Construction Design-Using Fibrous Sub-Slab Insulation with In-Floor Heat

I’ve written about sub-slab insulation and heated floors several times over the past couple years, this post is about something I’ve been thinking about for a while now, using a fibrous insulation, like Rockwool’s ComfortBoard 80 or 110 under a concrete slab with hot water heat.  The questions I had about this strategy are one, will there be an issue with compression of the insulation when concrete is poured over the product?  And two, will a staple be able to hold the hot water tubing in place?

Rockwool ComfortBoard being installed below a concrete slab.  Photo by Travis Brungardt

In the past, I have used XPS or extruded polystyrene insulation (the colorful insulation) under slabs.  It’s fairly economical, has a good resistance to heat flow, easy to source, and has a good compression rate meaning there is little “squishing” of the product during and after the concrete pour.  (Squishing is a highly technical term).  A few issues, I’ve seen XPS absorb and hold water, it can be more difficult to seal around floor penetrations, and probably the biggest downside, it’s not great for the environment.  I have not used this sub-slab insulation strategy in several years.

I’ve also used EPS or expanded polystyrene foam; this is the stuff that looks like it snowed after cutting, this insulation is also a common packaging material.  Typically, in sub-slab installations, we are using the type IX product which has a higher density.  It will install much like and is a little cheaper than XPS.  EPS has a slightly lower R-value but is also available in GPS, which include a graphite additive to increase the resistance to heat flow.  It can also be slightly harder to source, most lumberyards I work with don’t stock the product but can get it within a couple days.  EPS has the lowest global warming potential (GWP) of all the insulations discussed in this post, ComfortBoard is close with regards to its GWP.

The third sub-slab insulation I’ve worked with is closed cell spray foam (CCSF).  It also has both advantages and disadvantages.  First, it’s the most expensive of the choices I’m talking about in this post and chances are, you will have to hire an insulation contractor to perform the work.  Another issue, depending on the blowing agent used, it can have a higher GWP, bad for the environment.  I do like that it has a higher R-value per inch than the other insulations and it works great for air sealing under the slab.  It also has good density, and you can spec a higher density if needed.  I don’t know if CCSF absorbs water like XPS, something I’m hoping to do a back yard wingnut test on in the future.   You can read more about my adventures with sub-slab CCSF here.

Now to the fibrous sub-slab insulation.  I’ve used Rockwool’s products on several occasions.  I really like both their cavity products and their board insulations, such as ComfortBoard.  We used ComfortBoard 80 as exterior insulation on the Concreteless Slab on Grade project.  This insulation is made from basalt, a type of rock and steel slag, a biproduct of the steel industry.

The ComfortBoard product has many properties I like, it’s hydrophobic, meaning it doesn’t care if it’s wet or dry, it drains easily, and water usually beads off the product.  It doesn’t burn and is vapor open.  Bugs don’t particularly care for it, and it does not grow biologicals.  There are a couple downsides, ComfortBoard is harder to source.  My lumberyard does not stock the rigid boards, they are a special-order item.  Current lead times are long, around a month depending on the product but my contact at Rockwool says those lead times will be reducing in 2022 as the new factory that recently started producing product catches up on the backlog.  Another downside is the “squishiness” of the product which I will touch on in a bit.

ComfortBoard can be used both above and below grade.  In 2017, Rockwool issued a technical bulletin which reads:

Rockwool ComfortBoard is rigid mineral wool board that is used as a continuous layer of insulation in residential building envelope systems.  It has the mechanical properties to be used in residential sub slab applications as an alternative to rigid foam.”

The bulletin goes on to say it can be used under a poured concrete slab above crushed stone and between a slab edge and foundation wall, acting as a thermal break.  It should not be used under footings and load bearing walls and not used in applications where less than a four-inch slab is poured.  It should also not be used in a high-water table environment.

You can read the technical bulletin here.

Pretty straight forward, one note I would like to expand on is the crushed stone required under the product.  The crushed stone will allow any moisture that accumulates in the insulation to drain out.  This stone layer may also be required by code as the radon mitigation system.

Here’s a quick design I sketched up using a stem wall for the foundation and then pouring a concrete slab inboard of the foundation.  A typical detail in residential construction but more common in commercial builds.  The drawing has the stone under the insulation and shows the tubing in the concrete slab.  One item the drawing is missing, the sub-slab vapor control product, typically 6-mil or thicker polyethylene sheeting.  It’s important the poly be installed on top of the insulation, not under.  The location of the poly in a sub-slab assembly is a very common mistake in my market.  When the poly is under the insulation, any water from the concrete pour that does not come to the surface and evaporate will drain into the fibrous insulation and probably take many months or years to fully dry upward.  The poly will prevent any downward draining.

