Insulating Outdoor Air Conditioning and Heat Pump Units

Outdoor units of climate systems, such as air conditioning or heat pumps, can cause two types of noise problems: contact and/or airborne noise. Especially attached to a lightweight surface, such as a wooden construction of a dormer window or another type of extension, contact sounds can cause a lot of nuisance in the form of a constant hum. On this tips page you can read about the question of a customer concerning his outdoor unit, the solution we provided and the achieved result.

The customer's question

"Dear Sir/Madam,

I hope you can provide me with some advice. Recently a heat pump was installed with an outdoor unit placed on top of my dormer window. Ever since the heat pump functions, there is significant noise pollution in the attic room with the dormer window. When the outdoor unit is fully operational, you can clearly perceive a low hum. I supsect the outdoor unit to cause the roof to vibrate resulting in contact noise. I have listened near the dormer window myself and can hardly imagine that this airborne noise coming in through the window and being so audible.”

Solution for the customer (contact sound)

The situation

In the customer's situation, the outdoor unit is located on the outside of a light construction. The customer also indicates that he perceives a hum sound, which indicates a problem caused by contact noise. Many outdoor air conditioning and heat pump units are placed on plastic beams, or on profiles, which form a (rigid) connection between the unit and the surface. In this case, the unit is equipped with profiles that function as legs (see photo).

The solution

In order to reduce this contact noise, a vibration-isolating product was used containing sufficient stability and resilience: Regufoam D300 with a thickness of 25 mm. Underneath the corners of the profiles, blocks of Regufoam vibration insulation were applied to reduce the nuisance. The Regufoam mat was cut with a sharp hobby knife into four squared blocks that have the same width size as the profiles of the unit (minimum size of the blocks must be 100 x 100 mm). From one mat of Regufoam more than eight cubes can be cut.

Fine-tuning the solution

Because there are countless different air conditioning units, roof constructions and situations, you may have to adapt this solution to your situation in order to achieve the optimal result. So in case your result is a bit disappointing, you can improve the result by applying the following tips.

• Tip 1: The subsurface is too light
  On a very lightweight surface, such as a wooden roof, improvement can be achieved by adding extra mass. Remove the gravel from the place where the unit will be placed and place several concrete tiles on this spot, in such a way that they do not wobble. Put the gravel back around the tiles and place the unit on the concrete tiles, with the blocks of Regufoam between the unit and the tiles. This usually results in an improvement of a few decibels.

• Tip 2: Still not good enough…
  If you can still hear low hums, the 25 mm cubes may not be thick enough for the lowest frequencies. Double blocks provide a longer 'travel way' and enhance the result. Glue the two Regufoam cubes together with Sikaflex. Do not make the blocks thicker than 50 mm, which causes the unit to wobble and allows for clearance on the pipes as a result!

Customer response to the result

"Dear Sir/Madam,

I have received the Regufoam, and it has clearly reduced the noise pollution. The sound is not completely gone yet, but I suspect that the remaining sound rather consists of airborne than of contact noise. So, thank you so much for the solution!”

Reducing airborne noise

As you have read, the problem with this outdoor unit consisted of both contact and airborne noise. The emission of airborne noise can be reduced by placing a partial housing around the device. A housing can consist of several walls made of heavy sheet material (preferably four) such as concrete plywood, also called waterproof plywood. The inside of these walls should be equipped with Merfopol, an LDPE plastic product that is suitable for outdoor applications. Finish the ends of the Merfopol with a profile so that the rain can minimally penetrate the plates. Because even though Merfopol withstands moisture, the drier the material remains, the higher the absorption values. Make sure that the walls cannot be blown over by wind! Provide enough space around the unit for the supply and exhaust of air, about which you can find information in the documentation of your device.

Installation tips

Regufoam is rot-resistant, hydrolysis-resistant and insensitive to fungi, but should not get too wet. Water inside the product greatly reduces the vibration-insulating properties of the material, and frost could cause damage to the material. Before the Regufoam cubes are caulked, tape them on the sides with Acustitape. This thin tape ensures that moisture cannot penetrate. Ideally, Regufoam is mounted with Sikaflex sealant and can be cut to size with a sharp hobby knife.

Industrial applications

As for the application of Regufoam underneath large units with multiple fans or industrial applications, a calculation must first be performed in order to determine which type of Regufoam with which size can be used. To this end, we need information with regards to the weight of the unit(s), the interface with the surface (the size of the legs for example), and if available, the speed at which the unit rotates. When applying vibration insulation (Regufoam), make sure that the unit is stable so that no clearance can occur on the pipework.

Questions? 
Do you have any questions about this topic? Please contact us, we will be happy to answer your questions as soon as possible.