HorizonTV Featuring ProRemodeler: Watch Episodes On Demand At HorizonTV

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
insulating an old house

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.

How to Insulate an Old House

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 pre-World War II homes 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.

Adding Insulation to Walls

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?

Insulating from the interior when you are exposing significant portions of the exterior wall is fine. It is a good opportunity to also look and see if you've got bulk water that has been entering the structure and amend it so it doesn't continue. If you're just exposing a couple cavities, using a batt insulation (installed properly) paired with some can spray foam for penetrations works fine. If you've exposed a larger section of wall, then considering using a Spray Polyurethane Foam is worth it for the added air sealing benefit. Be sure to find out of your region requires a vapor retarder which can be either in the form of a primer or a sheet product over the foam. I strongly prefer the primer version.

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,

I would leave the walls alone until the time that you need to replace the stucco, and then replace the windows, stucco, and building paper then. In the interim, I'd strongly suggest spraying the rim joist (where the basement ceiling joists meet run over the top of the foundation), sealing penetrations through the bottom and top plates, adding gaskets to outlets and switches on exterior walls, and addressing the ceiling insulation. In the ceiling, I'd remove whatever is there, add 4-6" of spray foam (high density - closed cel foam. The soy stuff is a marketing gimmick and has a trace amount of soy in it, so use whatever foam you want), and then 12" or so of loose fill fiberglass or cellulose. This will give you maximum bang for your buck, dramatically improve the performance of the home, and keep your walls dry. One other tip is not to use a set back thermostat if you have radiators. Set the temp at 68 or 70 and leave it there during the winter.

I have purchased a 1867 second empire style house with a good bit of water dagame to the upper & lower sills of the walls. I have since stripped the plaster/lathe, removed the siding, replaced all the lower 8x10 sills, & 4x4 upper sills with reclaimed heart pine. The wall studs have been lap jointed & mortise & tenon ends reapplied at top & bottom for connection to the sills. The floors have been carefully removed & some floor joists have been replaced where needed. Again reclaimed members were used & re-notched as they originally came. 3/4" t&g plywood has been added for subfloors. The bottom of the floor joists clear the dirt foundation by 3'-0" since the house sets on brick piers. The crawlspace remains open and unobstructed between the piers so it has constant airflow. Basically I've had to rebuild from the ground up in several first floor rooms of the house. Since I've gone through all the trouble to rebuild the structure of the house, id like to make sure that when it comes time to close up the walls, add siding & insulation that what I've worked for isn't all in vain.
So if I'm understanding correctly, on older homes that need insulation added to pre existing conditions, the "motive", "murder weapon" & "M.O." mentioned in your article above would cause damage to the structure. However, in cases such as mine, where the house is under renovation and has been stripped down to the bare bones.. In that case would it would be ok to add plywood sheathing, a breathable moisture barrier, a rain-screen and the original wood cladding. Would also be ok, in this case, to add a closed cell spray foam insulation to the wall cavities.

Wow! Sounds like you've really done incredible work on this home! 4x4 heart pine sills! That is the stuff of beauty. I'm impressed with the hard work you have put into the home along with the detail in which you've described the conditions. You've been doing your research!
1. Where is the home located? Your climate changes the solution. I'm going to guess you are somewhere in the South with that foundation type, and closer to the Eastern Seaboard with that 1867 build date?
2. If the house has been stripped of everything you have a unique opportunity to add insulation to just the exterior of the home. If you are in a climate where your pipes don't freeze under your home, then you don't need much insulation at all. You could probably use something like Huber's ZIP R-12 system and achieve continuous exterior insulation that would make the home efficient and keep the wood framing happy and dry.
3. If you don't want to change the exterior dimensions, soffit connections, reveals, etc. Then you will be looking at conventional in the wall insulation. Spray Foam (SPF). Open or Closed cel, they both work just fine (one requires the addition on vapor retarder).
4. You are spot on! A good WRB on the exterior in combination with a ventilated rain screen is perfect! Benjamin OBdyke's Home Slicker+ has an integrated Typar and rainscreen that goes up simultaneously. Putting on a ventilated rain screen under wood siding regardless of where the insulation is located is an excellent system for keeping the home dry.
Anytime we insulate we add risk to the durability of the wall system. Not insulating causes other problems. In a space like yours, and with the care and detail I'm hearing you taking, I think you can safely insulate in the wall cavity with the insulation of your choice.
Hope this helps!

Thank you for all you comments & support. This has not been an easy project, and it still has a long ways to go, You were correct about being in the South. It's located in Eufaula, Al 36027. The house is 150 years old, and has been abandoned for nearly 40 years. I'm hoping with the repairs done and the attention to detail , that it will help keep it in tact for another 100 years.

Any advice on adding insulation to our kitchen which has its original plaster walls? We've added cellulose to our attic and insulated the knee walls, sealed & insulated the rim joist, sealed all accessible openings, installed storm doors, and retained the original windows plus use storms. None of our walls are insulated to our knowledge and we are the second owners. Our kitchen has always been the coldest room in the house (and I'm from WI). We are about to tile the backsplash and I am wondering about the possibility of blowing in insulation from the interior. This would involve 2 10' walls (one has the back door in it so 7' functionally) and would be from the floor to the bottom of the wall cabinet. We are in the Mid Atlantic and a historic district. Thanks for the very helpful article.

Add new comment

Overlay Init