Now, on to the questions I had.  Question number one, is the insulation dense enough to support the weight of the concrete and any load from above?  According to the manufacturer, yes under normal circumstances.  At 10% compression, ComfortBoard 80 is listed at 439 lbs. per square foot.  ComfortBoard 110 has 584 lbs. per square foot.  Rockwool does say to expect a little compression of the product during the concrete pour, which may slightly reduce the R-value.  I don’t think I would use this insulation where heavy loads are placed on the concrete slab.

Question number two, will the product hold a tubing staple?  Yes!  I recently tested with the help of my brother.  We bent a tight radius of 9 inches in a 1/2-inch tubing and stapled it to a 1-inch sample of the ComfortBoard 80 product.  I was amazed at how hard it was to pull the staple out once installed.  I don’t believe there will be any issues with installing hot water tubing to this fibrous insulation using the staple method.

I recently had an Instagram post on this topic, one of the questions that came up was why change, the foam insulations are performing just fine.  Doug Horgan commented:

“…Foam insulation has some issues which have many of us looking for alternatives.  It’s made of not-great-for-us stuff including a shockingly high amount of fire retardants and even the base styrene plastic isn’t great.  Until very recently XPS was only available with high greenhouse effect blowing agents.  Long term moisture performance can be an issue…Rockwool isn’t perfect (high embodied energy for example) but in certain regards it avoids some of the foam issues.”

As Doug said, Rockwool isn’t perfect, but it is a good option if you are looking to eliminate the plastics and other chemicals used to produce the alternative insulation products.  Hopefully I will find a customer that is looking for an alternative option for sub-slab insulation and will have the opportunity to use Rockwool below grade.  In the meantime, if any readers have used or plan on using this method, please let me know.  I’d like to hear your feedback.

6 Replies to “Construction Design-Using Fibrous Sub-Slab Insulation with In-Floor Heat”

  1. So with needing to put the poly down above the rockwool, did you just lay it over the pex for the radiant?

    Thanks for all the great info too!

    1. Hi Jay, thanks for the question. I would either place the poly over the pex, or as an alternative, install the poly, then place rebar or wire mesh, the pex can then be tied to that. What we don’t want is a bunch of holes punched in the poly by the pex staples. The location of the poly in the sub-slab assembly is constantly being installed incorrectly in my market, it’s usually under the insulation. We want to keep the water present in the concrete during the slab pour from absorbing into the insulation. That water takes years to move back out of the insulation and through the slab, which can increase indoor humidity levels.

  2. Thanks for the information on this product. The concrete subcontractor that is pouring the slab for me had the vapor barrier below the rockwool. If you’re interested in any user experience with Comfort Board 80, I’ve been waiting on this product for two months. Delivery date kept getting pushed back because no one keep this in stock. Finally got a delivery date this past Monday, and only half of the order was delivered. Neither Rockwool, not the supplier seem to know what happened to the 40 sheets that were not delivered. We were planning on double layering 2″ Comfort Board 80, but now we have decided to use one 2″ layer of Rockwool and one 2″ layer of foam board. Do you have any thoughts or expertise on which should go on top, Rockwool or foam board?

    1. Hi Nathan,
      Thanks for sharing your experience with the sourcing of Rockwool, if it is okay with you, I’d like to forward your comment to someone with Rockwool, they would want to know about your frustrations in acquiring the Comfortboard. My understanding is there is only a week or two lag in deliveries for Rockwool.

      As far as your sub-slab assembly, it shouldn’t matter which order the foam insulation and Comfortboard go. If you are installing tubing for an in-floor heating system using staples to hold the tubing in place, you may want to consider placing the foam insulation on top. Both insulations will hold the staple, but they are designed for the foam insulation.

      It’s very common for the vapor barrier to end up under the insulation, though it’s not the best place for it to be. I always specify that it goes on top of the insulation in projects I’m involved with, it forces the moisture in the concrete to dry up and out instead of absorbing into the insulation where it takes years to dissipate. If the poly is already in place, I wouldn’t worry about changing it, just make sure it is well sealed, its purpose is for both vapor and soil gas.

      Good luck on your project,

    2. Hey Nathan, I sent you an email as well but wanted anyone reading this to know that I work for ROCKWOOL and will gladly assist with the purchase of our products. We have a few national distributors that usually can get our products in a few weeks depending on location.

      Also, we recommend the vapor barrier to be on top of the insulation. I have seen it in the middle of the layers of the Comfortboard also to protect it from the holes (although feel those holes on top are negligible to the vapor barriers performance.

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