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Community and Q&A

Residential frost-protected shallow foundation in central Mass.

Kenneth Gartner | Posted in Building Code Questions on

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

References –

Revised Builders Guide

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Replies

  1. D Dorsett | | #1

    That's a lot of premium priced XPS, all of which degrades to ~R4.2/inch well within the lifecycle of a house. You could replace a lot of that (or all of it) with 1.5lb density "Type-II" EPS (also R4.2/inch) and save some money, and it's quite a bit greener too due to the drastically high impact of the blowing agents used for XPS, and the comparatively benign pentane used for blowing EPS.

    You can save even more money (saving more than half, sometimes more than 2/3 the material cost) if you buy all the foam on the reclaimed foam & factory-seconds market. In Worcester there is Green Insulation Group (http://www.greeninsulationgroup.com/ ), and in Framingham there is Nationwide Foam (http://www.nationwidefoam.com/ ), either of which will deliver bigger quantities to your job site if desired (for a price), or you can rent a truck and haul it yourself. Reclaimed roofing foam tends to run ~25% the price of virgin stock, in near-perfect condition. Factory second new stock is a bit more. Either EPS or XPS will work in your application, but you can't use polyisocyanurate below grade or under slabs due to it's hygroscopic characteristics.

    Radon hazard isn't faith based- the statisitical science is clear. There are many non-smoking non-believers who have succumbed to lung cancer when radon was the only risk factor. Believer or not, if your house tests high (and many do in central MA, due to the copius quantities of granite in this region), affects property values. It's wise to install a passive radon abatement system under the house, and test. Even if it tests at 4 picocuries per liter or higher (the remediation threshold), it can sometimes be dealt with by adjusting HRV ventilation rates up a notch, or by using passive venturi type vent caps on the radon stack, either of which could be tried before installing 24/365 radon fans.

  2. GBA Editor
    Martin Holladay | | #2

    Ken,
    Q. "How to best adhere some kind of metal mesh to the outside foam as a base for the stucco?"

    A. As far as I know, the best way is to use TapCons (through the rigid foam to the concrete) with washers.

    Q. "Very, very good drainage around and under the foundation."

    A. Yes -- you want good drainage. Use redundant drain pipes. A few 4" diameter perforated pipes in the crushed-stone layer (you will have a crushed stone layer under your rigid foam -- right?) sloped to daylight will handle the sub-slab drainage.

    On the uphill side of your site, you need the grade to slope away from the foundation to a swale. In the swale, you need at least two catch basins. I've had good success with plastic catch basins designed for 6" diameter drain pipe. (Don't be tempted to use plastic catch basins designed for 4-inch drain pipe.)

    Q. "Horizontal R-10 under the slab is the maximum?"

    A. If you have PEX tubing in your slab, I urge you to consider R-20 horizontal foam under the slab. Contrary to popular belief, a FPSF does not depend on leaking heat from the building to work. More insulation is better.

    Here are some relevant links:

    Frost-Protected Shallow Foundations

    Foam Under Footings

  3. Brian P | | #3

    I'm slightly more experienced than newbie after overseeing and helping with the build of our small net positive house in NH, zone 6. We have a FPSF and are using a single ductless mini split for primary heat. I'll let others with more experience comment on most of your post, but a few comments based on your desire for simplicity and having budget constraints:

    Make your roof overhangs extend out a good amount to keep water/snow away from the house, 2' wouldn't be unreasonable.
    You don't want comments on this, but...the ductless minisplit(s) will be more than enough for primary heat in your climate. I don't see the benefit of installing another expensive heating system, I would even skip just installing the pex as a "maybe we'll add it later".
    The thermal sensors are an unnecessary expense and complication. Many people in colder climates than you have successfully built houses with FPSFs without problems.
    Take the standard radon mitigation steps now, so that it doesn't become a more expensive/complicated situation later. Even if you don't believe in radon health risks.

