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Tips for Insulating Old Houses

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Construction Practices

Tips for Insulating Old Houses

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

By By Michael Anschel June 28, 2016
insulating old houses
insulating old houses
This article first appeared in the June 2016 issue of Pro Remodeler.

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.) 

insulating old houses

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).

insulating old houses

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.

written by

Michael Anschel

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

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