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Musings of an Energy Nerd

All About Basements

Is your basement a damp hole in the ground or a delightful place to retreat and tinker at your workbench?

A basement workshop is a pleasant retreat where homeowners can temporarily escape from their ordinary above-grade concerns.
Image Credit: Donaleen

Foundation discussions can get heated. For some reason, builders often dig in their heels when the topic of slabs versus crawl spaces versus basements comes up. It’s time to declare a truce.

It’s perfectly possible to build a great house on any one of these three foundation types, as long as everything is properly detailed. Each type of foundation has advantages as well as disadvantages. If you have a foundation type that you prefer, that’s great. I’m not going to try to change your mind.

Basements are handy

Before the development of central heating systems and the electrical grid, most cold-climate homes in North America included a basement or cellar. Why? Because a cellar was the only room in the house where temperatures wouldn’t drop below freezing during the winter. Homeowners could store potatoes, carrots, and turnips there without the risk that these foods would be spoiled by frost.

Of course, most of us no longer have to worry that the food stored in our kitchen cabinets will freeze. However, basements still have virtues:

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  1. GBA Editor
    Allison A. Bailes III, PhD | | #1

    Don't forget the warm and hot climates!
    Nice article, Martin. You're showing your cold-climate bias again, though, especially in the section on sub-slab insulation. If a house, or just its basement, doesn't have air conditioning, then your recommendation about using sub-slab insulation to prevent condensation on the basement floor is good advice. Here in the South, every new home has air conditioning, so with a good design and proper control of indoor humidity, sub-slab insulation isn't needed to prevent condensation. It may not help a whole lot with the heating load either and is not required by code for IECC climate zones 1-3.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Allison Bailes
    Thanks for sharing your Georgia perspective -- always welcome. (By the way, what percentage of new homes in the Atlanta area have a basement?)

    You're right, of course. I have edited my article to reflect your point.

  3. GBA Editor
    Allison A. Bailes III, PhD | | #3

    Basements in Atlanta
    Martin, I don't know the actual numbers, but we have a lot of basements here. My guess is that there are more crawl spaces than basements and more basements than slabs, but we have significant numbers of all three.

    Also, I forgot to mention in my first comment that by not putting in sub-slab insulation, you help with the cooling load a bit, too. That's not a huge driver, though, because the basement load is already small and reducing it further can make controlling humidity without a dehumidifier harder. But we're getting to the point where low-load homes in the South need supplemental dehumidification anyway.

  4. mfredericks | | #4

    Dehumidifier link?
    Thanks Martin, another great resource with lots of advice pulled together in one article - just the way I like it! I'm hoping you can repair the link to the article you posted about dehumidifiers - right now this goes to a 'page not found' error. Thanks

  5. GBA Editor
    Martin Holladay | | #5

    Response to Mark Fredericks
    Thanks for reporting the bad link. I have fixed it.

  6. kevin_in_denver | | #6

    Advantages of Unfinished Basements
    I'm not too sure how a basement can be "honest", but leaving it unfinished makes plumbing and electrical improvements to your home much easier.

  7. user-1035959 | | #7

    Basements for the win
    Basements may add upfront construction costs, but they are inexpensive to heat and cool especially if insulated well. The risks with water intrusion are easily controlled with common sense stuff - overhangs, gutters, grading, waterproofing, sump pump. There's risk for your stuff upstairs if you have a roof leak.

    Basements are also an excellent place to put your mechanical equipment and to store things. Also, if you live in a tornado area, it's a safer place to take cover.

  8. RZR | | #8

    “A layer of horizontal rigid foam on top of the crushed stone to insulate the slab from the cold soil below;”

    Not a good idea, consult this with a PE (structures engineer) ask they look at all combined loads, not just compression, tensile cracks from bending from flexure and deflection, fatigue life, creep, just to name a few…If you can find an indi test and field study disproving my concern I’d LOVE to see it, otherwise, save the hype. Foam in a load bearing application should intuitively make no sense, nor placing a home on foam. It’s not that structural or rigid.

    An aerated concrete mix will provide much higher values as any PE should determine with the proper allowables, and r-value. Perlite in a concrete mix drains well and is a good choice against soil. Use a portland cement type IA (aerated) as a binder with sand and 1 inch to 3/4 limestone rock...30% perlite and portland, 70% rock. Test the mix first. Don't over water it sould not stick to shovel or hand, use a rammer to pack it.

    Speaking of structures I did not see mentioned, get a soil analysis or geo-tech out to take a look at the soils mechanical (compression, shear) and thermal properties and ability to drain, atterberg limits, and plastic index, etc, from a lab, take that info to a PE. First order of business. If you have a good soil, ask the PE about incorporating it into the concrete mix since it has a high ability to regulate and control moisture, along with other aggregates, rock, sand, lime. Denser surfaces with high Portland cement loose this capability, increase the wall thickness with less expensive soils will get back to required compression and shear wall strengths, tensile with more rebar. Be a good idea to know where the water table is too. Thinking outside the box I know :)

    “A layer of polyethylene above the rigid foam (directly under the concrete slab) to act as a vapor barrier;”

    Agree, above the perlite or aerate mix, no foam.

    “A 4-inch-thick layer of crushed stone under the basement slab as a capillary break; the crushed stone layer needs to be vented through the roof to help control radon;”

    “At least one 4-inch-diameter drain pipe running horizontally through the footing, to connect the crushed stone layer under your basement slab with the exterior footing drain;”

    Structurally, I don’t like holes in my footings where some of the highest loads are. Why? Because it is proven that the cross sectional area reduction unless replaced with a doubler can lead to stress concentrations and cracks. Structures Engineering 101. So we are hoping all stone layer drainage will make its way to these drain pipe(s), then out to the footing drainage. Of course the proper slopes would have to be designed in, an inch per foot? If so you just put your foam in bending, not uniform compression. If the intent is to drain high levels of water from heavy rains away from foam, don’t use foam, otherwise large rock (1-1/2 inch limestone) should provide drainage, and soil depending on density, without interruption from weak plastic perforated pipe.

