The possibilities (and Dangers) of Interior Insulation

What R-values can you (safely) achieve (part 2)

There can be a number of reasons you would want or need to use internal insulation to improve the energy efficiency of a building:

  • Building is landmarked because it is historically significant and adding exterior insulation would change the exterior character of the building.

    Brick wall, with interior airtightness, vapor retarding layer (INTELLO PLUS) that doubles as cellulose mesh

  • Owner wants to preserve aesthetics of the building exterior.
  • Exterior insulation is technically or legally impossible (eg because the wall was built on lot lines or because zoning regulations do not allow the building to get wider, longer (In NYC we fortunately just got green zoning in NYC regulations adopted, which recently lifted some of those kinds of restrictions)
  • Interior needs to be able to be heated quickly (for example a Church that is only used on Sundays) - so the thermal mass needs to be outside the insulation layer.
  • Work can only be preformed in stages (working room to room, because of cost/scheduling reasons)

Additionally, the benefit of interior insulation is that when it is properly executed it is often less expensive than external insulation. This is possible because often there is no need for scaffolding, the homeowner can do the work themselves and also because the work can be done in several phases.

Even though increasing the interior insulation thickness to higher R-values will create higher energy savings, the downfall of adding more and more interior insulation is that it can also lead to moisture issues in the wall and could thus trigger unwanted structural damages. The most important reason for this is that when you increase the insulation levels, it reduces heat flow from the interior into the existing exterior wall. Consequently this wall will be now be colder in the winter and will be exposed to frost conditions - something that can cause harm. Additionally, there is less heat available to dry out the wall to the exterior in case there is unforeseen bulk water penetration - for example, by driving rain or window leaks - both these elements makes protecting the wall against driving rain critical.

Interior insulation of brick wall with dense packed cellulose insulation behind INTELLO and service cavity

Depending on the insulation properties of the existing wall, in general the R-values of the renovated walls can be between R-19 and R-22 (0.25 to 0.30 W / m² K) - without additional risk to the structure. Of course this is highly dependant on the location, local climate and condition of the exterior wall.

Implementation of values > R-23 (U-value <0.24 W / m² K) as could be required by the building code, can only be recommended after proper investigation and examination of the wall's physical/material properties (tests) and potential hygrothermal modelling

Essential pre-conditions before one can start thinking about inside insulation:

  • Proper functioning rain protection - for instance cornices, protruding lintels, sacrificial water-repellent (stucco, stone veneer) layers and other elements that help to keep the facade dry are large advantages. Therefore, walls without protection of gutter and overhangs, or leeward facades (wind facing) are problematic to insulate on the interior.
  • When walls are brick/stone veneered (brownstone/limestone)/plastered on the exterior, the water absorption and permeability of this exterior layer should be examined (Which side is exposed to driving rain, are there cracks in seams/joint or are there current damages from rain exposure/absorption.
  • Existing walls should be dry - prevent capillary suction of ground water into the wall with a break if required.
  • The existing wall will become cold by the added insulation in the winter. All exposed elements should therefore be resistant to frost.
  • The insulation materials and other layers in the assembly should be designed and proofed by an architect/facade consultant.

This blogpost is based on a translation of the Proclima blogposts regarding interior insulation, some minor modifications and additions have been made to the text for USA construction methods. Please note that these are just general recommendations and that a climate & building specific solution remains crucial to safeguard the structure while adding insulation and reducing a historic building's energy use.

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