![]() ![]() In both Figure 1 and Figure 2 the insulation layers extend horizontally inward – typically 4 feet with a thermal resistance of R-10. Don’t even think of placing a sand layer between the polyethylene vapor barrier and concrete (see BSI-003: Concrete Floor Problems, May 2008). Both require a polyethylene vapor barrier in direct contact with concrete. Įxtruded Polystyrene (XPS) or Expanded Polystyrene (EPS)įigure 1 and Figure 2 show rigid board insulations - foam plastics such as extruded polystyrene (XPS) or expanded polystyrene (EPS) and semi rigid board insulation such as mineral wool/stone wool or fiberglass respectively. This is – and I use the next word with emphasis - absolutely necessary to provide a “termite barrier” and “insect barrier” and to provide air control layer continuity between the perimeter wall and the slab itself which is acting as the air control layer of the foundation. You get a three-for-one….Īnd you need to seal the top of the slab – across the rigid insulation bond break – to the top of the stem wall using a masticed stainless steel mesh membrane strip. It gets better, this capillary break also acts as a “drainage pad” where ground water can be drained through the foundation wall to the exterior. It provides for control of radon and other soil gas ingress when it is passively vented up through the roof to the atmosphere with a radon or soil gas vent. This capillary break serves a second function. ![]() All of the insulations are capillary active even extruded polystyrenes (XPS). Mineral wool/stone wool or fiberglass needs to be of sufficient density to support the slab – typically greater than 10 lbs/ft 3.Īll the insulations need to be installed over a granular capillary break. Internally insulated stem wall foundations typically are insulated with two types of insulation – rigid board insulations - foam plastics such as extruded polystyrene (XPS) or expanded polystyrene (EPS) and semi rigid board insulation such as mineral wool/stone wool or fiberglass. They do not have to be connected to each other….except when they have to. Amazing, both the stem wall and concrete slab are supported by the same thing….the soil. The concrete slab is supported by soil…it is bearing on the soil. The stem wall is supported by a footing that is bearing on the soil. In this approach the stem wall is structurally disconnected from the slab. The easy way to do things is to insulate a stem wall on the inside and thermally decouple the concrete slab from the stem wall. And we will go there…but first the easy stuff. Can we structurally connect stem walls to slabs and not get major thermal bridging without external insulation? Yes. And wait till we talk about termites and rodents and external insulation. This is not necessary…and it results in major thermal bridging unless you insulate them externally. But some folks refuse to believe it and are making folks structurally connect stem walls to slabs. Second, for not good reasons, when we have good soil conditions, it is easy to make stem walls work…structurally. I still think it is a pain in the hind quarters. They are a “bear” to insulate….but it can be done… Note this is a change in my opinion of a couple of years ago where I pretty much said it was not possible to do so practically. You are going to end up with a monolithic slab/grade beam that may also be post tensioned. Other structural engineers hate stem walls…not for good reasons…įirst, for good reasons, when we have bad soil conditions, stem walls are difficult to make work. Some structural engineers hate stem walls…for good reason. The absolute easiest slab-on-grade foundation to insulate is a stem wall….except….when it is not easy…. It is apparently time again to go over what is working and what is not. We were here more than a half decade ago ( BSI-059: Slab Happy, April 2012) and again a couple of years ago ( BSI-096: Hot and Wet But Dry, June 2016). Concrete slab-on-grade foundations should be easy to insulate…but they are proving not to be easy to insulate. ![]()
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