Residential frost-protected shallow foundation in central Mass.
We are planning to provide a monolithic pour frost-protected shallow foundation (FPSF) for a single story 1300 sq ft 2 bedroom home in central MA. Air Freezing Index is 2000. I have read through many different case studies, research papers, codes, etc and there is a bit of conflicting and locale-specific info so I am looking to see whether I understand what is required here. I am a newbie owner-builder. Any constructive criticism would be appreciated, but I don’t want to foment any religious wars.
Grade/Fill We are on a few percent slope, so we plan to excavate to subsoil then back fill with appropriate compacted materials (per R403.3) about three feet so we sit above natural grade at the highest point. Adding this much material is expensive and mitigates some of the low-impact of the FPSF, but it needs doing. Still looking for a structural engineer for various sign offs.
Exterior Vertical InsulationWe plan to have XPS R-10 2 inch as external vertical foam (R403.3 requires >= 5.6). My current thought is to place the foam *inside* the concrete forms and have some appropriate screws protruding such that when the grade beam is poured it will adhere well. I’ve a little anxiety that the foam might float during pour so need to quell that concern. After, the mechanical protection for above/below ground vertical foam will be a fiber-infused ‘stucco’ product so one question is how to best adhere some kind of metal mesh to the outside foam as a base for the stucco, perhaps already attached to the foam inside the form makes the most sense? Another possibility is a peel&stick aluminum flashing after the forms are removed, instead of stucco. I do not believe our locale is termite-paranoid, which leads to angst about vertical foam according to some accounts.
Grade Beam Dimensions From what I understand our foundation must protrude >= 8 inches above soil (MA building code) and <= 12 inches (MA energy code). From FPSF tables, the grade beam depth must be >=14 inches. So my math says that the grade beam must be >= 22 inches in height. I also believe that the base of the grade beam must be >= 12 inches in width as it rests on the ground. That all means 24 inch tall forms to accommodate 22 inch of grade beam concrete and 2 inch of XPS thermal barrier underneath it. Yes?
DrainageWe will aim to have very, very good drainage around and under the foundation (and downslope daylighting) since we are at the bottom of a hill with a perched water table. Yearly spring thaw has water nearly to surface for a few weeks. The roof is 5/12 shed style, metal. We do NOT plan to have gutters and will instead rely on the perimeter drains to keep moisture moving, with hardscape on surface to prevent ruts. Advice here is welcome. I don’t want gutters due to mechanical stresses put on by snow season, though we might consider ‘fair weather’ gutters to capture rainwater for the garden if droughts become more prevalent in future.
Horizontal Perimeter InsulationPlaying it conservative, I am going to hedge my bets on ‘climate change’ and add 2 feet of 2 in R10 XPS horizontally on the entire perimeter, though it is not currently required for our locale. We should be >=12 inches soil depth covering so I am not required to have hard (gardening accident) protection. The perimeter drainage gravel will be under the horizontal foam, daylighting downslope. The horizontal foam will be taped to the vertical foam after forms are removed, sloping away from foundation.
The SlabInside, I am planning on 4 inch slab (with embedded hydronic pex — let’s skip comments on this as a heating choice unless it directly bears on the FPSF performance, anyway there will be ductless minisplits as primary). A slab makes plumbing a more hair-raising experience since things have to be planned a priori and less tolerant of failures. Various plumbing intrusions (well supply water/electric) and outgoing drains complexify life. I imagine that all of these will traverse underneath the grade beams, sleeved&snaked where advisable for future maintenance. Various plumbing verticals will protrude above the slab, to be connected later, but are relevant because they will pierce the slab and thermal/moisture barriers.
Here is where there is some controversy. My choice is to have R10 2 in XPS board as a base, then 6 mil barrier on top. The 6 mil will be immediately underneath the poured concrete. XPS boards will be taped to each other to prevent wandering. The plumbing/electrical piercings will be taped to keep reasonable integrity of 6 mil. I think their might be some in-floor ‘boxes’ for plumbing that needs to be adjusted/concreted later (thus not part of initial slab pour) and I am still under-clued about best practices in this area and would welcome URLs. There will be XPS under the grade beam, both in the angled slope and under the 12 inch wide base — ie, the concrete is poured on top of 2 inch XPS. I will have these all taped to prevent wandering. The 6 mil will be continuous from underslab, under grade beams and then up the vertical exterior wall to aboveground adjacent to the vertical external boards. Except for intrusions, this should cover moisture protection thoroughly, no? Will typical practice in MA allow R10 XPS *under* the grade beam, structurally — I think yes, but would feel better if I saw in writing.
Some people suggest adding >> R10 under the slab, however the ‘Revised Builders Guide’ on page 6 specifically states that R10 is the *maximum* in order for the heat of the building to contribute to the FPSF magic using the Simplified Method. Thoughts?
I don’t really believe in radon hazard, but I am willing to place a perforated pipe under the foam board in each section of the slab (we have an internal grade beam, so I would add the vent pipe for each), through the slab and capped in case some ‘soil gas’ mitigation needs to be done later.
Instrumentation I would like take this opportunity to bury thermal sensors at various areas of the slab and underneath so I can have early warning if we are entering a frost-danger, despite all the best intentions of the FPSF. Since this is something that will want to be useful for decades, I was thinking of capped pex ‘sleeves’ and then just snaking down simple wires with thermal probe, have all of them come back to the utility room (and protecting against critters). I do not plan to have such feedback into the heating system, but rather for purely human consumption. Knowing that budget is tight, does this seem a reasonable plan for future-proofed thermal measure?
Seeking WisdomI know this was long and hard to follow without diagrams. I appreciate the GBA forum to ask such questions. This is our ‘forever home’ and I am very constrained by budget so need to keep things very simple and such that I can perform much of this work with guidance or at least oversee it with proper understanding. I want to prevent water/heaving issues and still be have reasonable energy efficient. I do not want to have any serious failure of the concrete for my next 50 years. But most of all, I need to appease the MA authorities that this is a credible design that meets all the building/energy codes — none of the building professionals I have contacted have seen FPSFs in practice and so this does not give me warm fuzzies. Is FPSF really still only for ‘pioneers’ in MA or is it becoming more mainstream?
Best regards, Ken
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