    While on the subject of drain lines, radon can get into the home through the sump pump line that feeds the footing lines, perhaps, a check valve?

    “A ring of perforated drain pipe on the outside of the footing, surrounded by crushed stone and wrapped with filter fabric to make a “burrito,” drained to daylight, to a distant drywell, or to an interior sump”….

    Like the fabric idea, can also be used to keep drain rock free of soils and debri in a lining…also drain to a rain catchment recycling irrigation and grey water systems if code allows.

    “An application of dampproofing compound or waterproofing compound on the exterior side of the concrete foundation walls”

    Here is a toxic free one I tried on Rammed Earth, did well, didn’t darken like Acrylics. On the inside I’d go with this or a permeable lime wash so the wall can breathe and absorb CO2, especially with soil in the mix.

    “A layer of dimple-mat drainage board installed on the exterior side of the foundation walls; failing that, the foundation should be backfilled with coarse, free-draining material like crushed stone, topped with an 8-inch layer of dirt (ideally, dirt with a high clay content)”

    HDPE with additives that can break down, leach into water supplies, erode over time, not a good idea. Use an aerated concrete or stone. Some high clay content soils are VERY expansive and can produce high lateral shear loads when wet, resulting in cracks, efflorescence, fungi, again, consult a PE.

    I’ll read the rest later, got to get to work :)

  9. GBA Editor
    Martin Holladay | | #9

    Response to Terry Lee
    You appear to be a naysayer who likes to post several comments a day on GBA. I really don't want to get bogged down in long discussions on these issues, because your opinions aren't based on facts.

    Suffice it to day that builders have been installing a layer of horizontal rigid foam under basement slabs (and slabs on grade) for decades, and that engineers are aware of the practice and routinely stamp construction drawings with these details to indicate their approval.

    This is not an "idea," as you label it, but a routine construction practice -- something you would know if you had spent much time on job sites.

    If you think that a horizontal 4-inch pipe installed in your footing will cause structural problems in your foundation, there is something seriously wrong with the quality of your concrete, the thickness of your wall, or some other element of your building assembly. Get real.

  10. jinmtvt | | #10

    on the subject of basement energy ..
    I have been looking ( just a bit ) for sources of information on heat loss to ground
    from sub grade walls with location dependent data ...anything available ?

    As anyone compared the heat loss of basement walls and deeper slab VS out of ground walls and grade height slab ??

    I used to think that underground walls must be more energy efficient in my cold climate,
    but something keeps telling me that it could yield diff results than expected once calculated.

    That said, would be nice to know if a basement would be better in terms of energy than an additional floor of the same dimension , in a house design.

    As for water table.... almost every house ( and i mean something aka 90% ) in Quebec province, is built with a full , heated basement ( used to be 3/4 unheated in the 50's and before.. )
    The need for 5ft+ deep frost proof walls in southern up to central Quebec provides for occasion to use this space as a full basement with not much more investment.

    Nowadays, basements are almost all fully finished within a few years of a new house built date,
    and most receive either kids rooms, play rooms, gyms , home theaters, laundry rooms and "man workshop "

    The use of dehumidifiers is very common here, to be able to keep correct level of humidity in the basements, outside of the heating season ( usually required ) and is not so much of an energy burden as soon as the heating season starts.
    ( could be a good argument to use a HP WH during the 2/3 of the year where humidity is higher than 60-70% here )

    Terry: please explain me how using foam under a basement slab , which will probably see next to NO loads ( we are discussing residential basments here ) might pose any structural problems.

    I have poured several 6" slabs on 2" or more not soo rigid EPS , on which we used control joints,
    without any structural issues ( not even a single structural crack )

    I don't know about where you live,
    but here the basement slabs are never connected to the footings and are always "floating" on a bed of compacted rocks .

    problems with water and humidity is almost always caused by improper height location of the slab/footing VS natural water table ( people tend to dig WAY too much ) and or failure to identify type of soils ( and the required additional arrangements ).

    I live on a "sand" island where the older houses were set a few feet down,
    and they all have their sump pump running 3/4 year long.
    I dig only 12-16" ( only removed forest top soil ) when i set up my own house,
    and filled the frost depth with clean sand all around ...
    water never got higher than the base of my footings ( which happens to be the in and out drain height ) ... a good 12" lower than the bottom of my basement slab .

    ( have installed a 6' deep "well " hole to peep at water table level while building )

    Basements rocks for HomeTheaters neway ..
    and just for that they SHOULD BE MANDATORY in every house built from now on ! :p
    ( both basement and a very efficient (read LOUD ) home theater setup !! )

  11. GBA Editor
    Martin Holladay | | #11

    Response to Jin Kazama
    Q. "I have been looking (just a bit) for sources of information on heat loss to ground from subgrade walls with location dependent data ...anything available?"

    A. This is a very complicated question, and it's not clear that location is the most important variable. The number of variables is huge.

    Here are some resources to get your started:

    Marc Rosenbaum, "Heat Loss to the Ground"

    Carl-Eric Hagentoft, "Heat Loss to the Ground from a Building"

    Another similar paper by Carl-Eric Hagentoft

    There are a lot of interesting comments on this topic posted on this GBA page: Can Foam Insulation Be Too Thick?

    In that comment section, John Straube noted, "The difference between earth-slab and wall-air heat transfer is encompassed in the 'surface film' coefficient. This coefficient (fudge factor) accounts for the insulating effect that a still layer of air near the surface provides to a wall. It is not that important for well insulated walls, as its values is less than R-1. Another possible difference is the dynamic nature of the above-ground air temperature variations, but for lightweight structures this mass is not that important. Otherwise, the physics of heat flow above and below grade ARE the same."

    There was a lot of back-and-forth on ground temperatures. John Straube wrote:

    "Mr Semmelhack's question points to the root of the confusion: the lack of good models because of a lack of measured field data. I agree that PHPP gives too low a temperature, and that the 'average annual ground temperature = average annual ambient air temperature' also misses the target (mostly because it ignores the fact that there is a heat source above the soil — the house — providing heat for 12 months of every year). This is the reason that I was basing my numbers on measured data, even though it is sparse.

