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.

Leave a Reply

Your email address will not be published. Required fields are marked *