UPDATED on May 22, 2015
Many green builders want to build a foam-free house — that is, a house without any rigid foam insulation or spray foam insulation. The reasons behind this desire vary: some builders dislike foam because it is manufactured from petroleum; some because of off-gassing worries; some because of foam’s relatively high embodied energy; some because of the negative environmental effects of the blowing agents used to make foam; and some because they prefer to use natural building materials like straw bales.
I believe that the use of some types of foam insulation is often defensible, and that there are valid counterarguments to many anti-foam positions. However, I’m not going to debate these issues in this article. Instead, I’m going to provide recommendations for those who want to build a foam-free house.
Avoiding foam is usually easy
To build a foam-free house, designers and builders can choose from a variety of foam-free approaches to accomplish the following tasks:
- To insulate a vented cathedral ceiling;
- To insulate an unvented cathedral ceiling;
- To insulate an attic floor;
- To insulate above-grade walls;
- To insulate a floor above a ventilated crawlspace;
- To insulate the walls of a crawl space or basement;
- To insulate a slab on grade or a basement slab; and
- To seal air leaks.
Some of these tasks — for example, insulating an attic floor — are easy. Others — for example, insulating a slab on grade — are more challenging.
Vented cathedral ceilings
As long as the rafters are deep enough, vented cathedral ceilings can be insulated with a wide variety of fluffy insulation materials, including cellulose, denim batts, mineral wool, or fiberglass.
For more information on ways to detail vented cathedral ceilings, see How to Build an Insulated Cathedral Ceiling.
Unvented cathedral ceilings
Creating a foam-free unvented cathedral ceiling is challenging. The only approach that I can think of is to install a thick layer of semi-rigid mineral wool insulation above the roof sheathing, followed by another layer of roof sheathing. The mineral wool insulation would need to be thick enough to keep the lower layer of roof sheathing above the dew point during the winter.
While it’s fairly common to install semi-rigid mineral wool insulation above roof sheathing, the method is usually restricted to low-slope (flat) roofs. Using this method for sloped roofs would be considered experimental, so builders should consult an engineer before proceeding with this technique.
This is an easy one. Foam insulation is rarely used for attic floors, so almost all of the usual materials — including blown-in fiberglass, blown-in cellulose, fiberglass batts, or mineral wool batts — will work in this location.
There are lots of ways to build foam-free above-grade walls. Options include:
- Double stud walls insulated with fluffy insulation.
- Larsen truss walls insulated with fluffy insulation.
- Klingenberg walls.
- 2×4 or 2×6 walls with a layer of semi-rigid mineral wool insulation on the exterior side of the wall sheathing.
- Straw-bale walls.
A floor above a vented crawl space
In dry climates (for example, in areas west of the Rocky Mountains), vented crawl spaces can work well. Even in more humid climates, this approach can work as long as the crawl space openings are large enough to allow wind to blow under the house. (In other words, this approach works better for a house on piers than for a house with an enclosed crawl space.)
If you want to insulate a floor assembly above a vented crawl space, specify deep I-joists or open-web floor trusses, and fill the joist bays with dense-packed cellulose, dense-packed fiberglass, or fluffy batts.
Crawl space walls or basement walls
It’s not a good idea to insulate the interior side of crawl space walls or basement walls with a fluffy insulation material like fiberglass or mineral wool. These air-permeable insulation materials allow warm, humid interior air to contact the cold concrete, and this can lead to condensation and mold.
If you want a foam-free house, the solution is to insulate the walls on the exterior with semi-rigid mineral wool insulation.
Slabs on grade and basement slabs
It’s hard to come up with an affordable foam-free way to insulate slabs on grade or basement slabs. If you are building a foam-free house, you may prefer to avoid this problem by building a house on piers or a house with a crawl space foundation.
While it’s an expensive type of insulation, one solution is to insulate under the slab with Foamglas. For more information on this insulation product, see Foamglas – My New Favorite Insulation Material and On the Jobsite with Foamglas.
Roxul, a Canadian manufacturer of mineral wool, recently announced that it will support the use of horizontal mineral wool insulation under non-load-bearing concrete slabs. Builders who decide to use mineral wool for this purpose will need to use EPS or Foamglas under load-bearing concrete footings. For more information, see Sub-Slab Mineral Wool.
If you are willing to try an experimental method, you might want to insulate under your slab with perlite. For more information on perlite, see:
- Foam-free builds
- Does a viable (performance and cost-effective), non-foam option exist for under slab insulation?
- Perlite compressive strength
- Underslab Insulation Using Perlite in Bags
One final point: in a warm climate, a slab on grade may only need vertical insulation at the slab perimeter rather than horizontal insulation under the entire slab. If you are building in a warm climate, you might consider insulating the perimeter of your slab on grade with vertical pieces of semi-rigid mineral wool insulation.
Sealing air leaks
On most job sites, builders seal a wide variety of air leaks with canned spray foam. If you want a foam-free house, however, you’ll have to come up with other ways to seal air leaks.
For thin cracks, use caulk.
Martin Holladay’s previous blog: “Redefining Passivhaus.”