Construction Design-Heating and Cooling Systems in a Slab on Grade Home

Single level slab on grade homes are becoming more common in my area.  Being in a very cold climate, choosing a heating system for this type of structure requires some thought.  A common choice is radiant in-floor heat.  This type of heat works by heating the floor, which in turn heats anything in contact with the floor.  The air is heated by the heat radiating off these warmed surfaces.  A very comfortable heating system when installed correctly.  One problem, a second, faster reacting system will be needed for heating during the shoulder months when just a little heat is needed. And then there’s cooling.  Most homes in my area are installing air conditioning. which will require another system.

Hot water radiant heat in a slab on grade home.

A great choice for a slab on grade home is a cold weather air source heat pump.  Most of these systems are called “mini-splits” meaning they have an outdoor condensing unit and a wall hung indoor air handler.  Both heating and cooling are satisfied with one single unit.  Super-efficient, affordable to install and low operating costs are the benefits.  The problem is they won’t quite produce enough heat during the coldest periods of the year to maintain comfort, at least not in my very cold climate.  A second source of heat will be required.

How about forced air heating.  There are all sorts of forced air systems to choose from.  The problem becomes the ductwork, no basement or crawl space where the ductwork would traditionally be installed.  This could leave the ductwork to be installed in unconditioned spaces, not a great idea.  I’ve lost track with how many homes I’ve been in over the last year where ductwork is placed in the unconditioned attic space.  Whether it’s the main heating system or a second source to provide a back-up heat or air conditioning, I’m seeing lots of ductwork in the attic.

So, what’s wrong with ductwork in the attic?  There are many reasons this creates issues.  First, there are the penetrations through the ceiling air control layer.  The ceiling is the most common place for air leakage to the outside to occur during the heating season.  All that ductwork from the furnace will need to both exit and enter through the ceiling at some point.  All these holes must be sealed, and often times they aren’t sealed well enough.

Ductwork chases entering the conditioned living space from the attic.

Second, any ductwork that leaves the conditioned space must be insulated.  In my very cold climate, this insulation is required to be minimum of R-8.  (In my opinion, this isn’t enough insulation.)  Most of the insulated ducts being installed in unconditioned spaces are insulated flexible duct.  A better system is solid metal ducting, but costs and difficulty installing and insulating often prevent this type of ducting choice.

Third, all the ductwork that leaves the conditioned space will need to be meticulously sealed.   Air leaking into the attic from the supply side of furnace ductwork can create a negative pressure within the conditioned space of a home.  Ducts that leak on the return side when placed in an attic will create a positive pressure within the home.  All these ducts need to be sealed to limit air leakage.  As a matter of fact, the building codes have addressed this issue by requiring ductwork that leaves the conditioned space of a building be tested.  This test, called a duct blaster test, uses a fan to create a pressure inside the ductwork.  Pressures are then measured and a leakage rate is calculated.  No testing is required if ductwork stays within the building envelope.

Other issues with ducts placed in unconditioned spaces is the possibility of the ductwork condensating.  Temperature differences between ducts and the space around them can cause moisture to form either inside or outside the duct.  Water dripping onto building materials can cause issues.  Ducts placed in attics also displace insulation resulting in hot or cold areas that I can often see with my thermal imaging camera.  I have also seen building components such as manufactured trusses cut to allow the placement of ductwork.  This can create a structural issue with the home.

What options do we have to keep ductwork within the conditioned space?  One option is to install the ductwork inside the home and build chases and soffits to hide them.  Chases and soffits can look good if properly planned, they can also look out of place if not well thought out.

Another option is to use a plenum truss.  I used this option on a recent project I worked on.  A plenum truss is simply a notch made in the truss to allow ductwork and other mechanical systems to remain inside the building envelope.  The truss is then filled in with framing to provide a flat ceiling.  One limitation is the notch size will be limited to the truss design and there will be some design limitations for the mechanical systems based on the location and size of the notch.

