially composed of recycled paper products like newspaper that's been treated with some mineral salts and borate to deter insects and inhibit mold growth and to provide some fire resistance dens packed cellulose is roughly R4 per inch and when installed properly it's effective at preventing air leakage it doesn't eliminate the need for a continuous air barrier by any means but it significantly enhances the overall performance of the wall assembly cellulos is also surprisingly moisture resistant despite being made from paper and this is mostly due to the addition of borate or boric acid during the manufacturing process now it's important that we're using a dense packed cellulose and not a loose fill cellulose since loose fill tends to settle over time it's air permeable and we get poorly insulated or uninsulated cavities towards the top of the wall assembly which can cause some pretty serious issues related to convective looping condensation and mold dense packing the cellulose so that it's one solid unit will help to prevent this from happening on the interior side of the wall we're calling out a taped smart Vapor retarder membrane in this case intell plus which will serve as our interior air seal as well as an interior Vapor throttle that will prevent moisture from the interior from diffusing into the double stud walls and potentially condensing on the back side of the sheathing allowing for any potential moisture to dry out of the wall assembly back to the interior which is highly beneficial to preventing any kind of rot or mold problems within the wall assembly this is what makes it unique from a standard polyethylene Vapor Barrier that doesn't allow for drying in either direction now I have to emphasize that this smart Vaper retarder must be taped at every single joint seam and penetration to ensure that we don't have air Le through into the cavity again air leakage is a major cause of failures in these high performance wall assemblies since air leaks transport moisture this is why we've called out this service cavity on the interior that's created with this horizontal 2x3 strapping or furring since this will reduce the amount of penetrations in that membrane and allow us to run electrical conduit and other services in this cavity behind the drywall it also allows for easier access to these services for maintenance without having to cut into the smart Vapor retarder membrane and potentially violating the Integrity of that air barrier during the repair and finally we have our drywall layer this can just be your standard gypsum board in this assembly we don't need a fiberglass matte face gypsum since we have our smart Vapor retarder helping to throttle inwardly driven moisture which helps to keep costs down we don't want to install any finished materials with foil facings or vinyl facings on these exterior walls since the walls need to be able to dry through the gypson board this also means we want to avoid locating showers on exterior walls or any other specialty rooms that require the use of impermeable finish materials that can impact how these double walls function now we need to treat Windows a little bit differently in our double wall assemblies especially if we're super insulating to higher R values first of all we really should be considering upgrading our Windows to triple glazed units especially if we have a large temperature gradient between the interior condition space and the exterior environment we don't want condensation forming on our Windows since they would be the coldest component in the assembly if we can't use triple glazed Windows the location of the window within the rough opening becomes extremely important as we would want to inset the window further in to keep the glass closer to Interior temperatures what can happen in these thick double walls is that if we locate the window unit flush with the sheathing as in the case of a typical flanged window we can sort of create a microclimate within the rough opening where conditions are slightly colder now there are ways to make flanged Windows and flangeless European windows work in a double wall assembly however I think the flangeless windows allow for a lot more freedom and ease of installation and far less headaches since using a flange unit would require building out an additional inset nailer to accommodate the windows nailing flange as far as prepping the rough opening we need to essentially join the exterior framed walls with the interior framed wall using a plywood Buck to help close off the wall cavity and to provide a uniform surface for the window installation 3/4 in plywood or OSB works well here but we want to make sure that it remains dimensionally stable and strong enough to hold back the weight of the dense pack cellulose the sill is sloped to the exterior with a beveled wood plate or a beveled piece of siding to drain out any water that happens to leak through the window unit or through a flashing failure and and shims are installed over the beveled plate to provide a level surface for the window installation we also want to provide a back Dam to prevent any water from leaking inside into the interior space from wind driven rain then the entire rough opening is coated in a fluid applied flashing that's compatible with the weather resistive barrier product and that will provide a monolithic water and air control layer that can transition to the weather resistive