Michael Anschel is president of Otogawa-Anschel Design + Build, in Minneapolis, and a founding board member of Greenstar, Mich.

How to Kill a House

Insulating older homes is an all-or-nothing proposition. Just blowing insulation into the walls can create moisture problems that will rot the wood structure from the inside out.

August 31, 2016

Wood has only a few enemies. An encounter with termites, fire, or a weekend warrior is almost always fatal. But when it comes to a fourth enemy, water, wood is amazingly resilient. Wood can get wet over and over and over. There’s just one catch: It must be given the chance to get dry.

German post-and-beam homes used straw infill as insulation and a lime-based plaster parging as the cladding and air seal. The assembly could hold moisture without causing it to condense, and the plaster could dry very quickly. Water in the walls was rarely an issue. There were no building scientists or ventilation standards, and yet the buildings performed incredibly well and lasted for hundreds of years.

The crime scene. Homes built before the 1950s worked in much the same way. The walls were wrapped with overlapping layers of paper with little or no flashing at openings or horizontal exterior trim elements. That allowed the sheathing and wall cavities to get wet fairly often, but heating the homes would dry out the wood. Though this was inefficient in terms of energy use, it wasn’t lethal to wood walls.

The motive. Since the 1950s or thereabouts, this simple wood wall has gained another enemy: the quest for energy efficiency. Not only did we add insulation, we often did it in the worst possible way: by drilling holes from the outside, blowing in cellulose, plugging the holes, and painting over it all. As it turns out, this is the worst thing you can do to an old house.

The murder weapon. Insulation placed between the studs of a pre-World War II home is the single most dangerous element in the wall assembly. This may not be what the energy-efficiency community wants to hear, but physics is physics. Uninsulated, unsealed walls dry out because they “breathe.” But adding insulation—and doing nothing else to manage bulk water, vapor, or ventilation—destroys this cycle.

The M.O. Adding batt insulation is bad enough, but if you really want to kill a house fast, drill some holes through the cladding, the drainage plane, and the sheathing to completely destroy the wall’s first line of defense and pave a path for bulk water to enter the wall cavity. Then fill the cavity with an absorbent material like cellulose, which holds moisture, so that any leaks will go undetected and the wall will stay wet for longer. Now sit back and wait. In no time—sooner in a stucco wall, which depends entirely on the integrity of the drainage layer through which the holes for the insulation were drilled—you will find moisture levels exponentially rising in the wall cavities.

And if you really want to maximize the damage, install a set-back thermostat that drops the temperature to 65ºF at night. You’ll be sure to get some good capillary condensation action on the sheathing, framing, and insulation that will further help speed the process.

The solution. These days we don’t want any part of a home’s enclosure to breathe. The only breathing a home should do is through open windows or mechanical ventilation. The rest of the house should be sealed up tighter than a mausoleum.

To insulate older homes, you must remove the cladding and weather barrier, drill the sheathing and blow or inject loose fill or foam insulation, replace the windows, install flashing properly integrated with the water resistive barrier, and replace the cladding, ideally adding a rainscreen. On the interior, you must air seal penetrations, replace window millwork, and repaint with vapor-retarding primer.

But wait, there’s more. This is hugely expensive in itself, but you must also perform a combustion spillage test, which will fail and require replacing the furnace/boiler/water heater. Plus, your not-so-leaky house will not meet ventilation requirements, so you must add a heat-recovery ventilator/energy-recovery ventilator, or watch the house rot from the inside.

The Upshot
In other words, done right, insulating the walls of an old house is a serious undertaking that’s really an all-or-nothing proposition. Unless cost is no object, the next-best solution is to do nothing to the existing walls, and instead work from inside to air seal and insulate the lid, the rim joist, and the basement (See “Tips for Insulating Older Homes” for details). If you can air seal other stuff, great, but do not insulate the walls!



Just bought an old house. It's completely gutted to the outside walls. Why would I not insulate this and install new windows and doors? I don't understand why you wouldn't recommend this.

Dear 1850’s house.

The primary concern stems from what we refer to as bulk water, entering from the exterior. This typically happens around windows, doors, and horizontal trim details. Water finds its way through nail holes, cracks, and a general absence of flashing details. Once inside, the water sits in the insulation, or in the sheathing, and because the insulation is reducing the amount of heat that finds it’s way to the exterior, those materials stay wet. If wood stays wet, it rots. The reason those beautiful 1850-1940’s homes last so long, is because they have been baking for 100+ years. The wood is generally so dry that you can’t pull nails from it without shattering the wood.

If your case, what you really want to do is remove the exterior cladding (siding), install a Weather Resistant Barrier (something like Hydro-Gap or Tyvek Drain Wrap), install your windows with the proper flashing details so it is integrated with the WRB. This becomes the “drain plane”. In a perfect world you’d add a layer of drainage mat (something like Home Slicker) that allows for air to travel between the siding and the WRB, ensuring things stay dry. Then install your trim boards and siding material of choice.

On the inside, you can use any number of insulation types. Low Density Spray Polyurethane is my favorite for old homes like yours. It is flexible and will stay bonded to the wall studs as the home moves during the seasons. It also achieves an ‘Air Seal’ which is very important from both an energy and moisture management perspective. Finally, depending on your climate, you will want to install a vapor retarder on the interior. You can use a membrane product that goes under the drywall, or if you are using Kraft Faced Fiberglass Batts the Kraft facing is a vapor retarder. Another option is to use a vapor retarding primer over all the new drywall surfaces. This is my favorite solution, since it is so simple, inexpensive, and you have to prime anyway.

I hope this helps clarify things!

Hi, thank you for your advice! What are your thoughts on insulating exterior walls during a partial renovation to a 1912 house? i.e. former bedroom conversion to a bathroom, with the plaster walls being removed. Should we re-insulate those exposed walls? or will that cause a problem where new insulation meets the old original walls that are not being touched?

I am just about to purchase a 1920 heritage stucco ext finish house in Montreal Canada. I am concern about energy efficiency but do not want to make the wrong move. Last thing I want is to trap moisture within the walls and damage the house. I do not know yet if the exterior walls have a massonry or wood structure. I will check this as soon as I am back from the notary. The house heritage status makes an exterior intervention thought as it has to go through the governement approval. If it has ext. massonry walls, is it healty to insolate from the inside (was thinking of removing interior cladding, addiging a 2x3 framing inside, soy polyurethane foam spray, vapor barior spacer & gypse). If insulating the walls is not a good move for the building's healt, what about the roof?
thanks for your feed back,

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