    Take the money saved and put it into...better building envelope, solar PV, windows, etc. My $0.02.

  4. Kenneth Gartner | | #4

    @D Dorsett, excellent links to local foam resources. Bravo. That could result in tangible savings. I solemnly vow to keep fully paid on my new GBA membership during my construction phase with such savings ;-)

    I have not heard that EPS could be used underslab. The literature from XPS manufacturer Owens Corning seems to go to great lengths to say that EPS is not good due to many reasons. Is this merely marketing spin in favor of XPS or is there a real reason to worry about foundation failure if EPS is used? This is part of what confuses me about the contrary recommendations I have. Can you provide a reference where EPS is recommended for under-slab please, if you know one? http://ww3.owenscorning.com/content/docs/FOAMULAR%20XPS%20vs%20EPS%20extrusion%20matters.pdf

    As to the statistical data on radon, I suggest you keep an open mind about it. It seems an over-reaction to a non-problem became embedded into code/practice and entire industries sprung up to support it. Alas, that does not make it true. Here are statisticians that say that the prior statistical work that led to radon worries in the first place was based on incorrect math models. New models based on radon having a hormetic effect completely change the interpretation dramatically such that radon is actual *good* for you in small doses, so removing it is actually the wrong thing to do, healthwise. Aye, caramba! http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3315166/

  5. Kenneth Gartner | | #5

    @BrianP. I am glad to see a FPSF owner on the forum. My architect has seen it work in many states, like NH, but here in MA, it is not the weather that concerns me most, it is my mandatory HERS rating score and the various code/inspection hurdles with such unfamiliar practice.

    We are hopeful that minisplits will work as primary, but that involves a bit of faith. What subslab insulation did you build in? Do you live on the exposed concrete or do you have carpets or other flooring that isolates you from any concrete chilliness? Thanks for letting us know a success story.

    Since I expect to be in this house for many decades and there will not be a do-over for the foundation, the pex does not seem an expensive item to add before the pour. I would design a simple manifold, single-zone and eventually consider SHW assist if the minisplit does not pan out and the floor heat comes into play more enough. It is all electric heat -- no combustibles -- so I don't want to depend on the subfloor heat. However, the reason for the thermometers is because you 'cannot manage what you cannot measure'. As Martin mentions, it is not the house heat that prevents frost heaving, but the insulation on the earth's ambient temperature itself. If I have some simple temperature measurements I can decide when I need to take evasive action. So for my current thinking, cheap pex solution will be the insurance just-in-case there is some unexpected glitch in the environment (whether polar vortex induced or not) as well as basic comfort. Keeping an open mind, though.

  6. Stephen Sheehy | | #6

    Kenneth: My house in Maine is on a slab, with a traditional foundation down below frost line. We used 4" of reclaimed XPS under the slab. We got it from the Worcester location Dana mentioned. Even with shipping to Maine, we saved a lot. The reclaimed insulation was a little dirty, but otherwise nearly perfect.
    The slab was polished and is the finish floor. We have never felt that the floor gets cold. The house is so cozy that I'm usually in bare feet, even in winter. The house is one level, about 1650 square feet. Walls are about R45, roof about R70, Windows R11.
    We heat with 2 Fujitsu RLS 3 minisplits. I believe you'll find that radiant heat is a total waste of money. The minisplit(s) will heat ( and cool) just fine. With a good envelope, the radiant heat would rarely be on. Spend the money on envelope improvements and/or solar.

  7. GBA Editor
    Martin Holladay | | #7

    Stephen,
    Thanks for your convincing testimony.

  8. Expert Member
    Dana Dorsett | | #8

    Thanks for the link to that NIH paper! I'll be reading it in detail (with great interest!) as time allows.

