Foam-Free DER: Wall & Foundation Assemblies

In this post we examine the deep energy retrofit (DER) of existing wall & foundation assemblies.

As we outlined in Foam-Free DER: Series Introduction (here), we are looking at the recommended assemblies provided in the Mass Save Deep Energy Retrofit Builder Guide (the Guide), and offering foam-free alternative approaches.  Also as noted in the introduction, we are starting on the "white board", literally - and look forward to comments as we elaborate and refine these options.

We follow the order and categorization of the Guide for easy comparison:

  1. Cavity Insulation and Insulating Sheathing with Water Control at Face of Insulating Sheathing
  2. Cavity Insulation and Insulating Sheathing with Water Control Between Sheathing and Insulating Sheathing
  3. Foundation Walls

Note: All suggested approaches below are only a starting point in the process, and must be carefully considered  by the project professionals in relation to all the interrelated general, as well as project specific conditions.  Airtight high performance assemblies must maintain airtightness to function properly and require robust detailing and blower-door verification. 

1. Cavity Insulation and Insulating Sheathing with Water Control at Face of Insulating Sheathing

Page 56 of the Guide shows a wall section with insulating foam sheathing outboard of the existing wood sheathing with water control at the exterior face of the foam.   The air control layer is called out as housewrap or liquid applied membrane between the existing sheathing and the new foam boards.

The Guide warns that the foam boards are vapor impermeable and raises new potential moisture risks for the assembly.  The wall can only dry in (against the naturally dominant vapor flow outward) - so be sure not to impede the inward drying potential at inboard layers.

Instead of wrapping the house in layers of vapor impermeable foam, we suggest vapor open fibrous boards.  These fibrous boards may be mineral wool, wood fiberboard, fiberglass or even cork.    Between the fibrous boards and existing sheathing install an air tight membrane that is either vapor retarding (DA) or vapor intelligent (INTELLO X) - as this is now going to be your interior air and vapor control layer.   Outboard of the fibrous boards place a waterproof, airtight and vapor open membrane (SOLITEX Mento 1000).  Or if you are going to have an open joint rainscreen then use SOLITEX Fronta Quattro.  Over the membrane install battens and back-vented siding.   In this way you now have a super insulated enclosure that is vapor open to exterior and vapor retarding and/or variable to interior with the insulation surrounded by airtightness.

Water Control at Face of Insulating Sheathing

Note:  In these and similar assemblies where the vapor control layer has substantial insulation inboard as well as outboard it is important to put a majority (50% or greater) of the insulation outboard of the vapor control layer.   For maximum safety  a ratio of 70% insulation outboard to 30% inboard is recommended.   Using hygroscopic insulation like cellulose will also provide safety and allow ratios to be adjusted accordingly.  A WUFI calculation is recommended if appropriate ratio is uncertain.   We (475) can help with the WUFI - just let us know by email at

2. Cavity Insulation and Insulating Sheathing with Water Control Between Sheathing and Insulating Sheathing

Page 57 of the Guide shows an assembly that is virtually identical to the first wall with the exception that the face of the foam boards aren't taped, necessitating the need for a drainage plane behind the foam.   (However what's really driving the water control layer change of location is the type of window being installed and how it is placed in the wall -we'll unwrap this line of thinking in our later post on windows and doors.)

Anyway, we can also have a foam-free option:  put the waterproof, airtight and vapor retarding DA membrane on the existing sheathing and the fibrous insulation (mineral wool, wood fiberboard or cork) over that.  Then install the battens and back-vented siding.  This is not as ideal as the first foam-free option above, but it's better than foam.

Water Control Between Sheathing and Insulating Sheathing

3.  Foundation Walls

Pages 60, 61 and 62 of the Guide shows foundation sections with various forms of foam  (XPS, Polyiso, closed-cell/open-cell spray foam) insulation at the interior.    There are two particular items of note with these assemblies:

  1. As the guide notes again, the foam can inhibit or completely eliminate the assembly's ability to dry in its direction - in this case, the foundation's ability to dry inward - and can consequently potentially pose a freeze-thaw risk.
  2. The airtight connection at the sill plate/rim joist on top of the foundation, to the wall above, is universally addressed with closed-cell spray foam.  This approach blindly tries to seal three dimensional joints in a manner, one might consider akin to fishing with hand grenades.

We suggest installing mineral wool or dense-pack cellulose at interior of foundation wall.   Mineral wool in conditions where wetting is a greater concern and cellulose where wetting is not a driving concern.  Inboard of the insulation on a stud wall place an airtight, vapor intelligent membrane (INTELLO).   The intelligent membrane minimizes the wetting and maximizes the drying - while providing airtightness.   Attach the INTELLO to the floor slab below and sill plate above with adhesive caulking (CONTEGA HF).   If dense-packing place horizontal battens on the INTELLO, forming a protective service cavity.   The sill plate becomes the critical transition point to the wall assembly above - so replace and repair as required, filling holes and gaps, making it continuous - it is then a verifiable and repairable control component.

Foundation Walls

Note:  Water control is the most important control element.  Prior to installing mineral wool or cellulose at foundation walls, water intrusion must be satisfactorily addressed - including potentially:  drain mats, interior perimeter drains, sump pumps and so forth.  A robust drying capability is job one.

Next post in the series is on ground assemblies.

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