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R-value, safety, and cost
Foam plastic is required by International Building Code and the International Residential Code to be separated from the interior of a building by an approved thermal barrier to protect against fire. Cellulose insulation is not required to be separated from interior space because it will not ignite readily when exposed to fire. Actually, cellulose insulation is officially classified by building codes as fire blocking material, which slows the spread of fire through closed cavities in building assemblies. Tests also qualify cellulose insulation as an effective fire stop around steel through penetrations in fire-rated walls.
More specifically, foam insulation installed in attics and walls must be covered by a fire ignition or thermal barrier such as: gypsum wallboard, corrosion resistant steel, wood structural panel, mineral fiber insulation, intumescent paint or cellulose insulation. However, once a house is engulfed in flame, ignition and thermal barriers are of little benefit because the foam insulation will be directly exposed to fire or will be ignited through auto ignition (i.e. temperature of ≥ 700 ºF).
The reason there are such rigid codes concerning foam insulation is that once ignited, fire spreads rapidly, creating very high temperatures and dense toxic smoke in a matter of seconds. In a NFPA 286 corner test conducted at a prominent fire testing laboratory, a supposedly Class 1 (i.e. can be installed in a residence) foam insulation material reached flashover conditions in 44 seconds. This test is supposed to run for 15 minutes before reaching flashover.
The fire hazards associated with foam insulation are real, and everyone, from the architect, building code official, building inspector, building contractor, and most of all, the home owner, should be concerned that building codes directed at foam insulation are strictly enforced.
While foam insulation has become popular because of its unique ability to expand and its appearance of being ‘high-tech,’ there are other facts that should be considered. The three most important factors of residential insulations are: R-value, safety and cost. As already noted, from objective evidence and testing, it is clear that foam insulation’s safety is problematical due to its fast burning and toxic smoke producing characteristics in home fires.
To surround your house with foam insulation, a product that is highly flammable and produces large quantities of bellowing toxic smoke when ignited, there would need to be good reason for doing this. One might think that the reason would be the energy savings gained by having a better R-value. Actually, foam insulation doesn’t have an appreciable higher R-value than cellulose insulation. Both of these insulation products carry a 3.6 to 3.7 R-value per inch. To make matters worse, foam insulation is rarely installed to completely fill the wall cavities because in so doing it requires much more (1) labor expense to shave flush with the framing the excess foam insulation to allow the gypsum wall board to be properly attached and (2) extra foam insulation material that then must be disposed of. What this means to you, the homeowner, is that you actually get less than the full R-value quoted by the contractor.
Could the reason be that installing foam insulation is less expensive than installing cellulose insulation? Actually, the facts are quite different. Installing foam insulation costs 2-3 times more than installing cellulose insulation!
So why choose foam insulation over cellulose insulation? You wouldn’t if you knew the real facts!
A quick comparison
|CHARACTERISTIC||FOAM INSULATION||CELLULOSE INSULATION||COMMENT|
|R-Value||3.5–3.7 per inch||3.6–3.8 per inch||Applied Foam generally does not completely fill wall cavities thereby shorting claimed R-value.|
|Air Tightness — Air changes per hour (ACH)||0.20||0.30||Building experts recommend that ACH not be less than 0.35 in order to maintain a healthy environment.|
|Greenness||Little||The greenest of the green||85% of cellulose insulation is recycled paper. Foam is made from petroleum distillates. Foam has no recycled content. Foam requires much more embedded energy to manufacture than cellulose.|
|Smoke Developed||≤ 450||≤ 25||Due to excessive smoke developed index foam must be covered with thermal/fire resistant barrier when installed in inhabited areas|
|Hazardous||Yes||No||Some components of foam insulation are toxic, although the finished product is not.|
|Fire Resistance||Yes, but ...||Yes||See “Smoke Developed” above.|
|Fire Block & Ignition Barrier||No||Yes||Cellulose approved as both. Can reduce cost considerably.|
|Flame Spread||≤ 25||≤ 25||Widely-used Class 1 foam insulation failed the 15 minute NFPA 286 Test in 44 seconds. See also“Smoke Developed” above.|
|Cost (R-30 Attic)||$2.75/sq. ft.||$0.60/sq. ft.||Foam costs are 2 to 3 times higher than the cost of installing cellulose without providing any offsetting benefit.|
|Cost (2X4 Wall)||$1.20/sq. ft.||$0.55/sq. ft.||Foam costs are 2 to 3 times higher than the cost of installing cellulose without providing any offsetting benefit.|
|Sound Reverberation Control||NRC = 0.70||NRC = 0.80||Cellulose reduces sound reverberation (echoing) more than foam.|
|Sound Transmission Control||STC = 39||STC = 49+||Cellulose has greater sound blocking capability than foam on similar wall assemblies.|
|Thermal Barrier Required||Yes||No||Foam must be separated from the interior of a building by an approved thermal/fire barrier.|