    There is no basis for most of the objections to EPS under slabs( or other below-grade applications.) There are decades of experience with EPS insulation under slabs of 10s of thousands of buildings, and between concrete foundations and soil. Type-II EPS is the most commonly used material for insulated concrete forms and EPS is used under roadbeds and airport runways as frost-heave prevention too (in case you're using the slab as part of the landing pad for your C-130. :-) ) EPS crumb (loose beads) is also used as backfill (with a landcape fabric wrap) as drainable backfill on large commercial foundations in extremely cold climates for frost heave mitigation too.

    If it's in an absolute un-drained swamp and the foam is below the water table Type-II EPS will see a 10-15% degradation in thermal performance when saturated, but if the water table is only 6" below the top of the slab you already have MUCH bigger problems to worry about, eh? (Like heading for high ground while you still can!) And when the tide goes out, the water leaves the foam quickly. There isn't a real advantage to XPS in this application- it's only marketing. For the counter-marketing fluff from an EPS manufacturer, see: http://www.concreteconstruction.net/how-to/site-prep/choosing-between-eps-and-xps-rigid-insulation_o

    Assuming you are building code-legal drainage under the slab (and foam), and using reasonable surface drainage and backfill on any stem walls & slab edges, there is no way for the EPS to be taking on copious amounts of water. Just about every PassiveHouse design has EPS under the slab.

    If the slab is being used as the heating radiator it's cost effective to take it to at least R15 even with new virgin stock foam, and R20-R25 isn't crazy when using reclaimed foam. If the PEX is going to be stapled to the foam, make the top inch of foam XPS, or Type-IX (2lbs density) EPS for better staple retention.

    Mini-splits work just fine as primary heating (without backup!) in central MA, even in not-so-super-insulated buildings. A deep energy retrofit project I was involved with in ZIP 01610 a few years back is heated with one mini-split per floor (x3.) The load numbers on each floor were within the published output of an FE12 Mitsubishi, but the owner was concerned that there wasn't sufficient margin and went with FE18s instead, which were probably sub-optimally oversized, but not by lot. They've cruised through temps as low as -12F since then without coming anywhere near maxing out. (The local 99% outside design temp there is in positive single-digits, like most of central MA.)

  9. Brian P | | #9

    We have 4" of XPS under the slab and on the slab edge. Logistics and time prevented us from using reclaimed XPS or EPS, but I would have liked to.

    Our first floor is the exposed concrete and it feels comfortable, we stained and sealed it ourselves.

    The best I can do is give you my advice after being heavily involved in our house build, researching many other house projects, and visiting a few. Properly sized minisplits will work well, skip the PEX and associated system, and definitely ditch the temperature sensors.

  10. Marc Labrie | | #10

    Ken, I can attest the frost prevention come from the heat present underground. I poured a slab on grade in New Brunswick (similar climate to Maine ) in October and only started the built next spring. Even with the slab exposed to the elements the whole winter, there was no frost heave. With 8 in of EPS underneath I skipped the in slab heat after calculating the surface temperature would remain within 1 degree Celsius of the inside air temperature. We polished the concrete slab ourselves and it is our finish floor. Note that we also installed a 4 feet wide perimeter skirt of eps.

  11. Jon R | | #11

    There are several FPSF designs and some do depend on building heat. Others use only heat from below and continue to work fine even if the building is left unheated.

  12. Ethan ; Climate Zone 5A ; ~6000HDD | | #12

    Ken, you write: "We are on a few percent slope..." I'm curious what the slope of your site is and if that presented problems with the FPSF design/construction?

  13. Kenneth Gartner | | #13

    Ethan, the slope is about 4% in two different directions. Since a perched water table runs near to the surface (heavy soil, and we are bottom of a hill) we are excavating about three feet of the native soil and replacing with very porous (non-frost susceptible) fill to a level surface slightly bigger than the foundation foot print, topped with about 6 inches of crushed stone as a capillary break. We take advantage of the natural slope of the undisturbed soil to act as a drainage plane and so will place perforated drainpipe in the low spot to move groundwater to a daylighted area downslope one hundred feet. For my peace of mind, there is intended to be geotextile fabric laid atop the undisturbed soil, underneath the fill. Note that the non-frost-susceptible soil is the key element here for the FPSF to work and be code-compliant without footers. Adding that much material is pricey and I hope the touted virtues of the FPSF manifest for us.

