Building with SIPs in Humid Environments

Structural Insulated Panels (SIPs) are one of the most energy efficient building systems you can use to build a high-performance building envelope, such as a net zero home. High performance building envelopes are air tight and incorporate continues insulation systems like SIPs and spray foam insulation. In humid environments, these well insulated structures pose some issues that designers and contractors need to consider when sizing the cooling and ventilation system.  

First, SIPS are much more energy efficient than the R-Values assigned to the EPS insulation. This means that SIPs have higher performance values, than the R-Values assigned to fiberglass insulation, which are the same values assigned to EPS insulation. Fiberglass insulation "PERFORMANCE" is well below its stated insulation ratings, while SIP performs way above those same rating values.

Additionally, a SIP building envelope is much tighter than conventional buildings. Blower door test for most standard SIP envelopes will range between ½” to 2”. A home constructed with conventional construction is commonly leaking 5” of air and more. However new energy codes in many states are setting leakage to 3 ACH50 or less, requiring designers and builders to do a better job sealing homes and reducing unhealthy air transfer.  

Having an air tight structure is key to quality indoor air, as leaky structures create unhealthy indoor air quality conditions. A tight structure allows the engineer complete control over the indoor environment as all the fresh air that is brought into the structure, is brought in by the heating and cooling system. This means that fresh outside air can be filtered, cleaned and dehumidified by the system before it is distributed throughout the building. Advanced filtration systems and UV lights, can remove mold, pollens and other contaminants, creating indoor air quality conditions that are far better than the surrounding outside air conditions.

A SIP building envelope is a highly insulated structure much like a walk-in cooler. Like any refrigerator, unplug it and close the door. and within a short period of time, condensation will appear as water droplets on the ceilings and walls. This is the moisture in air, also known as humidity.

The mechanical engineer that does not take the efficiencies of SIPs into account during the design phase, could be in for serious moisture problems when the project is complete.

In humid environments, the cooling system many times is the only ventilation and dehumidification system the structure has, so if the cooling system is too large, it will short cycle and fail to remove the humidity from the indoor environment. Humidity or moisture in structures is created by everyday living activities, such as cooking, showers and even drying clothes. It is very important that this moisture be removed and not allowed to build up over time creating unhealthy air quality and indoor conditions. If the cooling system is short cycling, it will be unable to remove the humidity from the return air, creating a buildup of moisture over time.

If the SIP is an OSB SIP with drywall finishes, the drywall finishes and the OSB facers can become saturated over time, causing mold and rot. If the SIP is a magnesia cement board SIP, moisture will build up on the surface of the magnesia board and inside framed wall cavities. Most magnesia boards absorb very little moisture and act like a mirror in high moisture environments, with the moisture building up on the cool surface of the board. As the EPS will not absorb moisture, lumber inside the SIP system and framed wall cavities become the only placed for excess moisture to accumulate inside the structure.

It is not necessary for a SIP structure to have a cooling system, but it is necessary for a SIP structure to have a proper ventilation system. The ventilation system must be properly sized to ventilate the structure to remove moisture created by living conditions and to equalize the humidity levels with exterior humidity conditions. The use of a dehumidification system along with a proper sized ventilation system, can make the conditions inside a SIP home without a cooling system quite comfortable. Ceiling fans are good for air movement but do not solve the fresh air requirements or offer the ability to exhaust moisture created by living conditions. 

For the Florida environment, our recommendation as a SIP manufacturer is that mechanical engineers specify humidistats instead of a typical thermostat. Humidity levels inside a SIP structure with a cooling system should be kept below 50%. Having the system monitor and control humidity is a good first step to having a comfortable and healthy indoor air environment. When the cooling unit starts, a good rule of thumb is that the unit should run a minimum of 20 minutes before it shuts off. If the unit only runs a few minutes, cools the home and shuts off, the unit is short cycling and not running long enough to properly dehumidify the indoor air.

Sizing the unit properly is the most important step and care must be taken to make sure that the unit is not oversized. Standard cooling system design programs will size the ac unit for a 1400 sf single family home at 3 tons. This is much to large of a system for a SIP home with 6” exterior wall panels and 8” SIP roof panels. The SIP performance values of this structure are R-38 for the walls and R-50 for the roof structure, almost twice the insulation requirements of the building code. Remember, conventional homes with R-19 fiberglass insulation for the walls and maybe R-29 for the roof are only performing at about 50% of these values for most conventional structures. This equates to performance values of R-8 or 9 for the walls and R12 -15 for the roof, which is substantially less than the SIP performance values.

The proper cooling system for a 1400 sf SIP home in Florida would be closer to a 1 to 1 ½ ton split system or a 2.5 to 3-ton, 5 head mini-split system. More on the mini-split units in another article, but for the traditional split system, the idea system would be a 1 to 1 ½ ton (depending on exposure, windows) with an in-line dehumidification system all controlled by a humidistat. The outside fresh air and the return air can be dehumidified before it reaches the cooling unit.

If the budget allows, variable speed units are also highly recommended as they save power and will increase the efficiency of the cooling and ventilation system.

With a proper sized, high efficiency unit, heating and cooling cost for a 1400 sf home in South Florida can be reduced to as little as $30 a month or less.

The use of SIPs on your next project will reduce the size of the heating and cooling unit, along with the electrical cost to operate the system by 50% or more compared to the cost of operation of a larger unit with a conventional leaky structure.

SIPs envelopes are so tight and well insulated that heating and cooling the structure should not be the #1 design concern, the number #1 design priority should be ventilation and removal of moisture caused by everyday living such as cooking, showering and washing clothes. With proper dehumidification of the indoor environment below 50%, occupants find that they are more comfortable in the home at higher temperature setting, which further reduces electrical consumption cost.