    "The closest example I have is the Finnish paper 'Thermal and Moisture Conditions of Coarse-grained Fill Layer Under a Slab-on-ground Structure in Cold Climate' from the Journal of Thermal Envelopes and Building Science, Vol 28, no. 1, July 2004. The coldest temperature measured was 45°F, rising to 68°F by the end of the summer, before falling again.

    "A more recent paper, 'Heat, Air, and Moisture Control in Slab-on-ground Structures,' Journal of Building Physics, Vol. 32, No. 4, April 2009, shows even higher temperatures, with averages of 15°C/60°F in the winter, except near the edge where they dropped to 10°C/50°F (winter average over 15°C though).

    "In Norway, another cold climate that builds lots of slab on grade, a paper in the 7th Nordic Building Physics Conference on slab heat loss reported, 'In Norwegian climatic conditions, with a yearly mean soil temperature varying from 2° ~ 7°C, we can use 12°±1°C as a default value for the inner zone reference soil temperature". 12°C is 53.6° Fahrenheit.

    "In Britain, a study of a commercial building is reported in 'Temperatures in and under a slab-on-ground: two- and three-dimensional numerical simulations and comparison with experimental data,' in the journal Building and Environment, Vol. 35, 2000, found temperatures (in a milder climate near Cardiff, Wales) in the 14°-16°C (57°-61°F) range during winter periods. I also have some papers from Sweden, and basement measurements from Canada.

    "Suffice it to say, if I base my design on real measured results, rather than someone's model, I repeatedly find that the temperatures of the sub slab range from the 10°C to 15°C range (50° to 60°F).

    "I would be interested in hearing about any published measured data to add to my small but growing collection."

    As a "pretty good house" advocate, I think that these issues are mostly a concern of building scientists, not builders. It's perfectly OK for builders to use rules of thumb for insulating basement walls and slabs, and to leave the THERM modeling to the scientists. Here is some advice for designers and builders: (a) Pay attention to airtightness, (b) Include adequate R-value -- equal at least to the most current version of the International codes, and (c) Make sure that your insulation layer is continuous, without significant thermal bridges.

    Q. "Has anyone compared the heat loss of basement walls and deeper slab vs. out-of-ground walls?"

    A. The answer to your questions is simple. During the winter in cold climates, there is more heat loss to the air than to the soil a few feet below grade.

    Q. "It would be nice to know if a basement would be better in terms of energy than an additional floor of the same dimension."

    A. Assuming that the R-values are the same, a below-grade building will be easier to heat than an above-grade building.

    A. "The use of dehumidifiers is very common here."

    Q. New homes with good water-management details shouldn't require basement dehumidifiers. My guess is that these homes with basement dehumidifiers have sub-optimal details and specifications.

  12. RZR | | #12

    Jin, slabs do have live and dead loads, that is why foam manufactures give a compression allowable. When they are monolithic-ally tied to a footing-wall it creates a strong design since load can pass between the slab and footing-wall, if the joints are detailed correctly. Load is reacted by the sub-structure and it is not usually distributed evenly in 100% compression (that is why we use rebar). If you had access to the slab and footing-wall and put strain gages on it you would see this. The lower surface(where you can not see) is more prone to tensile stress cracks. With that said, an proper engineering analysis needs more than a compression allowable.

    Martin, I'm not a "nahsayer" , I am one of the structural engineers you refer too and I've been on MANY job sites. You have conservative engineers, and you have the liberals. If you were one, you would know this. I'm a conservative. My "opinion" is based in fact, here they are, again, if you were an engineer you would understand this....

    Loads are developed in a "load cell" or coupon bench test that produces operating loads or limit loads or material "allowables" For an Engineer to do a proper analysis, they need more than compression; ultimate tension, ultimate shear, modulus of elasticity, deflection, fatigue over time, yield strength, creep, are also required. Most all building materials have these allowables in code, or in span tables, etc, I have not seen for foam....The liberal engineer will stamp the drawing since they will not be there in 25 years when you have settling issues from a lack of understanding of the slab, footing, and reactive sub-structure below it. The conservative engineer will do all the load checks combined, and take the material to ultimate failure than back off the load to limit with a factor of safety, and apply a generous safety factor 2+. It is usually.5- 1 in industries that better understand loads and their reactions, since the added dead weight is counterproductive to the design.

    I merely recommend that people do not assume it is generally ok to put your home on foam, especially the footing-load bearing wall, and to hire an engineer which is exactly what you said,

    "engineers are aware of the practice and routinely stamp construction drawings with these details to indicate their approval."

    Key word "Engineer" I did not read in the article.

    I do not advocate placing homes on foam and I'm entitle to my factual opinion until I see the allowables on a website and the proper testing period. I never asked you get into a long discussion you do not understand or note in your report which are critical. I looked on a few manufactures websites perhaps not the correct ones. I think I did find a creep allowable on one, but that has to be a known load in the design for a proper analysis.

    My questions are what is the best way to design a randon system? Anyone? Please explain the operation of your design.

    View out and walk out basements with a large percentage of windows are popular here. I was just asking a realtor if reducing the windows would still sell, I think that is an unknown people love windows. The tornado sites I have been to the only basements left were the look out small windows. I'd agree a safe room is still needed. The wood floor stays intact.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Terry Lee
    You call yourself a "conservative" engineer. In your case, I'm not sure what that means. I am sure, however, that your advice is well out of the mainstream for residential construction.

    Your advice does our readers a disservice, for several reasons. This is a green building web site that tries to advise people who are building houses that require minimal energy use.

    I couldn't care less whether you are a conservative or a liberal. But when you advise readers that a layer of horizontal rigid foam under a basement slab is "not a good idea," you are misinforming readers. Readers may end up omitting the insulation; in a cold climate, this will increase the heat loss from the building, and will increase the chance that their basement slab will be damp or moldy. It will preclude the chance of ever installing carpeting on top of the slab. It is a difficult-to-reverse decision that should not be taken lightly. Your cavalier dismissal of my good advice isn't helpful; it's potentially harmful. That's not conservatism; it's simply bad judgment.