    A note for people in MA area. I just got a great deal on reclaimed 3.5 inch EPS subslab foam for $7 each (new, these would be close to $50) with reasonable delivery via craigslist (look for eps in salem, ma). So, I bought more than enough to double the underslab foam to 7 inches ... R28!

  14. Kenneth Gartner | | #14

    For posterity, here is the followup of what I actually did for the FPSF foundation, it being poured last week. I truly was educated by the thoughtful responses to my inquiries and my implementation shifted directly as a result. Thank you, GBA!

    Having scored a huge reclaimed EPS bonanza, I purchased 125 sheets (an entire truckload) of 3.5 inch foam and paid $10 per sheet DELIVERED. I used nearly all of the sheets, most in triple layer for an average R36 across the whole FPSF.

    The 11.5 inch deep grade beams were 16 inches wide and sat on 3.5 inches of EPS (for R14). The slab averaged 5 inches thick and was reinforced by #3 rebar in 18 inch grid. The grade beams were reinforced with 4 #4 rebar. I used 2 inch 'chairs' made from recycled medical waste from small company in Colorado. I agree that this was likely over-engineered but I plan to live to be a centenarian in this house ;-)

    My initial confusion that the grade beams needed to be much taller was corrected by an assisted 'biblical reading' of the relevant codes, some of which were not in the public domain so I never had a chance to read them myself. Thus, the depth of the 'frost-insusceptible' fill was able to be be included in the minimum depths cited.

    The frost-insusceptible base turned out to be more than one million pounds of 'cobble' (mixed stones from 4-16 inches) plateau to a depth of as much as 4.5 feet to compensate for the 5% grade slope and hopefully leave us high and dry. We excavated only two feet of subsoil (hitting boulders so large the excavator could not lift them!) and laid down geotextile before the cobble. This was topped with 3-4 inches of 1.5 inch crushed stone and 3-4 inches of peastone. A plate compactor was run across the surface when all was reasonably level.

    MA requires radon piping, so I placed 10 feet of 4 inch perforated PVC and a vertical PVC reduced to 3 inches to pass inspection. This was set in the peastone layer. The peastone was covered with 6 mil poly ethylene sheet and I carefully taped up around all vertical piping. Note that we had so much rain that large puddles accumulated so I made small scores with a knife and held them open to watch the whirlpool of the drainage into the rock below. I did not seal these tiny slits since they were soon covered with the EPS foam.

    The underslab plumbing was the limiting factor causing months of delay -- hence I will be building this house all winter. I am a newbie owner-builder so getting the attention of a plumber willing to work for hours/materials billing and let me help was a slow process. Even though the excavator had made a huge trench through the cobble, it was not quite deep enough and our under-slab plumbing only got covered by a single layer of foam. Several p-traps were placed under the slab (and filled with antifreeze for now) for a floor drain and curbless shower and a tub. Getting the level heights for final layout were delayed until I could fill the plumbing trench with builders sand and build the forms. I wrapped some of the cheap sill sealer foam around all plumbing verticals that could suffer from expansion stresses as they are embedded in concrete and that was OK with the local plumbing inspector (technically there is requirement to put a PVC sleeve one inch larger in diameter, but that point is only enforced for commercial around here).

