The Pitfalls of Old Home Insulation

By Scott Sidler • February 4, 2019

the pitfalls of old home insulationOld home insulation is a controversial topic among old house owners and restorers. What are the right materials to add and where can I use them? Will it cause unforeseen problems down the line? There are lots of questions and it seems fewer answers than needed, so, with this post, I hope to provide some much needed answers.

Insulation keeps you warm in the winter and cool in the summer. It makes our homes more comfortable than they would be without it, and whether you’re using something like mineral wool, blow-in, or good old fashioned fiberglass batts, adding insulation will surely improve the energy performance of any house.

So, you should add it to your old house as soon as possible, right? Not so fast. Used to be I was someone who promoted the retrofitting of just about any old house with new insulation, but looking back, that was naive of me. There are certain times (and places) where adding insulation will be immensely valuable and other times when it can prove catastrophic. So, you certainly need to know what you are doing before you take on the task of insulating an old house.

The Problems With Old Home Insulation

I’m not really talking about the problems with existing old home insulation, but rather the topic of how and where should you add insulation to an old home. Homes built before the mid-20th century were not built with the same techniques that we use today.


These older homes were largely built without insulation and just open cavities in the walls where the house could breathe. Builder’s knew that water was the #1 enemy of any house and the way they built may have allowed water to get into the walls in minute amounts, but due to the extraordinary looseness in the building envelope, the house could always dry out safely and quickly.

Then a few decades later, we come by and stuff those cavities full of fiberglass which acts like a sponge and we wonder why our old house is having so many problems. Adding insulation where it was never designed to go (mainly the walls of an old house) causes a host of problems all due to the moisture issues it creates. Here are just a few of the highlights.

Wood Rot

Wood can wet without issue, but keep it wet and then the problems arise. Anywhere that water gets trapped causes wood moisture levels to rise and once they get above 20% to 30% those are ideal conditions for wood rot. Installing any kind of insulation in an area that gets wet with any regularity will almost inevitably lead to wood rot even if you follow my tips to prevent it. You have to remove the continuous source of the water.

Peeling Paint

When paint is peeling down to bare wood, you may think it was a shoddy paint job, but the cause is usually moisture related. Once again, the moisture gets into the wall assembly and gets absorbed by the insulation where it sits like a dirty sponge soaking into the wood. When excess moisture builds up in the wood, it will try to escape through the wood surface and push the paint right off. Wondering where you have moisture issues? Look for the peeling paint and you’ll be in the right spot.


Black, green, brown, it really doesn’t matter what color it is, nobody wants it in their house for the health hazards it poses. What does mold need to thrive? Warmth, oxygen, wood, and (you guessed it!) moisture. Ever hear about the mold outbreaks in the desert? I didn’t think so. Dry houses are happy houses. Wet houses…not so happy. Want help getting rid of mold? Check out this post.

How to Insulate an Old Home?

Let’s start with the basics. I’ll show you where to insulate and where to leave it alone. I understand that you may disagree with me about the risk vs. payback when I tell you to NOT insulate an area, but trust me, I have your best interests at heart. I have seen insulation go sideways too often to not share what I have learn from my years in the industry.

Start in the Attic

The attic should always be the first place you add insulation whether you live in Florida or Fargo. It doesn’t matter the climate, just start with the safest place that provides the biggest bang for your buck and that is always the attic. I’ll say that one more time. ALWAYS the attic! I hope I wasn’t unclear.

How do you insulate the attic safely to avoid problems? Well, the attic floor is a great place to start because even if there is a roof leak there is air flow between the roof and the attic floor that allows things to dry out and avoid the problems we talked about earlier. Blown-in insulation is a great option here so check out this how to post about installing it yourself.

Stay away from the underside of the roof if you have an older roof or especially a wood shingle roof which is designed to get wet and needs to breathe to dry properly. If you have recently had your roof replaced and have adequate waterproofing, underlayment, and flashing applied, then I usually agree that insulating the underside of the roof is then an option as long as you keep a diligent eye on your roof to make sure it stays in good repair.

Leave the Walls Alone

Don’t insulate your walls. What?? But it’s cold? I know and it pains me to say this, but the number one danger area that causes problems with old house insulation is your walls. Wood siding is usually not replaced every couple decades like shingle roofs. It likely has the same 100 year old kraft paper behind it that provides virtually no water proofing support.

There are a couple creative yet expensive ways I would consider retroactively insulating the walls of an old house if you are in a far northern climate where it might make financial sense. For anyone living below the Mason Dixon line it is never worth the expense to retrofit an old house with wall insulation.

One situation where it is okay to insulate the walls is if you remove all the siding and apply a new housewrap and then go over that with a rain screen before reinstalling your old siding. This may seem like a massive undertaking and it is. Full energy retrofits like this are expensive, but it really is the only safe way.

If you end up adding insulation without doing a full retrofit then you run the risk of developing Sick Building Syndrome (SBS) which saw a major spike in the 1970s after the energy crisis motivated people to stuff their walls with insulation indiscriminately.

What About the Crawlspace?

Insulating under floors is a great way to keep warm with very little potential issues. Sure it’s dirty and difficult to do, but you wont run into issues like with the walls. My preferred material for crawlspace insulation is Mineral Wool since it’s not rodent friendly.

