New Code Old Home

1. The Moisture Management Mismatch

This is the most critical and common issue. Modern homes are built with a continuous water-resistive barrier (WRB) and a vapor barrier to prevent both liquid water and water vapor from getting into the wall cavity. The goal is to keep the inside of the wall assembly as dry as possible.

Older homes, however, often lack these modern barriers. Their wall assemblies—often composed of solid masonry or a wood frame with plaster—were designed to get wet and then dry out. They managed moisture through vapor permeability; they were able to absorb and release moisture.

The Problem: When you try to make an old, breathable wall "tight" by adding a plastic vapor barrier or a layer of exterior foam insulation without a complete understanding of the wall assembly, you create a potential disaster. The new, impermeable layer can trap moisture that inevitably gets into the wall from the exterior (through wind-driven rain, for example). This trapped moisture can no longer dry to the interior, leading to:

• Wood Rot: The timber framing of the house can be damaged, compromising the structural integrity of the home.

• Masonry Damage: When moisture gets trapped behind insulation on a masonry wall, the wall gets colder. In cold climates, the freezing of this trapped moisture can lead to freeze-thaw damage, causing the masonry to crack and crumble.

• Mold and Mildew: Trapped moisture creates a perfect environment for mold to grow, which can spread behind the walls and impact indoor air quality, potentially causing health issues for occupants.

2. The Air Sealing vs. Ventilation Dilemma

Modern building codes push for extremely tight building envelopes to prevent air infiltration and reduce energy consumption. While this is great for new construction, it's a complicated process for an older home.

The Problem: A drafty old home has a natural ventilation system. While it's energy-inefficient, the constant air exchange helps to remove indoor pollutants and excess moisture from activities like cooking and showering. If you air-seal an old home without simultaneously installing a mechanical ventilation system (like an HRV or ERV), you create a tight, sealed box that traps moisture and pollutants indoors. This can lead to:

• High Humidity and Condensation: The home's interior can become overly humid, leading to condensation on cold surfaces like windows and walls.

• Poor Indoor Air Quality: Indoor air pollutants like volatile organic compounds (VOCs) from furniture or cleaning products, along with moisture and carbon dioxide from occupants, can build up to unhealthy levels.

• "Sick Building Syndrome": This condition is characterized by health complaints that are linked to the time spent in a building, often due to poor air quality.

3. Structural and Material Incompatibilities

Modern materials are designed to be used in modern systems. Putting them into an older home can create unintended consequences.

The Problem: The dense, slow-growth timber used in old homes is structurally different from modern lumber. Similarly, a modern mortar mix used to repoint a historic brick wall can be too hard and rigid, causing the softer, more porous brick to crack and fail over time. When retrofitting a building with new technologies like HVAC or plumbing, the sheer size of the new equipment may not fit into the limited spaces of an older home without compromising historic features.

Conclusion

Retrofitting an older home with modern building codes is not a simple matter of "sealing and insulating." It requires a holistic, case-by-case approach. A building scientist would analyze the existing home as a complete, integrated system to understand how its materials and design manage heat and moisture. By understanding these interactions, they can propose targeted retrofits that improve energy efficiency and air quality without causing damage. The key is to work with the building's original design, not against it, to create a healthier, more durable, and more comfortable home for the long term.

An uninsulated plaster-and-lath wall is naturally "breathable," meaning moisture vapor can pass through the assembly and dry to both the interior and exterior. When you add a modern, impermeable insulation system, you change how that wall manages moisture, which can lead to condensation and mold growth.

Here's an example of how this happens:

The Original Wall System

Before the retrofit, the wall consists of:

• Exterior: Siding, followed by a layer of unsealed wood sheathing.

• Cavity: An empty space between the wall studs.

• Interior: Lath (thin strips of wood) with a layer of plaster and paint on top.

This old wall system is very leaky and allows a lot of air to pass through it. This air carries moisture, but the wall's vapor permeability allows the moisture to move and dry out. There's no major moisture buildup because the system is in a state of equilibrium.

The Retrofit Failure

A homeowner decides to improve energy efficiency by insulating the wall from the inside. They remove the plaster and lath, install fiberglass batt insulation, and finish the wall with new drywall. A crucial mistake is made: they don't install a proper vapor barrier and/or air seal.

• The Insulation's Role: The fiberglass batt insulation is installed between the studs. While it slows heat transfer, it is still very vapor permeable. The new drywall is also more airtight than the old plaster.

• The Moisture Trap: During the winter, warm, humid air from inside the home (from activities like cooking and showering) migrates through the drywall. When this warm, moist air reaches the fiberglass insulation and travels toward the colder exterior, it eventually hits the cold exterior sheathing. This sheathing, which might be a single layer of boards, is now the first cold surface the moisture encounters.

• Condensation: The temperature of the sheathing is below the dew point of the humid indoor air. This causes the water vapor to condense into liquid water on the inner surface of the sheathing. This moisture saturates the insulation and the wood framing, creating a wet environment.

The Resulting Damage

The moisture, now trapped within the wall cavity, has no way to dry out. It sits there, and over time, it leads to:

• Mold Growth: Mold spores, which are always present in the air, find a perfect environment on the wet wood framing, the back of the drywall paper, and the exterior sheathing. This can happen very quickly.

• Rot: Over a longer period, the constant saturation of the wood framing and sheathing can lead to structural rot, which weakens the building and can lead to costly repairs.

• Compromised Insulation: When insulation becomes wet, its R-value plummets, making it ineffective at providing thermal resistance.

The lesson here is that modern insulation systems must be installed with a full understanding of the building's moisture dynamics. Simply adding insulation without a proper vapor barrier and air seal can turn a previously acceptable, "breathable" wall into a system that traps moisture and leads to mold, rot, and other serious problems.