    I chose to compose the forms myself from 2x6x12 foot and 2x4x8 foot pressure treated lumber. I wanted to be able to reuse these to make a leanto greenhouse later. I oriented them such that the forms were 14.5 inches tall (2x6, then 2x4 then 2x6). In theory this would have worked OK, but in reality the wood warped and bowed. I ended up trying to use duct tape to block the large gashes (lest concrete weep out) and eventually just shoved a layer of house wrap on the inside of all the forms to keep these boards clean for reuse. Leveling these was tedious and I had to purchase a used rotary laser level and perform lots of fiddling. The forms sat directly on top of the poly, with the poly extending out beyond the forms at least 6 inches on all sides. Inside these forms went the first layer EPS foam, carefully cutting out holes for the vertical protrusions. The second and third layers were inset 16 inches, making the deep grade beam around the edges by their absence. Each successive layer of EPS foam was held in by 6 inch landscape staples, being careful to not pierce the poly underneath.

    I ended up using 12 inch, 5/8th in anchor bolts. I attached the perimeter anchor bolts via a Simpson nylon 'arm'. The bolts averaged 8.5 inches deep into the grade beam and about 3.5 inches above -- I wanted the extra height above so I could add some temporary wooden supports as I try to lift the framing members into place. These anchor bolts were too beefy for these rather wimpy nylon supports and so did not hang down straight. More fiddling and wire tying and manual straightening on the day of the pour. I followed the seismic/high wind guidelines and all anchor bolts are within 4 feet of each other.

    My sills are 2x6s that are cantilevered 2 inches over the edge to accomodate the R10 2in XPS for vertical foam. The vertical side is only 14 inches in height, in profile, with 3.5 inch of EPS and 11.5 concrete to be covered. My forms were not so uniformly straight so not all the cantilevers are fully 2 inches -- sigh, so I will be making a foam wire cutter and trimming them later. I do not believe R10 is nearly enough, so I will take the last few sheets of 3.5 in EPS and slice an angle off the top for aesthetics and adhere them to the XPS, for an effective R24-ish vertical foam. As far as adhering the foam to the house, I think some PL300 or Enerfoam along the concrete edge and an upside down J aluminum "termite" flashing at the top for mechanical hold will do the trick (along with all the soil pushed against it).

    The sill anchor bolts are bolted with 2 inch plate washers for extra strength in high wind situations. The 2x6 sills have EPDM gasket underneath them, for air blockage and capillary break from the concrete surface.

    Note that the poly sheet that underlies the entire insulative layer needs to be extended and folded up along the concrete wall to above grade so that it does not become a 'swimming pool' collecting water that happens to pass through the soil.

    Since the concrete floor will be exposed for at least one year, to dry out (and for our bank account to replenish), we sprinkled 20 pounds of an iron oxide dust to provide a nice adobe color to the surface during the finishing step. I kept a pound or two reserved for repairs and perhaps to tint the caulking that will conceal the expansion control joints. BTW, we put in control joints every 9-10 feet, cut with a 'wet saw' on the evening of the pour -- approximately 1/3 - 1/4 the depth of the slab.

    My whole FPSF foundation will have approximately 6 inches of soil around it. I know there is a good argument that soil should slope away from the house, but I am tempted to have the entire six inch layer be level crushed stone for three feet along whole perimeter so that all surface water flows down and under the house, to continue it path downslope.

    So far all the professionals and the inspector are happily surprised with results, given they had never seen an FPSF in person before. I am still building it, so don't know how it will perform, but it follows the myriad building codes and gets the nod of approval to continue here in heavily regulated MA.

    As to some of the other specific points raised in the initial query of this thread:

    o I WILL put on gutters, perhaps on top of decorative wood corbels, once I figure out how to avoid snow damage potential. The metal roofing will project one foot on both the low and high side of the shed roof, so the gutter attached to the house have to project further than normal. The drainage for gutters will be directed to vegetative swales on the surface or to the garden.

    o I did NOT put the vertical foam inside the form (so that it could be affixed during the actual pour via some kind of tie/screw). I saw no advantage to do this and lots of opportunity for people to trip on it or break it meanwhile.

    o I do NOT have any explicit perimeter drain for this FPSF. I am sitting on a plateau 3-5 feet tall composed of rocks with no interstitial soil. On the downslope I have a 16 inch trench lined with geotextile and filled with crushed stone to wend its way downslope to a vegetative swale.

