Foundation Wall Best Practices

Check out photos from HERS rating projects in Western Massachusetts, with detailed descriptions.

For new residential construction projects, we almost always recommend insulating the interior of the foundation wall. Although there are benefits to insulating the exterior of the wall, and the HERS Rating won’t be impacted, it is more effective to insulate the interior. By including the basement space in the thermal envelope (insulated boundary), the home gains benefits for code compliance and functionality. These include:

  • Drier and warmer basement - insulated foundation walls, along with insulated slabs, keep the massive concrete from being a giant heat sink. Cold concrete surfaces are a condensing surface- as warm moist air hits cold surfaces, it can condense. We mitigate this by keeping the basement space warmer and the masonry surfaces warmer.

  • More useful space - insulated foundation will make the space more useful for storage and functional.

  • Heat delivery systems will be ‘inside’ the insulated volume - Energy codes (2012-2018 IECC) require very tight duct work. This system must be tested if any of the forced air delivery system is ‘outside’ of insulated space. By insulating the walls, basement ductwork and the central heating/cooling equipment, is ‘inside’.

  • Better blower door test result and lower ACH50 - With the basement walls insulated, a HERS Rater will typically be able to include the basement volume in the ACH50 calculation. ACH50 is a calculation based on the relationship between the volume of insulated space and the blower door test result (CFM50). This eases code compliance and enhances performance based rebates.


Included here are various ways to insulate a foundation wall.