Question

Is it feasible to construct exterior walls from CMU and fill with spray insulation? We're renovating a greenhouse into a guest house.

Asked by Samantha
Avondale Estates, GA

The existing foundation is constructed of CMU, so we thought it would be cost effective to continue stacking the CMU, as opposed to using conventional framing. Is it possible to achieve energy efficiency with this method, or would there be too many thermal breaks?

Answer

Consistency is a good thing, but however in this application it will not be cost effective or provide you with the results I think you are looking for.

This method of insulating the concrete blocks is cost prohibitive. The reason being that the spray nozzle on the spray foam applicator fans out the liquid foam to cover a large area.

  • Because of this spray pattern, voids inside of the block cavities will form which will result in no insulation.
  • Also as for feasibility, the block would have to be laid in multiple lifts to allow the cavities to be filled.
  • This will result in extended installation time and multiple trips by two contractors to perform their job. That results in cost increase. 

Your statement regarding energy performance was almost right. It would not have too many thermal breaks as you had stated but too much thermal bridging. It is this bridging which will allow heat and cold to travel between the interior and exterior.

If you would like to continue with block installation, consider using Insulated Concrete Forms (ICF).

  • These are foam blocks which are installed with reinforcement bar and then filled with concrete.
  • This will give you the insulation properties you are anticipating and allow you to apply a multitude of exterior and interior finishes.

For more information:

Read "What do you know about ICF construction? I am thinking about building a new home." a Q&A answered by William Janhonen.

Tagged In: residential foundation

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