    When I advised readers to install a short length of 4-inch plastic pipe through the footing to join the crushed stone under the slab with the crushed stone outside of the footing, your wrote that "I don’t like holes in my footings where some of the highest loads are." But I didn't advise a "hole in the footing," and your implication that a footing with this type of pipe will prevent the footing from supporting an 8-inch-thick poured concrete basement wall is ludicrous in the extreme, since a simple structural analysis shows that an 8-inch-thick poured concrete basement wall doesn't even need a footing (from an engineering standpoint, not a code standpoint). Again, your advice is misleading and may do harm if not challenged.

  14. RZR | | #14

    My advice
    Martin, I did not advise to omit insulation, I gave alternatives to foam, relax. Lets look again at what I wrote,

    "An aerated concrete mix will provide much higher values as any PE should determine with the proper allowables, and r-value. Perlite in a concrete mix drains well and is a good choice against soil. Use a portland cement type IA (aerated) as a binder with sand and 1 inch to 3/4 limestone rock...30% perlite and portland, 70% rock. Test the mix first. Don't over water it should not stick to shovel or hand, use a rammer to pack it."

    Now lets look at some properties on popular under slab foam,

    100 PSI compression: "Values at yield or 10% deflection, whichever occurs first" ...Flex is low yields at 5% deflection.

    Yield is a property of many, the stress at which a material begins to deform plastically, so you don't design to it has an operating allowable. It yields at 100 psi compression, 10% deflection...and it's relatively expensive. The 3000+ psi concrete slab above much higher.

    The aerate concrete mix I recommended, 3-5 time higher depending on ratio of portland cement, 100-500 psi compression, much higher flex modulus at yeild, much higher ability to manage water, resist fungi resistant, insects, no thermal bridging due to mass effect. My test concludes the same findings as Europe data I can show if you want to see it, an r-value of around 2.5 -3 per inch, not as high as foam, so I recommended more thickness if needed which I doubt you need more than 4" due to it's mass effect and ability manage water.

    Here is a net zero home that did excellent with an aerated concrete block, r-10 under, r-15 perimeter: There are many more types scattered throughout the world. I fail to see the harm in my recommendations, and I apologize if I go against foam being the GBA or as you say "mainstream" solution to everything.

    Holes in any load bearing structure needs to be looked at. I'm not sure I understand your design so no further comment. My stone bed remains flat to provide uniform loading.

    Listen, by all means use foam it's no sweat of my back. I'm absolutely certain I will not. I think the data speaks for itself.

    Here is an aerated mix that is hard as a rock, r-value of 30 ish, 12 inch wall...poor water in it it evaporates and drains fast, won't freeze, natural fungi resistant, etc....needs a render that I can change surface properties as I wish. When it dries it absorbs CO2, reduces racking and compression by around 100-150 psi depending on binder content...great insulation for a PWF. Very low cost, less than "mainstream".

    All right Martin, I'm not up for a debate, besides in your article you said "you were not going to try an change the minds of different options or opinions" :) You definitely won't change mine and I'm sure I won't be changing yours on the 12th day of never :)

  15. charlie_sullivan | | #15

    What about a semi-basement
    What about a 4' deep foundation with a 4' stud wall on top of it to complete a lower level with the floor below grade and windows above grade? That was very popular in the 70s for cheap suburban houses in "split level," "split entry" and related styles. It has some negative associations with those cheaply built houses, but it seems like it could make sense for a modern cost and energy efficient house as well, and could be a good compromise between the two extremes of slab on grade and an 8' deep basement.

  16. jinmtvt | | #16

    dude for someone that busy you sure post alot!!!

    You keep "pushing" your "soil" solution all over the place.

    Maybe you should start a blog with something like " Alternative " solution as title.

    What is the labor involved in your "solution " for mixing and placing a sub slab "insulated" slab .
    ( i'd love to do double-slabs!! )

    BAck to loads..

    i understand that foam as its limits when working as critical load bearing material
    but common, under a residential slab ...
    As soon as you poor the concrete slab on it , your set.
    What is it "creeps" 2-3% in the next few months..the slab will go down a few microns with it
    who cares.

    Anyhow, start a thread/QA/BLOG whatever about your soil mix,
    but please stop placing it on every blog possible.

    ( i am interested to learn more about it , but i'm not interested in ur advertisement )

  17. GBA Editor
    Martin Holladay | | #17

    Response to Terry Lee (Comment #14)
    For the vast majority of U.S. builders and homeowners, specifying a concrete slab that includes 30% perlite is simply not an option, for several reasons: (a) This is still an experimental material that many homeowners won't accept; (b) In most areas of the country, it will be difficult or impossible to fine a ready-mix company willing to supply this type of concrete; (c) In most areas of the country, it will be difficult or impossible to find a concrete contractor willing to install, finish, and guarantee such a slab; (d) Finishing this type of concrete is no easy task; (e) The resulting slab is going to look different from a conventional slab.

    That's just for starters.

    I have placed and finished concrete that contains a high percentage of polystyrene beads. This is a nightmare material for concrete finishers. I know, because I was on my knees with a trowel for hours, trying to figure out how to make the slab look OK.

  18. GBA Editor
    Martin Holladay | | #18

    Response to Charlie Sullivan (Comment #15)
    The approach you describe can work, but it has several disadvantages:

    1. It places the first floor too many feet above grade for a wheelchair ramp.

    2. It makes insulation of the basement walls tricky (assuming that you want a decent R-value for the concrete section of the wall, and assuming that you want the interior face of the concrete wall insulation to be co-planar with the wall framing above, so that interior drywall can be installed from floor to ceiling without an awkward shelf).

  19. Expert Member
    Dana Dorsett | | #19

    The sub-slab foam loading thing...
    ... is dead-easy, orders of magnitude different from a sub-footing foam problem. Using Calling it a "bad idea" is to have skipped the math on the actual dynamic loads of residential slabs.