barrier seamlessly and will eliminate the potential for any leaks we're essentially creating a gutter underneath the window unit without the issues associated with metal sill pans then we're free to to install the window unit flash it to the rough opening at the jam and at the head leaving the sill open for drainage we provide a head flashing or drip cap flashing at the window head that's integrated into the weather resistive barrier and then we can air seal the window on the interior on all four sides continuously and that sums up our window install now let's talk about cost and budgeting for these types of double wall assemblies I first ran the numbers on this back towards the end of 2022 and after prepping for this video I was absolutely Blown Away by the overall cost increase across the board over the last year and a half now I should mention that I'm located in Oregon where costs tend to be on the higher end so some of you might find your costs to be lower in other states some might find them to be higher but this is just to establish a rough idea of how much more a double wall assembly will cost compared to a standard wall assembly and walls with exterior insulation this doesn't include labor because cost of Labor greatly varies around the country and will vary depending on the experience and quality of the builders and their subcontractors so let's go over our Standard wall assembly for what I would consider to be the absolute minimum for a well-built wall and how it compares to a double wall we have 2x6 framing with Zip system sheathing which serves as our weather resistive barrier and our air barrier after we've taped the joints we have fiber cladding installed over 1x3 furring for our ventilated drainage Gap we're insulating with an R23 Rock wo bat within the stud cavities and we're finishing the interior with just 1/ in drywall optionally we have a taped smart Vapor retarder if we're building in a cold climate to control condensation without adding too much cost so our base cost for this wall assembly is around $6.38 per square foot or $6.88 per square foot if we're adding that smart vapor retarder and that's wall area not linear square footage just to be clear so to go from this to our double wall assembly with the specifications that we walked through earlier it's only about a 27% increase in cost to $810 per square foot and this allows us to get double or even triple the performance of The Standard wall assembly for a relatively small bump and material cost especially with all of the components that we're adding to the assembly again this is not including installed cost so labor is going to need to be factored in when you start collaborating with your contractor or your Builder but 27% increase in cost for double or triple the performance not bad at all now if we were to compare this to a wall assembly that was insulated on the exterior with a couple layers of rigid mineral wool to achieve a minimum of R40 we would see a 76% increase in cost from $6.38 a sare ft to $11.24 aare Ft that is an enormous increase in cost compared to the double wall assembly and you're limited to about 6 in of rigid insulation before the cost of the Fasteners becomes astronomical now granted you are getting some better durability by using rigid insulation since we're able to warm the condensing surface of the sheathing we don't have to worry nearly as much about condensation within the wall assembly or small air leaks that rigid insulation layer provides a lot of benefits but if you're trying to Super insulate your walls on a budget it's not exactly the most cost-effective option and I'm a huge fan of rigid insulation installed out Bo it's just not for every project now that was using rigid mineral wool which is one of the more expensive rigid insulation products that you can install what if we switch to EPS foam we cut the cost of the rigid insulation in half but it's still a 57% increase in cost from The Standard wall assembly to achieve the bare minimum of an R40 wall so double wall assemblies really do offer the best bang for your buck in terms of achieving a super insulated high performance home I hope this was enlightening go check out the full article on double wall assemblies on my website at ai- designs.com and consider picking up my cad details for double wall assemblies which include three different options for double Wall Systems along with the window details I'll put links to all of those in the description below we also have over 150 free building science articles that cover a wide range of topics that have a focus on moisture management and insulating properly to ensure long-term durability leave a like And subscribe if you haven't already for more weekly building science content and for now good luck with your projects cheers [Music]

Channel: ASIRI Designs

Views: 168,137

Keywords: double stud walls, double walls, double stud, double, stud, walls, construction, double wall, insulation, energy efficiency, how to build, how to, double wall details, double stud details, details, construction details, insulating, insulating double stud, double stud framing, waterproofing, double stud window, window details, double stud window details, specifications, building science, asiri designs, are double stud walls worth it, moisture, condensation, vapor barrier, passive house

Id: 8n08OIHCY3o

Length: 15min 7sec (907 seconds)

Published: Sat Apr 06 2024