sometimes really old walls didn't have sheathing where the wood siding was fastened directly to the studs and occasionally separated by a layer of tar paper if you're lucky these types of walls can be very tricky to insulate especially if you can't remove the sighting for historical purposes as there tends to be an enormous amount of water and air leakage in these types of walls remember we don't want to insulate until we've controlled water and controlled air leakage since insulating reduces the drying potential of the walls meaning that they stay wetter for longer so if we can't remove the siding one option that we have is to provide a drainage plane and an air gap on the interior side behind the sighing now I'll say up front that this strategy is labor intensive and requires a great amount of workmanship and attention to detail this can't be guaranteed don't bother insulating so behind the existing siding or tar paper we're installing a dimple mat this SLE mat serves a few purposes it serves as a drainage plane a waterproofing layer and an air gap behind the existing sighing is basically functioning as a rain screen for the existing sighting so any water that gets behind here will just drip down the dimple mat instead of ending up the wall cavity and rotting it out if we insulate it and then over the dimple mat we want to install an air barrier membrane in this horizontal wall detail you can see that we have our air barrier here that's stapled to the studs and air sealed with a highquality air sealing tape like that and that will prevent air leakage from the exterior getting into the stud cavities and potentially condensing on the back side of the drywall if we're in a more humid climate so after each stud Bay has been air sealed with that air barrier membrane then we want to insulate it with mineral wall this mineral wall allows moisture to dry out of the wall cavity if it ends up there it's vapor permeable it's moisture resistant and it's not going to impact the moisture dynamics of the building as much as something like spray foam as we're not encapsulating the studs and foam that could impact the Wall's ability to dry out again this is a very high-risk wall condition and we need to make sure that we are providing an opportunity for water to dry out then we have a taped smart Vapor retarder membrane here the smart vapor retarder is serving as our interior air barrier and our Vapor control layer as we still need to prevent interior vapor from diffusing into into the wall cavity and potentially condensing on the backside of The dimple mat in colder climates as then we'll have liquid water inside the walls and then we have 2x3 strapping which is installed to provide an airtight service cavity for the smart vapor retarder and then we have fiberglass matte face gypsum with latex paint you could opt for a typical paper-faced gypsum product but this fiberglass matte face gypsum will provide more resistance to moisture and mold growth in these high-risk walls now we also need to protect the seill plates in this wall assembly from getting wet since water that trickles down the dimpl mat needs it's a path to drain out can't just sit on the S plates so in this assembly you see that we actually need to remove the bottom course of siding to access the S PL on the exterior and that should be okay in most cases with historic buildings if you address it as a sighting repair and if it matches the existing conditions we terminate the dimat on top of a new stainless Z flashing with end dams and with a flashing membrane underneath so if we zoom in right here we have our Z flashing and then we have a new flashing membrane underneath which extends down and past the floor assembly down to the foundation and right over a new Bas flashing with a drip edge so any water that drains Down The dimple mat will hit that Z flashing and drain away from the walls and then we have an eighth of an inch or/ qu in entangled mesh right here and that provides a thin lowprofile drainage Gap behind the siding so any water that gets behind the siding can actually drain out and dry and not be held in tension we don't need a big drainage space but we need a little bit of a gap to provide that drainage you could also use strips of a drainable self adher WRB like hydrogap SA or typ hard drainable peel stick then if we zoom in here you can see how the smart vapor retarder is air sealed and integrated into the new floor sheathing and taped and sealed at the bottom and then our siding gets reinstalled over that so to summarize the diple mat provides our drainage plane and water control layer water drains down and hits the flashing and drains down and out through the entangled mesh without compromising the framing and then we're air sealing with the air barrier membrane and the smart Vapor rarer you can find more details about the solution in my most recent book a guide to moisture management for residential remodels where we discuss a wide range of solutions that work for a lot of different building conditions including old walls like this Mass masonry old basements crawl spaces and slabs roofs windows and much more links to the ebook will be in the description below