    Rather than creating a problem EPS under roadway is often used to SOLVE problems with loose or lightweight soils related to the dynamic loads of traffic:

    While its true that residential slab thickness & reinforcement is nowhere near that of a highway ramp, the actual loads are several orders of magnitude smaller.

    While there are the special case where slabs are thickened at the edge to effectively become the footing, that's an exceptional construction method, not the common standard. I've literally NEVER seen that method used for basement slabs, only for slab-on-grade in dry not-very-cold climates (though it may exist elsewhere.)

    Of course foam under footings requires a load analysis, but for basement slabs that are only holding up the contents of the basement (and not the weight of the house) it's just silly.

  20. pjmeg | | #20

    Foam for sub slab and sub footing
    As information, American Society of Civil Engineers has published guidance on design of Frost Protected Shallow Foundations and the use of foam insulation sub slab and sub footing.

    In short, foam under footings and slabs shouldn't be considered "fringe", but engineering calculations are required.

    1. nilst | | #49

      I would love to be able to read what ACE has to say about foam under footings but the link is broken. If it is easier to summarize the salient points I'm all eyes.

      1. GBA Editor
        Martin Holladay | | #50

        Here is the correct link: "Design and Construction of Frost-Protected Shallow Foundations." Note that the American Society of Civil Engineers doesn't distribute this book for free -- you have to pay for it.

        The most widely available free guide to frost-protected shallow foundations is published by the NAHB Research Center. Here is a link: "Revised Builder’s Guide to Frost Protected Shallow Foundations."

        The NAHB Research Center guide is based in part on data from the American Society of Civil Engineers. In the acknowledgements, the authors of the NAHB Research Center guide note, "We acknowledge the American Society of Civil Engineers’ (ASCE) for permission to publish values that are contained in Tables 4 and A1 in SEI/ASCE 32-01, Design and Construction of Frost-Protected Shallow Foundations, 2001, authored and published by the ASCE."

  21. RZR | | #21

    Reality Check
    FPSF are shallow (12-16” depths) less disturbed soil, less dead weight, more efficient load transfer, more evenly distributed bearing. Note vertical and wing insulation, not under footing or wall. Exception, “unheated” building as approved by code or engineer. The material properties do not change in other geometry(basements), the geometry and load transfer where the risk of edge bending (flex) is more efficient in FPSF so rick are lower. The same material allowables noted in table A1 apply to basements, footing’s, crawl spaces, etc.

    Appendix A: “When these insulation materials are subjected to compression from soil and building loads, the insulation shall be supported on suitable bearing materials and shall have adequate compression strength and long term creep characteristics to resist the sustained loads (live and dead) during the expected life of the structure. Compression (bearing) loads shall not exceed Table A1.”
    Take note: An Engineer needs a creep value. Where is it?

    Table A1 shows no creep value it needs to be analyzed along with other properties that cause “deformation”. A committee used limited test data from Europe to develop allowables in table A1. Europe has different soils than we do. Creep over time can put your concrete in bending, just as soil needs to be analyzed by a professional, so does foam creep.

    Flexure is a measure of deflection noted in the manufacture provided at yield, creep is an unknown on the manufacture’s allowable spec sheet.

    Foam Flex Modulus: 50 PSI (high allowable with 3 safety factor), concrete (400-600 PSI) @ limit load. Aggregate is high and not a “suitable bearing material” for foam, concrete of another alternative mix is. Foam can level out compression and reach its flex limit and yield strength while doing so easily.
    If tensile cracks in bending occurs on the underside there will be issues. Utlimate tensile strength is equal to flex ideally, which is low in foam (140 psi). Baffles me how some think the lack of proper field studies over 30 years that dug out the soil, rock, and aggregate to do visually inspection and develop creep and bending data is ok to ignore. Perhaps why we see no real creep allowable.

    Dana: I guess to some, the empirical evidence lies in the fact that it’s done on roads (the same industry that cannot prevent cracks for centuries now), other structures with different loads and allowables, so it HAS to be good. Commercial is a completely different environment than residential in case you did not know, where Engineers have to have ALL allowables, and all stresses HAVE to be proven good for public safety. Evident that non-structural Engineers designs in residential such as yourself point to goggle and factory sales hype, got to love it! How much in royalties are your getting from Royal foam sales? Pointing GBA readers to roads does not change the mechanical property of foam used in homes (140 psi flex, low creep and resistance bending and tensile @ yeild). Or perhaps you can show us how it does? And the “simple math” to show it good?

    Consider loads on a 9 foot basement wall of 2000 PSF, side pressure load of 4000 PSF @ 9 ft depth, or consider a slab on grade that varies in pressure points by 30 PSF over a small distance. In either case, a bending moment in foot pounds produced that exceeds foams flex modulus of 140 ish PSI rating at yield. The foam cracks in tension, chemical and mechanical properties continue to break down over time as creep worsens. The same insulating property (heat resistance) increases creep over time and lowers the foams melting point, and other chemical properties from aggregates, some corrosive. The mechanical stresses (combined loads) and heat combinations cause settle in the large foam gap. Hazardous materials leach out to the environment.

    This is not new to GBA readers, can see the concern of Engineers included here: Perhaps you or you or any GBA reader or supportive “Engineer” that failed can answer Ron’s (last post) question?

    In this respect, there are just as many unknowns as “alternative” foam methods I pointed to with higher mechanical properties than foam, GBA supports and has MANY articles on. Jin, perhaps you are the one that needs to stop posting about what you have no clue about and read more GBA articles that covers a wide array of materials and construction practices.

    Compression check only: A typical wire mesh “plain” slab on grade weighs or has a dead load of about 75 PSF. That increases as mix PSI rating’s goes up. Live loads vary from 20-50 PSF for a garage. 125 PSF average that is transferred to foam. The manufacture I quoted gives values at ultimate compression, knock it down by safety factor of 3, 100 PSI = 5000 PSF. Good according to table A1, TYPE 5 FPSF (12-16” in depth, not 9’). If there is no creep and 100% linear loading (very unlikely unless flatness of substructure is held tight for extended periods of time) and there is even load distribution from rock to soil, and the soil cannot take the loads, that have much different properties to look at than dimensional foam. For one, the way foam distributes bearing and shear, expands and contracts, reacts to bending, etc, is different than soils….you do not base foams ability to react loads on soils ability, or rock, or aggregate, or concrete.

