I was about to launch into an article on insulating empty wall cavities in an older house when I realized that the topic is best broken down into two sections: a survey of the products you can use to insulate your wall cavities, and a discussion of installation techniques and methods. I’m just glad I realized that in the first paragraph as opposed to the fifth page. So this article will focus on insulation materials.
Important Note: In this article, I am only talking about insulating existing wall cavities in an older house. This article is not about wall insulation in new construction. I would’ve had to take a “No Country for Old Men”-style air gun to the head to tackle that large a topic in one post.
Also, we’re only talking about closed wall cavities. We’ll deal with roof trusses and gut rehab jobs another time.
Short of gutting the wall cavities, which would allow full access (and all possible options), there are five insulation options: loose cellulose, dense-packed cellulose, loose blown-in fiberglass, dense-packed fiberglass, and injection foam. As with any project, each approach has its strengths and weaknesses; your choice will depend on your specific project details and your skill levels.
Loose-fill cellulose is that stuff everyone is familiar with as insulation settled into the bottom of the wall cavity. Maybe it was blown into the wall cavities 20 or 30 years ago by some enterprising owner and now it does a passable job at insulating (the old “better than nothing”).
Check out this article for information on cellulose. When I describe it as “loose-fill,” I mean that it was blown in under low pressure, so that it has not achieved the uniform 3.5 lbs/cubic foot density required for dense-packed cellulose. Loose-fill cellulose is most commonly installed by DIY homeowners who’ve rented a blower machine from the local Big Box store.
The insulation is blown into the empty wall cavity (we’re assuming an uninsulated wall here). You would drill a hole in the top and bottom of each wall cavity, filling it as best you can without seeing inside. (I’ll go into a bit more on the logistic challenges in the next article. Loose-fill cellulose, dense-packed fiberglass, and loose-fill fiberglass are all similar in installation.)
The best selling point of loose-fill cellulose (and fiberglass) is that almost anyone can do it after the intense (nearly 30-minute) training regimen that the Big Box stores provide. Fill the wall cavity as best you can, not worrying if the insulation is filling in voids or dense or even enough. Don’t worry… you can’t see in the wall cavity and your Big Box insulation rental is nowhere near powerful enough to dense pack the insulation anyway.
The real benefit of loose-fill is that it’s an inexpensive DIY approach. The R-value and air leakage reduction potential won’t be up to snuff because the cellulose density will be spotty and uneven. It’s simply the least expensive way to put some R’s in that wall without tearing out the interior walls.
Lastly, you wouldn’t want to install loose-fill cellulose (or dense-packed cellulose or dense-packed fiberglass) in a brick or concrete-faced wall unless absolutely certain about there being a vapor barrier or capillary break between them. Brick and concrete are moisture-permeable, and placing insulation like cellulose (remember, it’s shredded paper) next to it can be a huge problem.
Dense-packed cellulose is just like loose-fill cellulose except for how we install it. Rather than trudging to the store, loading up the insulation and towing the insulation blower, suiting up in a Tyvek body suit and HEPA mask, and then swearing when you drill through a electrical wire, you instead get out your cell phone and dial it. (When it comes to dealing with the contractor price, it may help to imagine that the insulation work is free but the phone call was $6,000.)
Dense-packed cellulose is cellulose blown into a wall cavity until it reaches 3.5 lbs. per cubic foot — a.k.a. “dense.” This approach has some great advantages and one hurdle: it should only be done by experienced contractors.
The insulation machines for dense-packed cellulose make the Big Box rental look positively Yugo-esque. If a homeowner is lent a commercial insulation blower by a contractor buddy, it’s very easy for him to blow out drywall and plaster walls by overfilling the wall cavity. (Sure, professionals still might do this, too — but they’d be obligated to fix it.)
When installed against porous and potentially wet surfaces like brick, dense-packed cellulose has the same moisture concerns as loose cellulose. The R-value is comparable, while the reduction of air flow is vastly superior (since the dense-packed cellulose retards air flow through the wall cavity). While dense-packed cellulose doesn’t meet the technical requirements of an air barrier, it greatly tightens the building envelope compared to loose-fill cellulose or fiberglass.
Loose-fill fiberglass has become quite popular for DIY insulation projects, as the Big Box stores (rhymes with “Dome Hepot”) are more likely to stock it than cellulose. The installation process remains the same, and if homeowners fill a stud wall cavity, they’ll achieve similar R-value, uneven insulation density, and lack of air leak controls across the building enclosure.
Loose fiberglass can be blown in quite tightly, and when blown to a high density (at least 2 lbs. per cubic foot), will result in a reduction of air leakage through the wall (like dense-packed cellulose).
Like other fibrous insulation materials, loose fiberglass, when installed against porous brick or concrete, can be problematic. That leads us to:
Dense-packed fiberglass is gets a bit tricky. Standard loose-fill fiberglass is lighter and less dense than cellulose, which is why even when packed tightly it is less able to retard air flow. However, there are fiberglass products engineered for dense-pack applications. These are able to closely mimic the characteristics and benefits of cellulose.
The challenge is that fiberglass designed for dense packing has a lighter fiber and doesn’t settle as readily in loose attic applications. So you need two different products for the different jobs where a single cellulose product can be used for both.
Sometimes called tripolymer foam, injection foam is — wait for it — injected into the wall cavity, where it expands around wires and outlets. When installed correctly, injected foam provides a relatively high R-value per inch. It can be safely installed with brick and stucco facings, and it reduces air leakage. (When installed correctly …)
The challenge, as with any insulation application, is correct installation. During Efficiency Maine’s rebate program in 2010 and 2011, I tested out about 15 houses where the homeowners opted for injection foam. The results were wildly varied, from near perfectly uniform insulation to one case where the shrinkage was so severe that a different contractor fixed the issue by pumping in nearly 100 bags of cellulose. It’s safe to say this is a still-maturing technology, and that one should work with contractors experienced with injection foam application.
A couple of notes on applications with brick, brick facings and stucco: First, we’re talking about insulating the framing walls, not the drainage plane. We don’t want to prevent the exterior from draining and drying.
Second, check out this article by Martin Holladay detailing all the many, many challenges with adding insulation to brick buildings.
There are many different material approaches to insulating wall cavities. Choose one which matches your budget, conditions, and skill level.