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Community and Q&A

Insulation under an exposed concrete slab/floor?

Barry_E | Posted in General Questions on

In the photos below you can see the uninsulated area. The uninsulated area above is currently a micro brewery and is the bar/seating area. Code did not require it to be insulated so the last owner decided not to when it was built. The building is five years old and was only used for a short period of time before wife and I purchased it. It has since sat empty until this year. When we have extended periods of cold it seems to me that this is almost like trying to heat the outdoors? No sun hits the concrete except for a little inside through windows that are probably coated? I believe I was told my area of north west NC is zone 5a? Elevation at building is around 2800′.

I have told the renters that I will insulate the floor before winter because I felt like it was the right thing to do. I believe the concrete is 4″ to 6″ but not sure, I am sure it’s no thicker than that probably 4″.

Should I insulate with sprayed closed cell and if so how thick? Is there anything mechanically to consider before spraying?

Thanks for any help.

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  1. user-1072251 | | #1

    I assume the metal is under the concrete? What's above the concrete - a heated space or an unheated space? In none of the pictures is it clear what we are looking at.

  2. Barry_E | | #2

    It is a heated space and it is concrete above the metal. I'll take a few more photos to make it clearer what I'm asking.

  3. Barry_E | | #3

    Here's a better photo I hope that will make it more clear.

  4. GBA Editor
    Martin Holladay | | #4

    Just to clarify -- the "renters" you are talking about are the same people as the people who are operating the microbrewery?

    Further clarifying: You have steel beams supporting a room with the great outdoors below the steel beams. Above the steel beams is a steel deck, and above the steel deck is a concrete slab. The original builders forgot to insulate this slab. Is that correct?

    Now you are renting the space above to a company that operates a microbrewery, and the space above is hard to heat. Right?

    You should be able to insulate under this suspended floor with closed-cell spray polyurethane foam. Remember that all of the area of the exposed steel beams needs to be insulated -- not just the steel deck. You'll have to find a spray foam contractor familiar with commercial work; once the spray foam has cured, it will need to be protected with a coating (a special paint) similar to the roof coating used when creating a spray-foam roof.

  5. Barry_E | | #5

    Yes Martin, you are correct. I apologize for not being able to articulate my questions.

    Does the special paint need to cover the entire area or just what is hit by sunlight?

    I'm going to start calling you "the insulation whisperer".

  6. GBA Editor
    Martin Holladay | | #6

    I'm not a specialist in insulation coatings, but I know that roofing foam is never left exposed. You need to defer to the requirements established by two experts: (1) your spray foam contractor, and (2) your local building official, who may have fire safety concerns.

  7. Jon_R | | #7

    Consider taped rigid foam applied over the beams. Or applied on the interior side. Don't apply foam only between beams (because of thermal bridging).

  8. Barry_E | | #8

    I'm afraid Im lacking on the experts you mentioned. The one spray foam contractor, I Know pretty well. Not an expert. The building inspector also, not an expert. Our regulations on building are just catching up and sometimes it's a few days before the inspector answers a question.

    I'll dig around and also ask as you recommended. Thanks again for the help.

  9. user-5946022 | | #9

    This looks like a metal building, and what is called SOMD (Slab on Metal Deck).

    You insulate this with a spray applied insulation. This is the exact same use case you see in the ceiling of a parking deck of an apartment building, mall or office building if there is occupied space above. However, the spray insulation does not need to be closed or open cell foam. There are a plethora of spray on products available to insulate this - spray on fiberglass, cementitious, etc.

    Based on the photo, the unconditioned outdoor space below does not have fire protection (fire sprinkler system). Depending upon the use, these areas are often required to have sprinkler coverage, which is achieved using a dry pipe system. If not, and if there is any chance that a vehicle may be parked in that space, there may be a requirement to protect the beams with spray fireproofing. In that case, I would have the fireproofer recommend an insulation product with which they are familiar that can be applied concurrently with the fireproofing.

    If you decide you don't need fireproofing, there are many spray applied fiberglass products which are probably the lowest cost.

    Another common way of dealing with this is to install an insulated dropped ceiling, and install a unit heater in the plenum space. Set the unit heater to go on and heat the plenum when the space reaches 55 degrees. If you end up being required to fire sprinkle the open space below, this might be the most cost effective option, because you can sprinkle it with a wet pipe system, and outdoor heads.

  10. Expert Member
    Peter Engle | | #10

    You can also drop soffits around the perimeter with cladding to match the rest of the building, then spray your fireproofing/insulation against the underside of the metal deck, using the soffits as end dams. Then hang a suspended ceiling below the structure just to protect the insulation against weather, vermin and such. If you need a fire sprinkler, use the heated plenum approach above.

  11. GBA Editor
    Martin Holladay | | #11

    As the recent comments indicate, you need to talk with contractors familiar with commercial construction like parking garages. Good luck.

  12. Barry_E | | #12

    Had a local to do this for me as it was ridiculously cold slab inside. They did cover the entire beams but didn’t do the edge under the slab. I think they didn’t do it because it required covering the siding and the guys doing it were not willing. Here’s a few photos. What do I need to cover this with to keep the (UV) from ruining it?

  13. Barry_E | | #13

    Not sure why the photos didn't post earlier. What Is recommended to protect this from(UV)?

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