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The Pitfalls of Old Home Insulation

The Pitfalls of Old Home Insulation

Old 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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45 thoughts on “The Pitfalls of Old Home Insulation

  1. i am renovating a old house with wood siding- no vapor barrior was placed before the siding. is there any thing i need to place between the studs on the inside of the house as moisture vapor prior to installing insulation and drywall

  2. HI, Scott,
    I got a question. My house was built in 1904, wood frame. It was added onto in the 1920s with a brick only (no wooden walls), one story addition. Soon after purchase we decided that THAT room was going to be our living room. So I built walls, insulated and drywalled the 20ft X 12ft room and dropped the 10 foot ceiling to 8ft. I used “kraft faced rolls throughout. Yes, I installed a roof vent above the one story add on. Within 3 years the ceiling drywall discolored significantly. Then it fell in. (Nope, not a roof leak.) The insulation DID show mold — on the side toward the drywall. So — where’d I go wrong?

  3. I have a solid masonry 1940 minimal traditional. It’s a mix of random course stone on the front and sides, with brick on the rear. When I was ripping up old carpet there was a hole in a corner and you could see straight to the stone exterior wall.

  4. Hi Scott, Awesome article and glad I found it. I just resided my circa 1890s 3 story balloon framed home that sits on top of an English basement in Chicago. I went to the extra expense of having all of the old siding removed (3 layers in some spots) down to the original plank sheathing. I had new half inch radiant foam board installed on top of the sheathing then wrapped with tyvek and then finished with .44 vinyl siding on the sides of the house and color matched hardie on the front. I will look into insulating the attic later after the pandemic is over – attic has some vermiculite insulation which will need to be professionally removed first. As far as insulting the walls am I safe to install spray foam? Is that the best option if I don’t want to rip out my plaster walls? I’m okay with patching holes every 16 inches or so and assume I’d only need to do it on the top floor since the house is balloon framed, each chase is unobstructed from sill plate to the roof line. I would rather not use any sort of loose fill/blown insulation as there are some walls I’d like to replace eventually. If foam will work, is there an eco friendly product you could recommend?

  5. I grew up in an older home and live in a house built in the 1950’s. I agree about being careful, I’ve seen plenty of problems crop up after insulating. I have a question about adding exterior insulation on a cinderblock home. My house is cinderblock with 3/4″ furring and drywall. I am currently insulating the attic and considering adding ZipR sheathing and vinyl siding. I think that’s the best solution for both insulation and aesthetics.

  6. We are remodeling an upstairs bedroom in a circa 1900 Michigan farmhouse. We have the walls opened up and were planning to insulate, but this article is making me second-guess that. I have to ask, what about rockwool insulation? I’ve read that it is water repellent, so it seems like it would be a viable alternative since it doesn’t absorb water like fiberglass. Any thoughts you can provide are appreciated!

    1. Michele,

      What did you end up doing? We have a 1915 farmhouse where insulation was blown in a few decades ago. We will probably open some walls for a variety of reasons, and will probably remove the cellulose. I’m concerned that once the walls are open, the county will make us put insulation in because of energy codes (we are in Eastern Washington)

      Any advice Scott on our situation??

    1. Thank you for the article!
      Hi, I live in Southern California and own a 1925 Spanish style house with stucco walls
      With a a crawl space and a descent attic space. Since the climate is dryer than other parts of the US. What are your thoughts on drill/fill insulation in the walls for this type of house.
      Thank you

  7. Super helpful article! Thank you! We are about to buy our first home and there are a lot of old homes in our area. We would love to move into an old home but living in the Pacific northwest we have mold concerns.

  8. Hi, just read your article: “The Pitfalls of Old Home Insulation” from FEBRUARY 4, 2019. I wonder if you can help me with some advice. My wood frame home in the Bahamas was flooded to the ceiling during hurricane Dorian. It has now been completely gutted to the studs on the interior. The exterior is T1-11 plywood. No vapor barrier or anything else between it and the framing. Should I insulate these walls from the inside? I value your opinion.

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