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slab shield vs. rigid zone 6

stephenr | Posted in General Questions on

Hello,  I am planning to do a frost protected shallow slab on grade foundation for my home.  I have a plumber friend who swears that slab shield is the way to go.   It will have radiant pex tubes. I am all about using less rigid foam, but was wondering if it complies with the codes and is a wise option for zone 6.  Thanks!

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Replies

  1. mr_reference_Hugh | | #1

    Hummn. All I would say is trust your gut, not your plumber friend on this one. Ehough said.

    1. Expert Member
      MALCOLM TAYLOR | | #2

      mr_reference,

      They picked an unfortunate name. When I searched for it I got a lot of pictures like this:

      1. mr_reference_Hugh | | #3

        Based on my internet search, it looks like this might be a foil faced insulating membrane product. I looked for someone credible writing about foil faced membranes. I found an article by Martin Holladay (he will do) who talks about using these types of membranes to make Halloween costumes!!!
        https://www.greenbuildingadvisor.com/article/stay-away-from-foil-faced-bubble-wrap

        I am really happy "stephenr" asked the question here before making a very important decision based on the plumber friend's advice.

  2. stephenr | | #4

    Thanks for your replies, gentlemen. Halloween IS right around the corner!! LOL... Slab shield is a newer product that, along with its heat reflective properties, also has grooves into which you can snap your radiant tubing. It also acts as a moisture barrier. Its well regarded in warmer zones. My question is... does it pass code in zone 6, and is it a replacement for 2 layers of rigid insulation up here. Also, would one roll it out on top of 1 or 2 layers of rigid, say, in order to take advantage of its tube snapping grooves. And finally, is it best used to make a robot or a tin man??

  3. johngfc | | #5

    Stephen, Could you describe the specific 'slab shield' you're considering (thickness, R-value, link to manufacture's specifications)? There are a variety of products with the 'slab shield' label and they're quite different. Some of these advertise the benefits of a 'radiant barrier'. That's total BS since conduction (not radiation) dominates slab-ground heat transfer. As for code: what code applies to your location? If you're mostly interested in avoiding rigid foam, there's a Rockwool product approved for sub-slab use.

    1. mr_reference_Hugh | | #6

      Stephenr, are we in fact talking about the samething?

      When I looked it up, this is what I thought it was.
      https://low-e.com/products/low-e-slabshield/
      https://low-e.com/wp-content/uploads/2022/06/SlabShield_Brochure_2022_Web-2.pdf

      The same compay makes what appears to be one of those think radian insulation on a roll
      https://low-e.com/products/low-e-tab/

  4. stephenr | | #7

    Thanks for the replies. Mr.R, yes that is the product, although I haven't gotten to the point of where I am selecting a specific type of slab shield. I have been reading in PGH and there is no mention of it, which seems like a pretty good indicator that it is not a good choice. And John, thanks for your explanation of conduction as opposed to radiation. Not to muddy the waters too much here, but it brings up a related question that has been floating around in my mind. In a high performance build, how much of the heat generated in a slab through hydronics will waft up to the second floor? I have reasons for not wanting to install radiant on the second floor and have an open stairwell and a high R-factor all the way around. Is there no convection (or ways to encourage convection) with a heated slab?

  5. mr_reference_Hugh | | #8

    Are you willing to share why not heated floors on the 2nd level?

    1. For the slab shield, PGH is about making the optimal choices for a budget conscious person. I rely heavily on their guidance.

    2 The next slab project I do, I will be looking in depth at the feasibility of using foamed glass insulation. My concern with EPS and XPS is the environmental impact and the fact that certain ants love to make it their home.

    https://www.greenbuildingadvisor.com/article/foamed-glass-aggregate

    3. I really like Rockwool and/or competing products of the same type. The one that Jonhgfc mentioned.

    4. In terms of hydronic on main level heating the second floor, there is something to what you say. You can read about “stack effect” on GBA. BUT other factors will make this an undesirable choice event in a PGH IMO. I guess it might work in a full on Passive House. (Again I am a fan of PGH over PH).

    5. First, check your building code. In our province in Canada, we are required to have a heat source in every room. So your solution is a non starter where I live simply for code reasons. A person i know did this in his house but she got it passed because the inspector in his area was extremely flexible. The person also had roughed in wiring to every room in case electrical heat was ever required in the future in the rooms that did not have an HVAC heat source.

    6. With your HVAC solution, you need to understand that your house will require you to have a certain number of BTU/HR to heat the cubic feet in the entire house. Let’s assume with a PGH that the total is 15,000 BTU/Hr to heat the entire volume of space in the house. You would need to produce all the 15,ooo BTUs from the slab. Now we know that about half over your BTU/hr will be required for the first floor and half for the second floor. If we have the hydronics in the slab produce 7,500 BTU/hr, that would keep the temperature on the first floor nice and comfy. We need to understand that producing 15,000 BTU/hr in the same slab, to heat the whole house, will in theory mean that your first floor will not be at the same temperature you would expect from the 7,500 BTU/HR coming from the same slab. I would expect your first floor level to be too warm and that it would take some time for that heat to reach the rooms on the second level through the “stack effect” and other natural heat transfer effects.

    Even if you still wanted this solution and your code allows it, your 2nd floor rooms may not receive the actual amount of heat each of them needs because you are not controlling the heat distribution. This might still be ok if you are prepared to live with that. Just remember that you likely will need to resell the house in the future and this may not be everyone’s cup of tea.

    6. Your question:
    How to encourage the heat to get to where it is needed if all the heat comes from the slab.

    Comment: the only thing I can think of is to have ductwork installed to circulate the air throughout all rooms you want to have at a decent temperature. You could have inline fans in the ducts, or a central blower fan in an air handler. But now you have duct work. So at this point you may as well have a ducted heat solution for the 2nd floor.

