Tips for Insulating Older Homes

An insulation approach that seeks to find the balance between the real and the ideal

June 28, 2016
How to insulate an older home

All illustrations: Dan Morrison

A lot of energy efficiency fanatics would have you believe that every old house is paving the road to Armageddon, and the only salvation is to wrap and pump it full of as much insulation as you can find. They would also like to see everyone ride bikes to work and adopt a vegan diet. 

The problem with extremist views like this is the absence of balance. I’ve been a green building advocate for decades because, unlike any of the single-issue approaches, green building strives to find a balance between the ideal and the real. 

Every trade has its “perfect” way of putting things together, and if they all got their way, the project would cost 20 times as much. Instead, as remodelers we look at the key components and find the balance point. Here are my insulation essentials.

Wear a Hat

We all heard it as kids: “It’s cold outside—wear a hat.” A warm noggin is not just a good thing for you, it’s a great thing for your home. If you have limited dollars to spend, insulating the roof is generally the most effective place to focus. 

In an unfinished attic, the process is simple. Air seal any penetrations coming through the floor [1A]. This is especially important around the perimeter, where electricians and cable guys routinely drill through the top plate. This is also one of those situations where spray polyurethane foam (SPF) really shines because of its ability to air seal and provide decent R-value. A 3- to 4-inch layer of SPF across the attic floor is a good start [1B]. On top of that, depending on your climate zone, you can add as much cellulose or fiberglass as you need to achieve anywhere between R-38 and R-60, total [1C].

Knee walls. Where attics and upper half-stories get into trouble is at the knee wall. The messed-up old-school method of throwing batt insulation behind plastic sheeting is overly complicated and full of opportunities for failure. Throw that section drawing in the trash can. 

Instead, the line of insulation and air sealing should remain at the perimeter of the structure at all times [2]. Trying to insulate and air seal halfway through the middle of the space is more difficult than keeping sand from getting in your swimsuit at the beach. 

[1A] In the attic, air seal vent stacks and other penetrations coming through the floor.

[1B] A layer of SPF foam in joist bays both air seals and insulates.

[1C] Finally, add as much blown-in insulation as you need to achieve between R-38 and R-60.

[2] Don’t try to insulate a knee wall; instead, insulate the roof behind it. Use SPF to completely fill the unvented rafter bays, bringing the foam into the joist bays and even on top of the floor sheathing. Fibergalss batts will work, but only with soffit and ridge venting and a continuous air channel under the roof sheathing.

Wear a Scarf

The second easiest and most effective place to insulate is at the rim joist (a.k.a. the “band joist” in some parts of the world). This 1½-inch-thick ribbon of wood is the only thing between Old Man Winter and the joist bays, and up to 15 percent of the energy loss in a home happens right there. 

Use the insulation of your choosing—SPF, fiberglass batt, rigid XPS, or EPS foam—they all work well [3]. Use enough to get somewhere between R-15 and R-23 and you’ll be doing fine. If you’re using batts or rigid foam, it’s a good idea to first caulk the seams between the rim and floor joists. With rigid foam, you will likely end up with an additional bead of caulk on the inner side after the foam is in place.

[3] The rim joist is a big source of heat loss. Unless you use SPF, which creates an air seal, caulk the seams in the framing first, then use any kind of insulation against the rim joist.

Who’s on Third

Technically speaking, insulating the voids around your windows is the third most important area, but it opens up a whole conversation about the cost benefits of window replacement and a debate around retrofit details that connect the window to the exterior drain plane. It deserves an entire book, so I’m going to skip it and go after the low-hanging fruit: the basement. 

I’ve provided details for insulating and managing water at the basement walls in an earlier Building Science article (see “Building Things Right: Finished Basement,” PR/Dec15) But what about the slab? I think you have three decent options.

Replace the slab. Bust out the existing slab and excavate deep enough to install at least 4 inches of ¾-inch to 1½-inch washed gravel or river rock; then 2 or 3 inches of XPS rigid foam taped at the seams (if you use two layers of foam, run the second layer perpendicular to the first); and finally, a layer of cross-braided 6-mil poly sheeting [4]. Then pour a new 3-to-4-inch-thick reinforced slab. Now you have a capillary break and a thermal break. (If soil gas is an issue, consult the EPA’s “Building Radon Out” for step-by-step construction details.) 

[4] When replacing a basement slab, add 4 inches of stone, then 2 or 3 inches of XPS rigid foam, taped at the seams, and a layer of cross-braided 6-mil poly. Then pour a new 3- to 4-inch reinforced slab.

Install Gyp-Crete. If replacing the slab is impractical, your floor insulation options are slim, but you can still control moisture. Lay cross-braided 6-mil poly over the existing slab and top it with ½ inch to 2 inches of Gyp-Crete. This will provide a capillary break as well as some resistance to heat loss, although the slip-sheet isn’t enough to constitute a thermal break. If headroom isn’t an issue, you could include a 1-inch layer of XPS foam under the poly.

