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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
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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!



We are buying a 1920s SW Michigan lake cottage and intend to live in it year round. It has been remodeled to some degree but has no furnace, T-111 exterior walls and a mix of drywall and wide pine plank interior walls. We plan to install mini splits for heating and cooling and replace windows.
What’s our best option for insulation? Foundation is a very shallow crawl space with some trap doors inside to access plumbing. It doesn’t appear to have insulation on the floors or anywhere else. We had planned to blow in cellulose but now I’m worried after reading this. We can’t afford to tear everything down to the studs and start from scratch.

Great information!

So, just today I had a company come to give me a quote on filling my walls with tripolymer injection foam. My home is brick and the space between the brick and wood wall is roughly 1.5-2”. Does that 1.5-2” space not provide enough air space to dry out the wood ? And do you feel like the tripolymer would be problematic for the plaster on the inside ?

I have I ranch home. I am not sure when it was built I am guessing the 1950's or 60's. I want to put in insulation into the walls from the outside or inside of my roman brick home for sound, My bedroom faces a very busy street. I wear ear plugs when reading or sleeping in my bedroom.. I am not sure if I should have a contractor for foam or cellulose. I am at a loss for what in the best thing to do. Please help me.

We've got one of these early 1960's homes without block wall insulation. The receptacles on our basement walls (part of the wall is above and part below grade) are drafty. Sometimes you can feel moisture on the cover plate. Before I read your article, I read about how dangerous it can be to add insulation into the enclosed cavity behind receptacles, so I opted to install simple foam gaskets under the cover plates. The drafts are gone. But after reading your article, I'm wondering if I just created a new and perhaps more dangerous/destructive problem.

Should we remove the gaskets and let the cavity "breathe"? If so, any suggestions?

Question for you. We have a house in Berkeley, CA that was built around 1905. It's wood frame with cedar shingles on the exterior and lath and plaster interior walls. In conjunction with an expansion and conversion of the attic to living space, we have to strip nearly all of the shingles and original 1x10 sheathing behind the shingles to install shear walls for seismic resilience. We are disassembling the walls from the outside because the original shingles are fairly worn -- and so we can avoid having to demolish and replace our interior wall surfaces. We intend to install new cedar shingles on the exterior. Current thinking about the wall assembly is this (from exterior to interior): cedar shingles, rainscreen (perhaps 6mm home slicker), water resistant barrier, structural sheathing, mineral wool insulation in the stud bays (R-15), existing lath and plaster walls (drywall in a few places). For the WRB and sheathing, we are considering using zip panels -- and also considering using either tyvek or a peel and stick WRB over structural plywood. (Note: existing house is two stories: we will insulate the joist bay on the first floor from the crawlspace, but are not currently planning to insulate in the joist bays of the second floor.)

Thoughts? I am not sure whether we need a vapor barrier near the interior side of these exterior walls. If we do, what options do we have, given that we do not want to disturb the existing interior wall surfaces and would also like to avoid a wholesale repainting of the house, if possible? Is the barrier necessary in all areas -- or only in walls facing rooms like bathrooms and kitchens?

Thanks very much for your suggestions.

Great article. I’ve always heard and believed this to be the case with walls, but I’m having a hard time understanding what the potential impact would be insulating floors with spray foam. I have 1905 raised home on the Texas coast where the floorboards are the only flooring. No subfloor, just long leaf pine nailed straight to the joists. What are your thoughts for this application?

I have an old vertical plank house. It is drafty and non insulated. To correct this I am basically destroying each wall one by one and rebuilding. The foundation is square log on rock for most of the house, but there has been some block laid. Each wall is a new challenge and will be faced as it comes. The ceiling joists, rafters and roof are in good shape and will be kept. In the 70's and 80's previous owners did a remodel and added aluminum siding and did very little to help the house. All walls are being replaced with 2x4 or 2x6 studs with batt insulation, Tyvek wrap and new windows, doors, and siding. The flooring will be rebuilt and replaced as I go.
This is going to be an expensive rebuild but will be worth it in the end. Since there has already been so much remodeling done to this old house, and none of it done correctly, this option was the better for me. Sure I could have done a quick flip and fix and resold, but that's like polishing a turd and selling as a diamond. Hard work pays off in the end. Don't just slap something together and say you fixed it. If it requires a destroy and rebuild, destroy it little by little and rebuild it little by little.
The main concern is not the cost, or the work involved, It is the longevity of a home. If you have any ability to build and have friends or family that have built, labor is not a problem. The most important thing is to build or rebuild it right, listen to people who have do it before, assess each opportunity (or challenge) individually and fix (not hide or scab over) each one. Money comes and goes, YOUR HOME is there to stay.

It is entirely irresponsible to frame the issue of insulated existing buildings as a "all or nothing" proposition, and there are literally millions of homes that are NOT being destroyed beyond repair as the author is suggesting that don't have perfect drainage planes. There are the 100,000s of homes that have been improved via DOE Weatherization Assistance Program, or otherwise insulated via renovation using programs like Home Performance with ENERGY STAR, as well as the millions of homes that were built with imperfect drainage planes and flashing details between the 1940s and current day with insulated cavity walls that somehow are not imploding.

The author is correct to point out that the issue can be more complicated than simply dense packing existing walls, and that the drying potential of walls is reduced when insulation is added, which makes it important to consider water intrusion into walls, but the manner in which this author is framing this "all or nothing" proposition is reckless, irresponsible and debunked by literally millions of homes that have insulated cavity walls with imperfect drainage planes that are not imploding or rotting away to abject decay.

Aghast that this author was given such a profile on this magazine to write such an extremist position piece without evidence to back up their argument. Surprise!!! The author specializes in design/build whole-building retrofits that are probably really expensive, where he suggests the only way to improve a building is through extreme renovation and cost!!! What a shocker!!!

I have worked on numerous houses that had cellulose insulation blow into cavities through the exterior done many years ago. I have seen zero moisture damage. What I have experienced is houses that are soooooo much easier to heat. They also hold paint a lot better. The houses where I see mold and moisture issues are the ones sealed up and covered in plastic. The shit growing in these houses is scary.

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