Hydronic Radiant Heating
Written February 29, 2012 by Hendricks Architecture: Tom Russell, Architect LEED AP • Filed Under Energy Efficiency
When designing a home, it is important to consider early in the process what type of climate control system the home will be using. Mechanical systems used for heating and cooling work best when they are properly sized, thoughtfully laid out, and have adequate space for all the components.
There are many options for heating and cooling a home. Many homeowners these days are opting for hydronic radiant heat systems, and we get a lot of questions from our clients about radiant heat systems and how best to configure them. Radiant heat is a great, energy efficient choice for homes where heating is the primary concern and cooling is secondary.
Hydronic Radiant Heating
Unlike forced air heating systems, radiant heating warms the objects in a space, not just the air. Because of this, the space will feel warmer and the ambient temperature can be kept lower than it would be in a space that is heated with warm air. Other benefits of radiant heat are the lack of moving air that can transport dust and allergens, and the ability of radiant heat to maintain an even temperature without noticeable fluctuations.
Hydronic radiant heating uses a central boiler to heat a fluid that is then circulated through piping concealed in the floor system. The choice of floor system is a major determinant in the performance of a hydronic radiant system, but the choice of floor system should not be based only on what type of heating the house will utilize.
The best radiant heat systems use a concrete floor slab as a thermal mass (see related concrete slab vs. wood framed floors). Heavy duty plastic tubing is embedded in a slab that is insulated both on the perimeter and on the underside. The amount of insulation depends on the local climate, the level of efficiency desired, and the budget. The biggest advantage of this system is the substantial thermal mass of the concrete slab, which will store and radiate heat over an extended period of time. The slab will also double as a collector and storage medium of any passive solar gain. On the flip side, the slab will take a while to heat up, so this type of system does not lend itself well to turning the heat down during periods of inactivity or absence.
Leaks and damage to tubing that is encased in a concrete slab can be costly and difficult to fix, but thankfully they almost never occur. Problems due to tubing failure can be mitigated by making sure the tubing is thoroughly leak tested and the slab subgrade is well compacted granular material.
When a concrete slab floor is not practical, radiant heat tubing can be embedded in 1½” or more of lightweight concrete or gypcrete poured on top of a wood framed floor. This is often done on homes that have hydronic heat on upper floors or where a crawl space and wood framed floor is necessary. Since a 4” concrete slab is too heavy to be supported by a wood framed floor, a thinner, lighter slab is used. It has significantly less thermal mass, but does provide some heat storage capacity and also helps dampen floor vibrations common with wood framed floors. Floor framing has to be more substantial for this type of application than it would be for a floor that doesn’t have to support as much weight.
For radiant heat applications where a wood framed floor is preferred or required and gypcrete overlayment is not used there are a few different options. Warmboard manufactures a plywood subfloor sheathing that has integral channels milled into it that allow radiant heat tubes to sit below the top of the subfloor. The channels are clad with sheet aluminum that radiates heat upward into the living space and makes for rapid warming of the floor above. In this sense, it outperforms the concrete embedment systems, but it lacks the thermal mass and ability to moderate temperature fluctuations. Warmboard is relatively expensive, but by most accounts it functions well and is a viable alternative when concrete or gypcrete is impractical. Misplaced nails or dropped tools can easily damage the tubing, so pressure testing is required before covering and after flooring has been installed.
The staple up radiant tubing application is the least expensive and easiest system to repair or retrofit. As a trade off, it is also the least efficient and easiest to damage. Staple up systems involve installing the tubes on the underside of the subfloor between floor joists. The tubes are held in place by staples and sometimes backed with foil faced rigid insulation or installed with integral metal heat transfer plates. In order to be reasonably efficient, a staple up system needs to have more than code required insulation in the joist bays, and shouldn’t be used where floors are cantilevered out beyond heated space below because of the potential for condensation.
Staple-up radiant heat is the least efficient.
For more on floor system options to use with hydronic radiant heating, see our article on concrete slab vs. wood framed floors.
Tom Russell, Project Architect, LEED AP
Hendricks Architecture specializes in custom mountain style homes. Our homes have been featured in Timber Home Living, Mountain Living, Green Building and Design, Cowboys & Indians, Cabin Life and other publications. We’re located in Sandpoint, Idaho. Subscribe to Hendricks Architecture’s Blog
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Concrete Slab vs. Wood Framed Floors
Written February 28, 2012 by Hendricks Architecture: Tom Russell, Architect LEED AP • Filed Under Architecture
One of the most common questions a residential architect is asked is, “Would it be better to have a concrete slab or a wood framed floor with a crawl space?” When choosing what type of floor system will be best for a project, several factors need to be considered. Site topography, where the house will be located on the site, seasonal groundwater levels, the number of floor levels the home will have, the type of heat desired, and budget constraints all need to be weighed before making a decision.
A concrete slab on grade works well on relatively level sites with a first floor (or basement) level that will be relatively close to existing grade. Since fill will need to be imported into the house footprint to bring the grade up to the underside of the slab, floors that will be high off the ground are not logical candidates for a slab on grade floor. On the other hand, if your site has a high water table, is subject to flooding, or has surface water nearby, a slab on grade floor might be your best bet. Since a concrete slab is always poured over granular fill that can’t wick water upward, they typically work well on sites where a crawl space might be susceptible to flooding, provided the floor level is high enough to be above any potential surface runoff.
Concrete Slab on Grade with Hydronic Radiant Heating
Concrete slabs are also the most efficient for hydronic radiant heating systems, as the thermal mass of the slab helps hold heat. Concrete slab floors do require that all subgrade utility rough ins be completed before the slab is poured and be accurately located. Retrofits for plumbing or mechanical changes later can be difficult and costly.
Wood framed floors with a crawlspace (or basement) below have the advantage of access to space that can be utilized for running utilities (plumbing, electrical, and ducts) and for storage. They also work well on sloping sites or where the floor level will be significantly above the existing grade. Wood framed floors require more labor and material to build, and have potential for squeaks, creaks, and vibration if they are not properly designed and constructed. A gypcrete overlay can add rigidity and a solid feel to a framed floor, but is only practical if it is used as part of a radiant heat system.
Wood Framed Floor
On wet sites or where flooding potential exists, crawl spaces and basements need to be carefully designed to reduce the potential for moisture problems. They also need to be properly ventilated or heated as if they were living space. The building code has special requirements for crawl spaces or basements that are below the 100 year flood elevation, and in some areas, homeowners insurance rates are significantly higher if a wood framed floor is used where the floor elevation is close to the flood plain elevation.
As a rule of thumb, the installed cost of a slab on grade vs. a wood framed floor with gypcrete are about the same. Installed costs are, however, subject to a lot of project specific variables that can make one system significantly more expensive for a particular application. Factors like existing soil conditions, fill requirements, hauling distances, and ease of executing the concrete pour can affect the costs of slab on grade. Similarly, local labor costs, required floor framing member sizes, and current lumber pricing will determine the cost of a framed floor system.
We are often asked if finish floor options will be limited if one floor system is chosen over the other. The short answer is “yes”, but not significantly. Some wood flooring options don’t perform as well on concrete slabs, and likewise for some hard flooring options on wood framed floors. In general, however, most flooring choices will work on either type of sub-floor and shouldn’t be a major determinant in which system you choose.
At Hendricks Architecture, we specialize in designing western mountain style lodge homes. We design homes with both concrete slabs and wood framed floors.
Tom Russell, Architect, LEED AP
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