The possibilities (and Dangers) of Interior Insulation (part 1)

Intelligent vapor control layer and airbarrier (INTELLO) during installation in historic building

(Note: This post is a translation by Floris Keverling Buisman from the original German, which can be found here.)

The majority of the existing building stock uses far more energy than even a code minimum new building. Especially if these buildings are landmarked or the owners do not want to alter the historic exterior of the building, the only way to reduce the heat/cooling demand of these buildings is to insulate these structures on the interior.

In this series of blogpost we will indicate the general guidelines  to safely insulate the interior of buildings, while preventing damages to the historic structure. We will first look at the damages cause by a conventional interior insulation job (investigation by Michael Wehrli, Pro Clima technical staff).  In the following blogposts we will discuss generalized technical limits of interior insulation & guidelines.

“Interior insulation is durable and secure, only when the science behind it is correct and the quality of the execution is verified.”

Large reductions in CO2-emissions of the exsisting building stock can be accomplished with correct planning and execution of the interior insulation - this will result in permanent solutions for the building which will cause great energy saving. Besides the energy saving aspects, this upgrade also increases the thermal comfort in the building.

The physical challenge is to control the moisture issues which is possible, if work is performed properly, and the existing conditions of the structure are correctly examined and evaluated based on the latest building science.

Old and new solutions: an example from Switzerland

After insulation was removed - moisture damage on wall/corner becomes visible

In this landmarked house in Switzerland, located in Marthalen (Zürcher Weinland), mold was discovered when a defective interior insulation was removed (see image 1: Renovation of interior insulation). These kinds of interior insulation jobs are quite common. For a long time the need to install a vapor barrier on the room side of the insulation was considered common practice and deemed a necessity. However air-tight construction was overlooked at that time. It was normal that the vapor barrier were butted right against the neighboring building materials and the overlaps of the barriers' sheets were not sealed. Furthermore, space the electrical boxes was simply created by cutting a hole in the vapor barrier. Warm and humid air had plenty of entry ways into the insulated cavity. (see image 2: old vapor barrier - not sealed properly by modern (passive house) standards).

Mold growth caused by improper airsealing

Through an 'opening' in the exterior of the wall, for instance at wood joists or leaky brickwork, creates underpressure in the cavity that pulls hot humid air through the holes in the vapor barrier. At the backside of the insulation, this air rises along the uneven brick wall and cools down. Once it gets close to the dewpoint on its way up, moisture levels will increase on this wall (see diagram: flow of air through faulty interior insulation installation).

Path of airflow through incorrect installation of interior insulation and air/vapor control layers

Because the installed vapor barrier prevented the inward drying of the accumulated moisture. The old wall paper became a breeding ground for mold as it got wet. When wind pressures reversed the airflows in this wall, the mold spores will be blown back into the interior.

Image 4: Mold formed at the top of the wall, about 7' above an outlet in the corner. Following the air path as discribed in the diagram above.

One of the inhabitants became ill because of the long term exposure to the spores. An investigation into the cause of this illness resulted in the discovery of the damages within the structure (see image 4: Mold formation on the coldest part of the wall - the corner approximately 7' above an outlet).

Correction of the interior insulation.

After the interior finish, the vapor barrier and the insulation were removed. The mold was professionally cleaned and the remaining wallpaper residue was cleaned off.  The crew rectified defects in existing lime plaster to increase the air-tightness of the exterior wall.

The existing studs were not replaced, even though the connections to the solid wall  showed some signs of corrosion due to the high moisture levels they had been exposed. Then the vapor barrier surrounding the components were revised. Sun stone walls was equipped with a flat line to serve as the basis for the air-tight connection.

Increased safety with intelligent vapor retarders

A moisture-variable vapor retarder increases the safety of the structure, especially when combined with  cellulose insulation. In this case 4.75" was blown in the behind INTELLO PLUS. The insulation material has the advantage that it is hygroscopic and thus actively engages any moisture that might be present. Reducing local humidity spikes by redistributing it - this is a aspect that is highly beneficial property when insulating on the interior. The vapor variable membrane (pro clima INTELLO PLUS), is very suitable for structures that have high physical demands. Its moisture dependent properties allow structures to remain durably dry. It allows walls to dry inward fast in summer  (perm 13) and protects the insulation in winter against humidty with its low perm value in Winter.

INTELLO PLUS membrane during installation - small portion of service cavity battens visible on the left

The service cavity and sealing penetrations

After installing the vapor retarder and assuring all connections between the membrane- joints and plastered interior walls are airtight (see image: INTELLO Plus membrane during installation). Horizontal battens are added to create a service cavity - this protects the membrane from the homeowner (hanging pictures) and also allow space to run wires, pipes and cables.

Where penetrations in the vapor barrier / air sealing layer were unavoidable, appropriate cable and pipe gaskets were installed to create permanently airtight connection. After all services are installed the cavity coverd by Sheetrock.

As final due diligence, several humidity sensor were placed in the insulation, on the wall and in their interior - which proved that the renovation and used methodology was a success -- the wall performed well and also the residents reported that they were much more comfortable.

NEXT: The possibilities (and Dangers) of Interior Insulation - What R-values can you (safely) achieve (part 2)

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