Plenum Truss

Next option is to frame the entire ceiling down to allow for mechanical systems.  I have read several articles where this design was implemented. Basically your building a ceiling cavity under the main roof framing.  This may require additional engineering and coordination with the mechanical contractors but will allow the entire ceiling to act as a service cavity.

Last option is to create a conditioned attic.  These are popular in southern states, but rare in my northern climate.  A conditioned attic is a space where the roof is insulated and air sealed leaving the entire attic space open for mechanical systems and storage.  Codes in my climate require ceiling insulation levels to be R-49.  This means there will be al least 7 inches of closed cell spray foam or more than 12 inches of the fluffy insulation.  Or…a combination of rigid exterior insulation on the top side of the roof and fluffy cavity insulation below.  I nice option, but like I said, rare in my climate.

There you go, several options for heating and cooling systems and keeping ductwork inside the conditioned space of a slab on grade, single level home.  Please leave me comments and questions.  I’d like to hear any suggestions I can make to improve this blog.

10 Replies to “Construction Design-Heating and Cooling Systems in a Slab on Grade Home”

  1. Conditioned attic? OK, I live in East Tennessee, climate zone 4, and want to build a slab-on-grade house with a conditioned attic (open cell foam in the roof deck). There are lots of articles out there, many with scary warnings. Can anyone point me to a definitive source describing how this should be done? I don’t want to just trust the foaming contractor. For example, is a special underlayment required? I’ve seen both yes and no answers.

    1. Hi Lester, You’re in a mixed climate zone, an area where I really can’t give good advice. I can say, in my climate, we avoid open cell spray foam. Even in your climate, I would probably pay the extra money and use closed cell for insulating a roof. You are creating a sandwich with the roof decking, two products with very low perm ratings encasing a wood sheeting product. We have lots of these assemblies in my area that work. Trick is to maintain the roof, keep the shingles or other roofing material in good condition and all penetrations well sealed so water can’t leak from the outside, something that is required regardless of the interior insulation. 2nd is to make sure the spray foam is installed correctly without any missed areas. These missed areas will be a problem. Hope this helps.

  2. We live in a cold province in Canada. Can the duct work be installed inside a slab on grade to be used with a propane furnace provided the duct work is protected with notched truss like 2” x 4” , insulation and covered with plywood then cement?

    1. G&M, Great question! There are manufactured duct systems for ducts that are installed below grade, which I would recommend over building your own. I’ve had the opportunity to work on a below grade duct system that was constructed back in the early 1980’s. The system was installed in a slab on grade office building. There was a main duct trunk line that connected to parallel ducts that ran the length of the building which were installed every eight feet. The ducts carried air and became an electrical and communication wiring chase. They were covered with plywood to allow access for the electrical. The concern with a site built system is it needs to be sealed for both ground moisture and air, at least in the states. Ground moisture can be transferred to the ducts and then distributed into the home which will elevate the interior humidity, not good in a very cold climate. Another possible issue, will a building inspector say the duct is inside or outside the building envelope. If outside, it will have to be pressure tested. I’ve seen a couple modern systems, one in residential construction and one in a group home. Both used polyvinyl steel for the ducts. I was at both projects conducting a blower door test. Both buildings passed. A source to start is Spiral Manufacturing Co., a quick internet search led to a couple other sites. Thanks for the question and good luck with your project. I would like to see how you proceed with your project. If you can, send me an update, northernbuiltpro1969@gmail.com.

  3. Thanks for this informative blog. I have a 1500 sqft house that was built on a slab in the 70s. No duct work, electric baseboard heat and no A/C. It has vaulted ceilings, about 11 ft at the highest point and another 23 inches of arctic space. I’ve considered mini splits or adding duct work to the attic and bringing in natural gas & adding central air. Sounds like there are many cons vs pros for both systems. I’m not sure which would be best or if there are other efficient and aesthetically better options. Do you know of any good contractors in the SW Michigan area? Thank you.

