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.

28 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

  6. I am building a new home this summer in northern Ontario. I’m doing slab on grad with i floor heat. I will using an air handler for blowing secondary heat from coil off the boiler and I will have an a/c coil in the air handler for cooling. Do I need to have the duct work blowing in front of each window. I’m trying to avoid the amount of low ceilings/bulkheads I can get a vent and a cold air return in each room just not in front of the windows. I am assuming because my main source of heat is my slab I should be ok could you confirm ?

    1. Hi Kris,
      We joke a lot in the building science community about how many times we answer questions like yours with “it depends”. I’m assuming you’re in either climate zone 7 or 8? My answer would also apply as far south as climate zone 6. It’s going to depend on the windows you choose to install and the interior humidity level you are hoping to maintain. Double pane windows will condensate in cold outside temperatures depending on interior humidity levels. If the humidity levels are very low, less than 20%, there will be little to no condensation. If you get to -20°C or colder, many double panes will start having moisture and frost at 30% indoor RH. A good triple pane window, with u-values under .2 may not condensate at 30% until temps near -40°C. Washing the windows with air will help reduce the risk of having moisture and frost form when lower performing windows and higher indoor humidity levels are present. If you choose not to use air washing to keep windows from forming condensation, you will need to adjust indoor humidity levels based on outdoor temperatures. This may require you to play around with the indoor humidity level until you can determine at what point at a specific outside temperature causes condensation to form.

      The materials used to build the sash and frame of the window will also come into play. Windows with wood interiors that accumulate frost will need frequent refinishing to maintain the window, where as a PVC or fiberglass window may not require any maintenance.

      I recommend building your home as tight as you can, test to verify, and install balanced mechanical ventilation, in your climate, that will most likely be a heating recovery ventilator or HRV. If the home is high performance, you might be able to move to an ERV or energy recovery ventilator. I would only use an ERV with well insulated walls, very tight enclosure and triple pane windows.

      Hope this helps,
      Randy

    1. Hi Anita,
      You’re in a warm climate with a smaller home. This is the perfect application for an air source heat pump. The majority of the time, the system will be used to cool and dehumidify, but can heat the home when required. The cold climate air source heat pumps that we use in my area will still provide some heat at -5°F, I’m guessing your location will never see temps that cold. Look at the mini-split type air source heat pumps, they have different air handler options, most are wall hung but there are also options for ceiling cassettes. One suggestion, the better insulated and better air sealed the home, the easier it is to cool and heat.
      Hope this helps and thanks for the question.
      Randy

  7. We have a 95 year old log cabin on a lake in northern Mn. Partial stone basement in 1/2 of the cabin narrowing to dirt crawl space. 1500 sq ft. with 2 brs 2bas and 2 stone fireplaces. Has been used seasonally but we would like to make it year round. Has old electric baseboards for heat. There’s also an old wood burning furnace in the basement with some floor vents that is not used. Natural gas is not available, but a propane tank could be installed. What would you recommend for heating?

    1. Hi Debbie,
      I’ve been involved with many log cabin energy audits and building science investigations. You have many choices in the type of heat, often the problem with a log cabin is keeping the heat inside the home. My advice is to first make any improvements you can to the shell, the roof is often the biggest problem, log cabins typically have poor insulation and air sealing at the roof which causes both heat loss and ice dams. Make sure the walls are well air sealed, this may require adding or replacing chinking. Windows and doors can also be problematic. Lastly, insulate and air seal the rim joist area. You will also have to make sure the foundation doesn’t have a connection to the outside. After that work is completed, a heat loss should be calculated for the home so that the heating and/or cooling system is properly sized.

      As far as what system is best, both propane and electric on an off-peak program can be good options. If you have existing ductwork, a forced air system may be the most economical option. Cooling can be easily added to this type of system.

      I would suggest first having an energy audit with a blower door test conducted on the home, most rural electricity providers offer the service at no or reduced cost, and then make a plan based on the findings. The best and most efficient heating systems will not overcome poor insulation and air sealing.

      Thanks for the question,
      Randy

  8. Hi. We’re thinking to buy a house built in 1975 and it’s on poured concrete. No basement. Heating source is baseboard. I want to install forced air and heat. How to do that. It has a huge 2.5 cars attached garage and a small attic. Please let me know.

    1. There are a couple options for heating the home you are looking to purchase. If you are in a more moderate climate without severe winters, an air source heat pump mini-split system might be all you need. Cold climate versions of these systems can heat down to around -15°F plus supply the home with cooling during the summer months. These systems can be installed with a wall hung or ceiling cassette option. They can also have more than one air handling unit per outdoor condenser. There are a lot of tax credits and rebates available to help offset some of the costs. A second option is to install a fully ducted furnace. The issue is how to conceal the needed ductwork. You want the ductwork to remain inside the building envelope if at all possible. Running ducts in unconditioned areas, such as in an attic that is outside the building envelope can create a lot of issues for the home. If ducts need to be in an attic, I recommend turning the attic into a conditioned space, making it part of the home rather than outside the home. This is done by insulating the sloped area of the roof rather than the attic floor. This is common in new construction in southern climates, not as common in colder areas. Something else to consider is the location of the furnace itself. If it is in the attached garage, the area where the furnace sits should be part of the home, not part of the garage. You do not want garage air, which may have carbon monoxide and other fumes mixing with the air inside the home.