Installing Mineral Wool between the floor joists takes a lot of work due to all the plumbing and electrical penetrations but it is very effective. Using a material like fiberglass batts works nearly as well, but fiberglass is a favorite nesting place for critters in the winter so I prefer Mineral Wool which is far less hospitable.

The Bottom Line

Now you know. Start in the attic, leave the walls alone, and attack the crawlspace if you’re up for it. There are lots of other ways to increase your energy efficiency that don’t have to do with insulation like caulking trim and baseboards which are notoriously drafty, weatherstripping doors and windows, adding storm windows, and even using thick drapes. These are all very effective at keeping your old house warmer and that is the name of the game.

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22 thoughts on “The Pitfalls of Old Home Insulation”

  1. Any suggestions for insulating a 1920s flat roof on a Philly row house? The second floor gets really warm in the summer in spite of the light color of the roof.

  2. Such helpful information throughout your site – we recently purchased a 1908 Foursquare that has had some “updates” (vinyl windows, aluminum siding, laminate flooring over the original hardwoods, etc) and we have been working to make the place our own. One of the projects on the long-term list is finishing out the walk-up attic. It is currently insulated under the floorboards with what appears to be blown in or vermiculite insulation – we aren’t planning to disturb it. Upon moving in, we also discovered a family of bats that had taken up residence for the winter. In the short-term, we are going to have a pest manager bat-proof the attic to prevent further issues and then plan to insulate. Wondering if there are specific concerns about the breathability of the attic we should be aware of before diving in.

  3. Just stumbled across this article….we had insulation blown into the walls of our circa 1890 Ohio home & now we have mold in the bathroom & cedar room. What can we do to correct this mistake?

    1. Gotta find where the water is getting into the wall cavity and resolve it. It may be vapor drive from the inside or leaky siding on the outside. Make sure the bathroom has an exhaust fan too!

  4. Super appreciate this article, am flagging it as a major resource. The prior owners of my home did the best they could, but the roof is asphalt shingle with no underlayment, vinyl siding, aluminum sheathing on the exterior window frames, and vinyl windows. I’m looking at reroofing with davinci, residing with Hardie board, and replacing the vinyl windows (only, there are still a few original that are being cleaned up but not replaced) with fiberglass. I’m still a little squirrelly about the fiberglass but I’m not 100% that all-wood from someone like Pella is going to be overly better. What is your experience with fiberglass for windows? I’ll be adding in the mineral wool as I go, not disturbing the plaster as much as possible.

  5. Hmmm. Thank you for this information Scott. We gutted the bathroom of our 1928 bungalow and the building inspector told us to insulate the exterior walls with R-15 fiberglass batts. I’m now wondering if I should question this (we haven’t yet closed the walls up). I assumed that it was code, but if what you’re saying is true, then I think perhaps we should remove the batts.

    1. Colette, I run into this all the time. They require the walls to be insulated and while I wouldn’t recommend it I can only tell you what I have done in the past. I will insulate to pass my inspection and then remove the insulation before closing up the walls. Not exactly up to code, but I’m more concerned with the long term good of the building.

  6. We have completely gutted an old house; removing the plaster and blown in insulation. We plan to use spray foam insulation on the underside of the roof which is new metal. We also plan to spray the outside walls. Any concerns with spray foam? The house has original siding from 1920’s. We plan to paint, caulk the outside this summer.

    1. No issues from me about spraying the underside of a new metal roof, but I would caution against doing the backside of the siding. Wood Siding is definitely not designed for spray foam.

  7. Good article Scott. My son and I insulated our attic two years ago. Some of the best money spent on our home built in 1925. Used blown in insulation with a air machine. Did not cost that much and boy you could tell the difference.
    Also did weather stripping on the original windows that have storm windows. Weather stripped all the doors.
    Just went through a winter here in Iowa with -55 below wind chills. I must say we did stay comfortable in this big home .
    Our wall do not have insulation in them and after reading your article they will stay that way. Makes total sense that the walls must breathe. We have original Cedar siding on this beauty.
    Keep up the good work. Always read your articles.
    Thank you,
    Laurence Sanford

    1. Does anyone have a source/supplier for retroactively weatherstripping old windows and doors? I assume this is done by routing the edges and then pushing in the weatherstripping? How hard is this to do? I cannot material for this work anywhere! Thank you

  8. Great post and thanks for the advice! Being in the northern part of the hemisphere, it makes since for us to insulate the walls but will attack it from the exterior. Also, it’s a great time to replace the wiring as well!

  9. Thank you for addressing these issues! As I begin restoration of our little 1910 bungalow, I am continuing a long exploration into what it means to avoid the assumptions of conventional construction in pursuit of a true restoration. I have a childhood memory of the interior of an old ski lodge in Sweden in the depths of winter… with no understanding of the ”whys”… the air being quite cold while feeling comfortably warm. I explain it like sitting on a sun warmed outcropping on a cool evening. I’m still trying to recreate that sensation. Radiant heat. As soon as we started mixing up the equasion with installing HVAC systems to heat the air in our old structures, we started down a slippery path.I find far more evolved solutions in what is called ”natural architecture” than what currently is regarded as ”green”. The LEED certified green metodology is simply evolved convention, and just doesn’t coexist successfully with old structures. So not only do I need to restore the original part of my house true to it’s purpose, but to understand the principles as I design an addition onto it. These two directions aren’t just a difference in style, but quite opposite in approach from foundation up… My journey continues.

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