    A basement wall can weigh many tons and see large lateral forces from expansive high clay content soils, frost heathing, seismic, wind, and lateral back-fill pressure loads foam cannot take. It will react differently than soils and concrete to loads. You can design plain slabs, walls, footing’s that transfer loads to substructure, or structural with steel (rebar or fibres) that does not, or is suspended (see ACI 318-14). That may be a safer alternative to placing foam under non-structural plain concrete at a cost.
    To each their own, but to not point to a proper analysis and/or prescriptive code, or design guide, is unsafe for GBA readers to believe that it is ok to just throw some foam under concrete and call it good, which most will do. That lack of knowledge not with another variable foam, has dominated the foundation communities and has resulted in large costly issues by not getting geotechs and Engineers that understand soils, now foams….I can’t wait to see how the unknown deformations and cyclic loads of foams adds to the issues as unqualified people slap it down just because some non-professional or goggle search said so. That should keep our and other restoration businesses strong for many years to come, as if business is not already booming in our concrete foundation dominated community already. $ Thank you!

    Your living in a dark clouded dream world if you think foam has no issues and solves everything.
    I’m sure the next post will ignore the structural issues with foam and defer the reader to many wonders of placing your home on foam. As Martin said, get real!

  22. RZR | | #22

    Another post for Jin.....
    FYI: No insulation needed. You’ll find “soil” in commercial concrete mix, or the aggregate it is naturally mined from. I have MGO in some of my mixes that do not thermally bridge.

    "Magnesium-based cements commonly achieve compressive strengths of 9,000 to 45,000 psi and tension strength of over 800 psi, many times stronger than that of conventional concrete. Magnesium oxide combined with clays and cellulose form cements that breathe water vapors electro-magnetically, a significant plus. The clay in magnesium oxide balances and enhances the movement of moisture. It never rots because it always expels moisture. Also, unlike Portland cement, MgO cements will not drain the charge out of a car battery left overnight on a floor made with as little as 20% MgO content. MgO cements are completely non-conductive of electricity, as well as heat & cold, and have been used for flooring for radar stations and hospital operating rooms throughout the 20th century."

    Martin, I never said perlite mix was the finish mix. Some people admire the beauty of natural finishes that do not always need a perfectly flat finish. Ready mix trucks are expensive, mortar mixers and some shovels is not going to take a month to fill a sub footing or slab 4” deep. But I guess if you are a naher and foam pusher you can always find more bad than good. Interesting, most of what I have learned about these alternative methods was right here on GBA. Recently, I tested them out and I’m here to tell you not rocket science or massive amounts of labor. Even if cost was higher, which it’s not, as data shows, so are the mechanical and thermal properties. AC block, gypsum block, cellulose /rock concrete, air entrained mixes been around the world much longer than foam without the embodied energy and toxins that rot the environment, plant and animal live, soils, water supplies, etc.

    You’ll also find ready mix companies getting interested in green alternatives vs Portland cement. Many have developed cellulose mixes that are available. Otherwise, mortar mixers are cheap as is the labor. Hiring a ready mix truck for a one-off will be expensive. Do the math in your area.

  23. GBA Editor
    Martin Holladay | | #23

    Response to Terry Lee (Comment #21)
    To repeat: the use of horizontal rigid foam under basement slabs has been a routine detail for decades. You're wrong.

    And your cynicism is both remarkable and disheartening -- when you asked Dana, "How much in royalties are your getting from Royal foam sales?" Such accusations are sad and inappropriate.

  24. GBA Editor
    Martin Holladay | | #24

    Response to Terry Lee (Comment #22)
    You're right -- there is nothing wrong with mixing concrete in a portable concrete mixer, and placing the concrete for your basement slab by transporting it in a wheelbarrow. I've done it. I've also mixed many, many loads of concrete in a wheelbarrow with a hoe. These methods work.

    For builders who want to use rammed earth, soil concrete, or special mixes with perlite, these techniques are fine. GBA is proud to provide information on these methods.

    However, suggesting this approach doesn't help most homeowners in the U.S. who are negotiating with their contractor over how to detail their basement slab. If you're hiring a contractor, and you inform the contractor that this job won't allow concrete from a ready-mix truck, and that the contractor is expected to mix all of the concrete on site in a portable mixer -- well, just be prepared for sticker shock when the contractor presents you with his time-and-materials invoice.

  25. GBA Editor
    Martin Holladay | | #25

    Response to Terry Lee
    OK, you've upped the ante with your latest unsupported claim.

    You wrote that horizontal rigid foam under a basement slab can cause "extensive damage."

    The ball is in your court, Terry. Since there are tens of thousands of basement slabs in the U.S. with this detail, you should be able to provide a link to a report of "extensive damage" caused by this detail. At least one story, right? Where did this happen?

  26. RZR | | #26

    Creep and flex
    Until someone can find creep and flex data that answers the MANY concerns found right here on GBA from it's readers to their engineers, it is safe to assume it is an unknown risk. I'm right! A risk that can cause extensive damage and cost.

    I know Dana is not a sales person, I was just kidding, as I seen him suggest some emoticons would help. I forgot here, :) :) :) :)

    If that is the only error you can find in my post that says it all. Case closed. :)

  27. BCinVT | | #27

    Since any comments I make always come at the end you may assume I'm sagely taking in all the above and can find the wisest thing to add. Actually, I'm not a member of the GBA website and get here at the end from the free email. Still, I DO sagely survey what has gone before and I have to ask Martin, do they pay you enough for this?
    Once I was heading home with a load of Exp foam and lost about 5 sheets at a stop light. After a bit I noticed and went back to clear the debris I left. To my amazement it was still laying in the road and traffic had been going over it for 20 minutes. A major intersection. I got it out of there and took it home and used it. Sure, it had some tread marks, but it had only been compressed by about 1/8 of an inch. Sure, I think it can handle a slab.
    To give Terry some credit, I like to play with cement mixes too, and I think perlite is a vastly underused resource. But in the real world it would take quite a lot of dedication to make it anything like a substitute for foam. In 100 years we can revisit this discussion and see about the durability of foam, but if it is used with some structural thought it is the best we can do now.

