crawl spaces are notorious for being nasty places that are generally an afterthought in the design of a home and are typically halfhazard ventilated to meet code animals and bugs love to find ways inside crawl spaces even if the vents are screened insulating a crawl space can also have a massive impact on the durability and the moisture dynamics of the floor assembly and insulating a crawl space incorrectly can lead to the deterioration of the framing can lead to mold growth and rot this comes down to whether the crawl space is part of the condition space or whether it's traditionally vented conditioned crawl spaces on the other hand perform significantly better than vented crawl spaces and tend to be easier and cheaper to properly insulate in this video we're talking about how to design a conditioned crawl space and how to insulate them using several different strategies let's get into it a conditioned crawl space is part of the conditioned space it receives fresh air and exhaust St air and it's pressure balanced with the interior living space with transfer grills or another strategy to reduce the chances of negatively pressurized space a conditioned crawl space is a essentially a short basement and it has to be treated like one we aren't introducing any unconditioned outside air I don't like to use the term unvented crawl space because it's not unvented it's part of the conditioned space conditioned crawl spaces are not insulated at the floor level but rather at the exterior stem walls and at The Rim joists and occasionally above ground level and we're either providing Supply and return ducts within the conditioned crawl space transfer grills or another means of air circulation since a conditioned crawl space is essentially a short basement we can use the same strategies to insulate a conditioned crawl space as we would a typical basement that means we can insulate the conditioned crawl space either on the interior or on the exterior of the assembly however insulating on the interior tends to be the most common strategy now the first strategy that we can use to insulate a conditioned crawl space is to install taped rigid foam insulation on the interior side of the stem walls and around the rim joists and sealing the connection to the rim joists either with a fluid applied joint and seam filler compound or expanding foam sealant for this strategy we can either use XPS EPS GPS or polyisocyanurate as all of these insulation types will provide the benefits of a vapor retarder to prevent condensation on the concrete stem walls and taping the joints will provide the benefits of an air barrier remember we still have to prevent condensation from forming on the cold stem walls from the warm moisture Laden interior air as we would for a typical basement the rigid insulation should be taped to the vapor barrier on the floor of the crawl space to prevent air leakage that could result in liquid water from condensation preventing air leakage is still a very important consideration in a conditioned crawl space as air can deposit moisture at a rate that's orders of Mag mag itude higher than diffusion we also have the option to insulate the floor of the crawl space with rigid insulation installed underneath the vapor barrier and pour a thin topping slab or rat slab over the vapor barrier similar to a basement this makes for an extremely high performance crawl space assembly and will allow for easy maintenance access to the Mechanical Services that are running through the crawl space instead of crawling on an uneven bed of gravel and Earth the slab provides a smooth level surface another common strategy that can be used to insulate a conditioned crawl space is to apply continuous layer of closed cell spray foam on the interior side of the stem walls and up onto the rim joist and on the underside of the subfloor to prevent air leakage and Vapor diffusion that could result in condensation this tends to be the most convenient option as it doesn't require taping each individual rigid insulation board and sealing around each penetration at The Rim joist however spray foam can offgas some nasty chemicals for an extended period of time even if it's installed properly and this can be a problem especially if we're including the space as part of our interior condition space and presumably running duct work through the space if spray foam is to be specified I'm recommending that it's allowed to off gas long before the building occupants move in and that both air samples and foam core samples are obtained and independently tested for irregularly High quantities of off-gassing chemicals specifically the blowing agents used luckily the spray foam will be protected from large fluctuations in temperature as it's located below grade or mostly below grade and separated by the thermal mass of the concrete materials like mineral wool and rigid wood fiber can also be used to insulate conditioned crawl spaces successfully when combined with a smart Vapor retarder mineral wo and wood fiber insulation are both air and Vapor permeable and require that the moisture Laden interior air is controlled and prevented from diffusing through the insulation and condensing on the cold Foundation walls now there are some rigid wood fiber products that are airtight but that's for another discussion in this case a smart Vapor retarder membrane can be used to prevent Vapor diffusion into the foundation walls but allow moisture to dry into the conditioned crawl space if humidity increases or if there's a leak through the stem wall if you're designing or building a home with a crawl space a conditioned crawl space is a must we eliminate all of the problems that we associate with crawl spaces the mold the rot the bugs the critters that love to make a home in there it's completely worth it and necessary to upgrade to a conditioned crawl space guys make sure to give this video a like And subscribe for more weekly building science content and head over to sird designs.com where we have over 150 free building science articles that cover a wide range of topics including conditioned crawl spaces vended crawl spaces p and beam systems and more for now good luck with your projects cheers [Music]