Install a vapor shield. If the slab is in good shape and headroom is tight, you can still seal out moisture. Over the slab, install and tape the seams of cross-braided 6-mil poly or any closed-cell vapor-impermeable underlayment, then install flooring over that (anything except for carpet). You’ll get a capillary break, a small amount of resistance to heat loss, and a vapor barrier, although you don’t get any integration with your drain tile.

What About In-Floor Heat?

In-floor heat in basements is very popular, especially in bathrooms. But unless you have a thermal break (minimum R-5) and R-10 to R-15 insulation underneath the heat distribution system, you’re going to spend fat stacks of Benjamins heating the earth below and the adjacent rooms. The solution is to completely cut out the slab where you are installing in-floor heat [5], then excavate enough to be able to isolate the new slab with a minimum R-10 insulation (2 inches of XPS foam) on all five sides (perimeter and below).

[5] In-floor heating should be isolated with rigid foam both underneath and at the perimeter. In an existing basement, cut out the area to be heated, excavate to make room for stone, a layer of poly, and insulation, and pour new concrete.

What About Walls?

I know someone is going to ask about this. If you’re working on an old house, the answer is simple: Don’t insulate above-grade walls unless you really hate the house and want to kill it. And that would be mean, so don’t do it. I’ll explain why next time.

About the Author

About the Author

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



You can simply insulate your antique house, however, below are some considerations which you must keep in mind. While insulating your attic make sure to seal off areas where you will be working with old insulation, caulk would be ideal for sealing off those spaces, and then remove old insulation and install the new cellulose insulation. Next install spray foam under your floor to stop any form of leak, or spider or bugs from getting inside the floor. Lastly, replace your old windows with custom storm windows, which would not only save costs but will also provide you with an unrestricted view. However, I would recommend you to use services of professional insulation service providers like Hire Custom, as they will suggest the best insulation materials suitable for your home, and that too well under your budget.

Hi this is mark and I've been remodeling my house and also Insulating and i notice my upstairs is very warm I dont have an attic and I did blow insulation up in the ceilings but not all of it because I cant even get to it I do have icicles bad during the winter, what can I do to make my upstairs more efficient and how to insulate the ceilings better?? Thanks mark

Our house was built in the 40s, and huge tall windows all around with that wavy glass that was common to that period. We love the look, and we don't really want to replace them. Our previous home has what I describe as plastic window, which are 1/4 inch thick panels of what I believe is plexiglass that is custom cut for each window and held in place by magnetic strips around the edges. It seems to provide good visibility, and a good improvement in R value for each window. Is this sort of window treatment still done today and is it cost and R level effective compared to double pane windows?

There are lots of options for interior storm windows, even DIY kits and "air panels" made with strips of wood and plastic window covering. There are many manufacturers, some of which also make historical wood storm windows. Historic Homeworks has a whole message board devoted to the DIY air panels and lots of other topics, especially windows. The Craftsman Blog is very good on a variety of topics, except insulation, and has a good e-book on historic home repair basics and on windows. He also stocks some supplies. Your Old House Fix is a good Facebook Group to ask a question on.

Keep those old windows. With care they last much longer than replacement windows, and with storms and reglazing can nearly match replacements in R value. Add an interior storm to up it even more.

How does insulating your above grade walls kill the house? My house feels very cold. I’ve a 1950s brink house and I suspect the brick is the only insulation I’ve got...

Thank all of you for the very useful input. We are more or less going to do all this to our circa 1870's house (Mississippi), especially like the idea of interior storms, and the "hat and scarf" approach. We were previously instructed that a house is basically a chiminea, so use the hat, storm the windows, seal the cracks and use an EPA rated cast iron stove to supplement the heat. Fun stuff.

Great help- We just had some guys over here on a great deal to insulate the walls. Our house from 1900 has no insulation. Basically a traditional farm house type. (We do wear hats etc. in winter) but it is all bearable. The question is the attic. We use it mainly for storage and the like. The floor has joists that are only 6 inches high, and we put batten insulation and covered it with plywood. We also put batten in between the roof joists, but the main issue is whether there is enough insulation in the floor to keep heat from leaking up from the bedrooms that are below. Should we take out the old batten and put in modern foam. Or should we add carpet padding to the attic floor? or both?

I have a large walk-in attic/third story that has a floor installed. There is no insulation between the 2nd and third stories. Would it be safe to add insulation here? The house was built in 1920 and has ZERO insulation anywhere.
If i were to pull up sections of the floor and insulate would that be a good investment.
Thank you.

Recently bought an older farmhouse. I sealed up every penetration possible with foam and caulk. The attic was then filled with about 24” of cellulose. We would like to do something with the walls but also don’t want to do something we’ll regret later on. One side of the house is wood exterior with vinyl siding over it. The other side has brick walls and the other is stone. The interior walls are all plaster l. There is a company touting injection foam (closed cell) to put into the walls. I’m assuming this would be also a terrible idea and a quick fix that would cost more money down the road.

Thanks for the logical and honest article. Much appreciated. It ties back to my experience owning old homes in Europe - which I was doubting here! And yes, I know the structures are different but science is science.

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