  28. BCinVT | | #28

    foam confusion
    I did mean extruded poly foam. Expanded foam would be road kill.

  29. GBA Editor
    Martin Holladay | | #29

    Response to Brian Carter
    Thanks for sharing your anecdote about the XPS with tire marks on it. Love the story.

    Q. "I have to ask Martin, do they pay you enough for this?"

    A. It depends on the week...

  30. Billy | | #30

    Excellent Article

    This is an excellent article, and good responses to Terry Lee. The basement in my house is built almost exactly as you wrote including a moisture barrier on top of the footing and it is dry and mold-free. No basement stink.



  31. [email protected] | | #31

    sub slab insulation
    On a recent basement retrofit I assumed insulation in the basement floor would be part of the package. I knew ambient ground temperatures would mean the benefit would be much less than insulating walls, of course, but I figured that it would still make sense to put a couple inches down. The results of the energy modeling were surprising to me--in upstate New York $1800 worth of insulation (2" of Roxul DrainBoard) had modeled energy savings of $14 per year.

  32. GBA Editor
    Martin Holladay | | #32

    Response to Norm Farwell
    Thanks for sharing the information from your energy modeling.

    Yes, the installation of rigid foam under a basement slab has an extremely long payback from an energy perspective. That's why I wrote, "The main reason to include the rigid foam under the slab — at least in cold climates — has nothing to do with energy savings; the foam is there to keep the slab warm enough during the summer to avoid condensation or moisture accumulation in the slab."

  33. jinmtvt | | #33

    Norm- Martin ..
    This is the kind of situation that misinform everyone.

    1800$ for what? labor included ? area covered?
    why choose one of the most expensive insulation ?

    without more info, Norm's comment is completely meaningless.

    I doubt that 2" of insualtion VS none is that far in terms of ROI,you aren't in NY city,
    and you aren't adding 2" to already existing/planned 2" .

  34. [email protected] | | #34

    The details you ask for, Jin
    Missing information is not the same as misinformation, right?

    960 square feet; negligible labor; Roxul because I am concerned about the toxicity of flame retardants in foam and the GWP of the manufacturing process. (Last I checked foam costs $.10 per square foot per R vs $.13 for the Roxul, so it's about 30% more.) And yes, 2" versus none, that's why I was surprised.

    The details are complicated because this is part of a larger retrofit, so there are interactive effects with other improvements, for example. But the take home looks pretty basic no matter what insulation material you spec and no matter what the other improvements are: unless you want to chase 100 year paybacks, don't bother insulating a basement slab in order to save energy. Better off buying a dehumidifier.

  35. RZR | | #35

    Lots of confusion to my comments....

    Martin: I already provided proof that foam underlayment has risk by pointing to manufactures material allowables that lack creep test and properties. Without it Engineers can not design properly. I also stated that there is lack of empirical info (ACI points to some limited lab test data in Europe for FPSF, etc) .....I also said to gather field data buildings would have to be dug out (hope you don't mind, but I'm not doing that anytime soon :) one can show the empirical evidence which is the risk, I said. Those that are installing foam are the futures empirical data, the foam industries thanks you for your cooperation. It is the foams industries burden of proof, I don't have the burden of proof nor does any end user, my homes won't be placed on foam foundations is not going to happen. .. I also said more than likely people will not seek a PE as proven by many that did not and have foundation issues, the foundation repair industry is alive and well. I already pointed to other options with higher material and installation values. You don't have to mix some of those options in a mixer, they can mixed in place with a skid loader and there are readily available manufactured blocks you can purchase or make that are better than foam in strength and insulation.

    I also provided calculations that compression checks out for Formula 100 OC (100 psi rated compression) in FPSF per the ACI design guide, vertical and wing, the spec calls for Engineering approval as underlayment for unheated homes. I used a conservative safety factor of three. This would explain why foam does well when supported by a road that limits it's low deflection property for short periods of time. That is not the case when it is on high clay soil and aggregated, not even close. Put foam on wet soil and loose rock run some cars over it...test it out report back. Roads crack for this reason. Creep happens over years so that is a life cycle test we do not find on any manufactures website. Some believe that just because buildings have foam under them or cars ran over it, that is all the empirical evidence you need. I'm here to tell you look at any design guide that is good and you will not see that as empirical data used to develop proof of designs, it means nothing! Design guides, codes, standards, are developed in a lab initially, bench tested by pros, that are put to closely monitored field test over time. Labs can accelerate creep to 25 year in a matter of weeks. It is an expensive test most manufacture won't pay for, so they release the design for consumers to test for them.

    If you put foam in CIC application the foam is well supported against deflection and creep but, you have not moved the dew to foam. I also stated foam would be better in an structural application where properly designed rebar supported the slab. An Engineer would design the span of diameter of the rebar per ACI-318.

    Geopolymer cements are gaining ground. In 2012 ASTM recognized a growing market that their standards did not include or the use of high embodied energy portland cement, so material allowables are underway. This is good news for the future, such that pozzolans, rock, and other cellulose infills are better tested and understood for code adoption. Now they need to add a creep test to C518 for rigid foam.

    I been reading about basalt rock rebar interesting stuff:

    So let me conclude by repeating myself,.....Foam compression is acceptable in the FPSF example I provided it checks out. It does not check out in deflection on substrate that is not held flat over time. A structural elevated slab properly designed by a PE with rebar would solve this, the only risk is chemical degradation over time. If that or creep occurs you can bet it will be costly.

    That's it for me folks, carry on, I got better things to do then repeat myself. This is a discussion for Engineers, PE,s(not that we always agree) but I wish there were more out here.... I highly recommend you find one due to the variables and unknowns if placing foam under a plain(non-structural) slab or footing, or call ACI for the proper advice and direction.