    You could also look at a small heat pump with a small air handler, or you could also look at low static pressure ducted units that are paired with a heat pump. If you have a simple gable roof, you can have the trusses built with a chase specifically to run you duct work in the ceiling within the conditioned space. It is not uncommon but there will be a small cost for the chase in the trusses.

    7. You could look instead at using your hydronic heating boiler to run piping to the second floor and install hydronic radiators on the walls (instead of heated floors that you want to avoid on the second level). This would optimize the use of your expensive boiler.

    Heat pumps are very efficient but if the up front cost is a concern and you don’t like hydronic radiators, you could look at putting electric resistant infrared heating panels on the ceiling. This last one is a super easy solution to implement. Electric heating units like these are out of the way and 100% efficient in terms of energy consumed vs energy produced.

    Of course there are other heating solutions…

    I typed this on my phone and no reading glasses so I hope it comes out ok.

  6. stephenr | | #9

    Thanks Hugh, That was educational, exactly the reason i am on this forum. Here is my situation and thinking so far.
    I am designing a 1000 square foot 2x's over 1 house that I am building into a stepped ledge (hence the 2x's over 1). It,s as close to a PGH as I can get on about half the budget. I am a contractor and do a lot of building alone and will be relying on this approach to keep my labor costs down. A slab makes sense for the ground floor since half of it will be given over to utilities (a 8500 pound water catchment tank, composting toilet tank, etc.) and I figure I might as well heat it with an electric water heater since its a cheap and comfortable option. I am hesitant to continue the radiant tubes under the floor of the second level mostly because I only want to have real wood (probably locally sourced maple) flooring and don't care for the engineered variety.
    The second floor is an open floor plan with only a single small bedroom in the corner. Vaulted ceilings, 2/12 pitch. 8' and 12' load bearing wall heights. I hear your logic on why relying on the stack effect is probably not a good idea. A heating panel makes sense in the bedroom but for the vaulted room....mmmmmm... i guess if I weren't doing a radiant slab I would just go with heat pumps. But it would be a shame to also buy a heat pump system if I am already forking it out to set up a radiant system. To have both in a 1000 square foot house on a strict budget seems foolish.
    My ears perked up at the hydronic radiator suggestion. This could work on the second floor. I don't plan on having gas, so I would be relying on an electric boiler. I wonder if having, say, 2 radiators in the vaulted space would be more cost effective than setting up a heat pump? Or, should I scrap radiant all together and just do heat pumps upstairs and down?? Anyway, thanks for your valuable input. Glad I have all winter to get this sorted.

  7. mr_reference_Hugh | | #10

    Stephen.

    I knew it was to install hardwood. I love maple and just installed 3/;4 real maple in my home. I hear you 100%

    I have a few suggestions. You may know all this stuff already but here goes.

    The floor in the “utility area” does not need to be heated (right?) This leaves the hydronic heated floors really benefiting the living space on other half of the first floor.

    Buying a boiler and installing tubing to heat the first level will be a very expensive option for what you will get out of the system. It would make sense if you have a very strong preference for this setup and/or you used radiators on the 2nd floor. When I priced them before the pandemic, they were already very expensive,

    If you like electric (as I do), you can heat your slab with electric cables. There are types that go on top of the slab (cover with tile for example) and some that go inside the slab. Being electric, it is very efficient because there is no wasted heat at all. I assume you will have some serious insulation all around your slab and it will be air tight also.

    If you are not married to the the heated slab, you could for sure opt for a cold climate heat-pump, which is more efficient than an electric boiler or electric cable heater slab.

    I looked at using heat pump heads in each room but it does not work with air tight well insulated homes. The units (heads or otherwise) produce too many BTU/hr for what is required in a single room. It might work if you have one on each floor in your case. If you think of buying a multi head heat pump, read up about the pros and cons. To better control temperature and save on operating costs, many people install two smaller heat pumps with one head each inside.

    There are “semi-ducted” units to attach to a heat pump but the issue is that they use very low static pressure and it can be hard to find someone to design the ducts and hard to find someone to do the install. Of course , you could call design firms until you find one who can take the job. You may already know that low static ductwork necessitates expert design.

    We opted for full electrical. Heat pump with air handler (with backup heat strip) and ductwork. Mitsubishi will have units as small as 1.0 ton. Ours is 1.5 ton.

    The heat pump solution is very likely cheaper if you plan on having a/c. If you have a boiler to heat, you likely need a separate a/c system- more money and more space.

    You may end up reading about “magic boxes” at some point. They provide lots of functions in one unit but the problem is that they mostly heat with an inline heating coils within the ducts. Heating coil is less efficient than a heat pump.
    https://www.greenbuildingadvisor.com/article/a-magic-box-for-your-passivhaus

  8. stephenr | | #11

    Thanks Hugh, All signs are pointing towards heat pumps at this point. Interesting to think that buying two smaller heat pumps (1.0 ton, perhaps) might be the way to go, as you suggest. The upstairs will be 675 square feet and the downstairs just over 300. Very tight and very well insulated. Open floor plan. I like to sleep cool, anyway, and might just vent the bedroom to let heat in. looking forward to your future posts. I am going to start a new question about insulation. feel free to chime in!

    1. mr_reference_Hugh | | #12

      Hi Stephenr. I was just reading reply #44 in this posted question (link below). A frequent GBA commentor explains his experience with heated floors. I echo that. My mother has full heated electrical floors over 2400 sq feet (1200 sf per level) and both my wife and I dismissed the idea when it came to building our house. I just thought I would share - but it is a personal choice and preference.

      https://www.greenbuildingadvisor.com/question/what-do-you-love-or-hate-about-your-home?cid=237632&discussion=response#comment-237632

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