    1. Hi Andrea, thanks for the comment. I’ve been in lots of ’70’s homes that have electric baseboards as they’re main source of heat. Electricity is no longer very competitive with some of the other fuels unless it’s either on a load control program or a very efficient heating and cooling system, such as an air source heat pump. I would check with your electricity provider and see if they have any heating programs. Here in Northern Minnesota, we have several providers that offer programs like dual fuel. With this program, the electricity provider will turn on and off your heating equipment based on their electrical demand. In return, you get a lowered rate of electricity for your heating (or possibly cooling) source. You would need a second source of heat capable of heating your home during the times when your provider has your heating off. We typically use thermostatically controlled gas fireplaces sized large enough to handle the heating load of the home. Improving the performance of your home would also be a good idea. Unfortunately, I do not know any builders in your area of Michigan. Wish I could be more help. Good luck!

  4. You’ve answered a lot of questions and provided several options, however, if you framed the ceiling down lets say in the hallway running the length of the house, how do you get your venting down to floor level in each individual room. I would assume the cold are returns could go in the ceiling hallway trunk line but I don’ know how to get the vents to floor level. Perhaps box in a vent in room closets or room corners ??? Building in Northern Ontario (-20 plus winters) with plans for a wood stove as well.

    1. Hi Tom,
      If you have a really well insulated and air sealed home, the vents could stay in the ceiling and away from the outside walls. We did this with the Concreteless Slab on Grade Home and the occupants have been comfortable with no complaints over the past two winters, and that’s with an air source heat pump for the main heat and an electric plenum heater for back-up in an area with a winter design temp of -22°F or -30°C. If you do decide to run ducts close to the floor, there are hard metal ducts designed to fit in an interior 2×4 wall cavity. We often use this type of duct to exhaust range vents that are on interior walls, but they are often used for supply and return air inside wall cavities. It’s not recommended to use the building cavity alone as a duct. Lower the hallway ceiling so it’s low enough for the ducts to fit under the top plate or use a plenum truss so that the ducts are above the ceiling and can be dropped down the wall cavities will also work. Your idea about using space in a closet is also a good idea.
      Hope this answered your questions.
      Randy

  5. I have a 1780sq’ foot 2 bedroom townhouse on a slab, with mostly 10’ ceilings and a great-room with 20’ ceiling at peak height. Home is heated with a gas boiler, driving an in-slab water piping radiant floor system. I have a great-room fireplace that currently is not efficient for heating.
    Strata forbids heating systems with external components, due to noise.
    I’m trying to think through alternative heating scenarios FITB the time when the in-slab piping inevitably fails.

    I live on Vancouver Island (British Columbia) where the climate is the mildest in Canada. Think Washington State without as much rain. Air conditioning is not a priority.
    I’m thinking a combination of:
    . A high-efficiency gas fireplace.
    . Baseboard heaters in bedrooms
    . Perhaps a latest technology heat pump that would be acceptable to the Strata.
    Any thoughts?
    Thanks!

    1. Hi Mike,
      The air source heat pump would be the way to go if it is allowed. Efficiency for the gas fireplace will be in the 80-90% range. The baseboard heat is 100%, but is one of the most expensive ways to heat unless there are rate reduced programs offered by your electricity supplier. (Check into if your supplier offers any programs, the suppliers in my market have rebates and reduced rates for many different programs such as water heating, space heating and even controlled air conditioning.) The air source heat pumps are 300-400% efficient because they move heat instead of create heat. The outdoor unit on the high end models are very quiet, but I do suggest a ground mounting system instead of having mounted to a wall.

      Why do you think the tubing under the slab will fail at some point? For the past 20+ years we have been using Pex type piping which is well suited for the use and should last a very long time. I have heard of older type tubing failures, but only a couple. Because of the climate I’m in, I try to have at least two different heat sources in the buildings I’m involved with. That way, if there is a failure, or a very large increase in energy costs for one heat source, the second can take over. This second system can be something like a thermostatically controlled gas fireplace or the electric baseboard heaters you mentioned.

      Hope this helps-Randy

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