      Hope this helps,
      Randy

  9. Great article on the considerations for heating and cooling systems in slab-on-grade homes, especially in colder climates. The explanation of radiant in-floor heat as a comfortable heating system was informative. It’s interesting to learn that in a slab on grade home, a second, faster reacting system is needed for heating during the shoulder months.

  10. we live in a condo and our upstairs neighbor wants to install a heat pump on a concrete structural slab which is our bedroom ceiling. Is there any concern to us regarding noise or vibration?

    1. Hi Joe,
      The wall hung or ceiling mounted air handler part of an air source heat pump are extremely quiet. I doubt you would be able to tell when it is operating. The outdoor condenser can be a little noisier, especially if it is mounted to the exterior wall of a building. This can be an issue in light framed construction, in buildings with structural concrete floors, I would guess there is some mass in the walls as well, and you may not even hear the condenser.
      Randy

  11. What a great and informative thread! I live in NJ. 1800 sf ranch house built in 50s. Forced air through ducts in the slab which heat unevenly. I’ve had termites and currently have drain moths. Hard to locate the source of the drain moths with no crawl space or basement to inspect the sewer lines, but the moths do seem to travel through the floor ductwork popping out throughout the house. A number of people in my neighborhood have gone to systems in the attic and have not conditioned that space. I have read all of your concerns about that option. I’m thinking about a gas fireplace and mini splits. My wife doesn’t like the idea of mini split registers all over the house. And I’m not sure mini split will be efficient enough to heat my home in the coldest time of the year. So, what do you recommend? My 3 choices are. 1. Keep the system as it is with forced air through the slab and deal with the uneven heating by supplementing it with mini splits or gas fireplace. (I also wonder if this old system exacerbates termite and drain mouth issues,) 2. Go with an attic system. If so, I’m not sure conditioning the large attic is an option. 3. Go with a mini split and gas fireplace system.

    1. Hi Michael,

      I’ve been in a few homes with ductwork in the slab, they can be problematic. If they are not sealed correctly, they can pull moisture from the ground raising the indoor humidity levels, possibly creating issues with the indoor air quality and mold growth. Another issue is the one you are experiencing, uneven heating. The ducts probably are not insulated, warming the ducts takes a lot of time.

      I like the air source heat pump mini split with the gas fireplace back-up idea. There are several options for the registers or heads. There are floor mounted, and ceiling mounted heads available from several different manufacturers. Some of the ceiling mounted heads also can have short runs of ductwork to feed adjacent rooms. These do protrude into the attic, so if the attic is unconditioned and vented, air sealing and insulation needs to be considered.

      The other option of using the attic space for the ductwork may require a change in the design of the attic space. Vented and unconditioned attics that are outside the conditioned space of the home are not a recommended location for ductwork. An option is to move the attic insulation and air sealing from the attic floor to the attic ceiling. Often this is done with closed cell spray foam, though there are other ways to accomplish this. I recommend working with a building science practitioner if you choose to go this route. The spray foam may tighten the home enough to where indoor humidity levels rise, creating other issues.

      Hope this helps,
      Randy

  12. Thanks so much for this phenomenal article! Amazingly informative! We are planning to build our first home in Clearwater, BC (winters with regular minus teens Celsius and occasional -30c) using a prefab builder called Winton Homes (Fulton Lake DO2 model). The most cost effective way to execute the kit is with a slab on grade foundation but I’m unsure as to what the most cost effective and efficient way to heat the house will be in our climate. Without a doubt we will have a wood stove supplementary heat source, and we do plan on a med size solar setup, but what would the most practical primary heat source be given the lack of a crawlspace? Heated slab? Is it possible to have a non-heated slab in our climate with just a cold weather air source heat pump? How do you duct it then? I assume baseboards would just be way to expensive to run. Just after the most practical solution balancing initial costs and operating costs. Could it end up being cheaper to just pay more for the kit with an engineered floor and put it on a crawlspace and do a standard forced air system? Thanks so much in advance!

    1. Hi Dan,

      Yes, cold climate air source heat pumps can be an effective heating system, even in a very cold climate, that being said, you will need a back-up source for when the temperatures drop below the capacity of the heat pump (somewhere around -20°C to -30°C). You have a couple options; one is the mini-split configuration with one or more wall hung units and the outdoor condenser. I would probably pair this with electric baseboard heaters as the backup. The woodstove could also be a backup, but someone needs to be present to light a fire. Personally, I prefer to see a backup system that is thermostatically controlled. Another option would be a central ducted cold climate air source heat pump. These are more traditional forced air heating systems with ducts supplying heating and cooling to the various locations around the home. They can be ordered with a factory electric plenum heater that takes over when the outdoor temps are low. The issue with this system is, of course, running duct work. It possible, have the home designed so that the ducts are inside the building envelope. My last several projects I’ve been involved with have been slab on grade homes, several with central ducted heating and cooling systems. Having a truss with a notch to keep the ducts in conditioned space can be an option. Here’s an article I wrote on that topic: https://www.northernbuilt.pro/construction-design-service-cavity/

      Hope this helps and good luck with your project!
      Randy

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