  36. GBA Editor
    Martin Holladay | | #36

    Response to Terry Lee
    You have provided more obfuscations, I'm afraid. You wrote, "My homes won't be placed on foam foundations."

    But in this article, I'm not discussing foundations -- only basement slabs. A basement slab is a type of finish flooring, not a foundation.

  37. RZR | | #37

    Martin, I believe you keep referring to generic terminology since you really don't understand 'all about basements" Look @ ACI 318-14, there are MANY ways to design basements, crawl spaces, slabs, footings, and hybrids there of. I hope you intuitively can see that as you go down in depth the loads get higher, lateral pressure loads double every two feet of depth, as does dead weight increase, that can deflect foam in bending, add seismic even worsens the potential.

    I'm done here, there is no changing opinons even when the data or lack of data prevails. Complete waste of time to disagree with you. If I were a supporter a foam, and your opinion, it be all good so be it. :) I didn't present the FPSF document I just discussed really does not matter, foam properties do not change if you put it at 2" or 10'...The creep data still does not exist. Repeating myself once again.

  38. jinmtvt | | #38

    please define from your engineering POV , the loads that a free floating concrete slab in a full depth basement might be subject too ?
    What might cause the great problems you point to, if one is to use sub slab insulation in foam
    with its mechanical properties.

    That is where you are loosing me in all your posts and arguments so far.

    I fail to see how a slab which will see only very light load
    ( probably not even exceding its own "wet" poured time weight )
    in a regular residential basement, could suffer from severe mechanical defficiencies if placed on creeping foam boards.

    Fully understand how foam under footings is something to be wary of,and need to be worked on by a competent engineer.

  39. user-270695 | | #39

    After reading this, I really miss Riversong.....
    In retrofits, we have solved persistent elevated basement humidity by installing a sub-slab depressurization system much like a Radon system. This was after all the roof water, yard drainage was optimized and foundation walls spray foamed.

  40. GBA Editor
    Martin Holladay | | #40

    Response to Robert Post
    Researchers have confirmed your observation: when an active radon mitigation system is retrofitted into an existing house with a basement, the relative humidity of the basement air drops. So sub-slab depressurization is sometimes a sensible way to improve a damp basement.

    For more information, see All About Radon. In that article, I noted, "In the case of an older house without any poly under the slab, an active radon mitigation system often provides a side benefit: lowering basement humidity levels. If your house falls into that category, you may be able to eliminate use of your dehumidifier — in which case the cost to run the radon fan may be more than offset by dehumidifier savings."

  41. justinmurphy | | #41

    Above slab insulation
    I'm sold on the benefit of subslab insulation, but still wanted to ask if laying rigid foam on top of the finished slab had comparable results? Mine will be a 7 foot basement (unfinished). Thanks.

  42. GBA Editor
    Martin Holladay | | #42

    Response to Justin Murphy
    Is this a new house or an old house? If it's a new house, you definitely want to put a layer of horizontal rigid foam under the slab.

    If it's an existing house with an uninsulated basement slab, you have no choice. So you can install a layer of rigid foam above the slab instead of putting it where it belongs (under the slab). If you install rigid foam above your slab, the next step in most cases is to install a layer of 3/4" plywood. The plywood should be secured to the concrete slab (through the foam) with TapCon fasteners.

  43. justinmurphy | | #43

    Thank you! It's a new house
    Thank you! It's a new house so subslab is possible. Just thought ease of insulation and continuous coverage* might warrant above slab.

    *by continuous coverage, I mean with sub slab, I would want to make cut outs for the column footings so insulation is not bearing the weight off the support columns above.

  44. GBA Editor
    Martin Holladay | | #44

    Response to Justin Murphy
    The problem of "continuous coverage" exists in either case. After all, even if you choose to install rigid foam on top of your slab, I don't think that you will install your support columns (lally columns) on top of the rigid foam.

    In fact, it's easier to detail rigid foam under concrete footings that it is to put a lally column directly on rigid foam. For more information on installing rigid foam under concrete footings, see this article: Foam Under Footings.

  45. sgbotsford | | #45

    The thought occurred to me that for a given climate and location the three foundation systems aren't created equal.

    E.g. In my part of Canada with 1750 C freezing degree days per year. (Snow on the ground usually from mid November to mid April, and cold snaps with a HIGH of -35 C) you have to go down far enough just to get below the frost line, that you may as well go two feet further and put in the rest of the basement.

    In places like New Orleans, I can see merit in putting down piles and perching the house 10 feet in the air. The space underneath becomes a shaded patio type space. DON'T put the utilities down there. Put them all upstairs. A sub panel for the pillar room. Outlets as high as reasonable to reach.

    When the next Katrina hits, pack the dog and the kids (don't forget the bird!) turn off power to the lower level, and abandon the house.

    THIS is a house that won't need major repair just to be livable. You will lose the barbecue, and the kids bikes may need some serious work before the wheels roll well (sea water doesn't do bearings a lot of good) but the living part of the house is above the flood, and is not an impossible source of mold.

    (I could see merit in requiring this sort of construction anywhere that floods.)

  46. hudson_valley_gregg | | #46

    Polyethylene failure
    Great overview, Martin. Thank you for the grounded basement insights.

    My most burning question revolves around the use of polyethylene vapor barriers. If the poly isn't breached during the pour, then it will break down over a number of years (not necessarily decades). I wonder whether you agree with this statement. If not, then no need to move ahead here. If yes, then the opportunity to review alternative approaches to moisture management arises. Thanks for anyone's consideration.

  47. Expert Member
    MALCOLM TAYLOR | | #47

    Poly is somewhat susceptible to UV damage, but remarkably stable when buried under a slab. There is no reason to think it will break down over time, or that small holes incurred during the pour are a problem.

  48. Expert Member
    MALCOLM TAYLOR | | #48

    A house built with the idea that the first floor is sacrificial makes sense in a variety of conditions. Here is one designed for Tsunamis:

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