    o I do NOT plan to place horizontal foam skirt around the foundation. I could add it in the future, but it is not required by code for our area.

    o I did NOT add radiant heat under the slab. This was an agonizing choice since both advocates and detractors were so vocal. I weighed all my options and decided that since I have a limited BTU budget (this is an electric-only house) that (MItsubishi?) minisplits will be the only heat source and radiant did not merit inclusion. Negative factors included worry that PEX left out in the sun for 2 months might degrade, another thing that could go wrong in the future, very limited utility room space to support a manifold, difficulty with devising a 'balanced' system of multiple zones and ultimately we will cover concrete with some snap-click kind of flooring since we don't want our elders to risk a fall onto concrete anyway.

    o In answer to my own question as to how I could tell that a used EPS foam product was 'strong enough' to be a support for concrete slab since they often have no markings. One answer is to identify the source of the foam. If it was used for any kind of horizontal surface (such as a roof) there is nearly 100% chance that it will work fine. When you walk on it, merely slight boot impressions (instead of cracking) are indicative of something that is strong enough for a slab.

    Best regards,
    Ken

  15. Steve Knapp CZ 3A Georgia | | #15

    Great recap, Ken. Thanks for sharing your process for building the FPSF. I admire your tenacity and attention to detail on such a tough DIY project. Considering all the effort this approach required, would you do it again?

  16. Kenneth Gartner | | #16

    That is a poser, Steve.

    I like the concept of monolithic pour, liked the idea of having no component of my foundation rooted in wet silty mud. FPSF offered lower impact/cost by not having a deep hole, which is an advantage too. In reality, since I had a slope, I was not going to have a 'low impact' benefit for saving manpower costs, but would avoid a neverending saga of huge boulders encountered (the glaciers were very generous to us). I liked being able to have insulative layer and to over-engineer strength to my heart's content, moreso than I could afford someone else to do if I paid labor by the hour.

    Acting as my own General Contractor and General Counsel meant I have relied on random conversations, websites and forums to gain my knowledge. I often found myself showing R403.x parts of the building code to tradespeople and inspectors and have them give me an odd, quizzical look. I hadn't meant to be a trendsetter or a maverick but sometimes have been placed in the role of having to be a "cheerleader" and "ambassador" for the FSPF concept in MA. I am happy to do this, but my enthusiasm is borne of anticipation, not experience. Many naysayers out there -- your feet will be cold, why didn't you put in radiant heat, you have to be below the frost line -- so were I to do it again, I would suggest really obtaining more confidence about the goals and the means to achieve those goals so that negative energy does not divert me.

    Building my own house is something I have wanted to do for a long time. I do not want to live in someone else's mistake. I want to be onsite for any contracted work so I can make sure it is done correctly. I have been lucky that my pros that help me know I am earnest and will not cut corners while I am in the trenches with them, so to speak. I find it slow going and doing it again I would have to get a handle on both "known" and "unknown" unknowns earlier.

    I am still on the roller-coaster ride, now stuck between HERS rating that says minisplits will heat the whole house and the minisplit installer saying I need to have a Manual J before he will install it and the HERS rater saying he cannot provide a Manual J. Looping in circles like this. At least, since most decisions already have been made, there are fewer circles these days and they each seem to have a narrower diameter ;-)

  17. Robert Opaluch | | #17

    Great, very informative thread. Deserves to be an article! Thanks all.

  18. Steve Knapp CZ 3A Georgia | | #18

    Thanks for the additional response. On the Manual J, a qualified third-party can do that for you. He/she will need your construction drawings, window schedule, and a few other details, but it should not be that hard to determine the most cost-effective approach. This article is pretty helpful: https://www.greenbuildingadvisor.com/articles/dept/musings/who-can-perform-my-load-calculations

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