Question

I need to insulate the roof of my 1904 two-family Boston, MA home and I'm looking at air-krete. Please advise.

Asked by Michelle Murray
Boston, MA

The walls have been insulated with blown-in cellulose. The attic has t3 rooms and was lived in 50 years ago. I plan to update the electrical and heat and make it liveable again. Air-krete advertises itself as the most eco friendly option.

Answer

Elizabeth DiSalvo

Answered by Elizabeth DiSalvo

Norwalk, CT

Trillium Architects

November 15, 2011

Hi Michelle,

Airkrete is definitely an eco-friendly option and very good for anyone with respiratory issues or chemical sensitivities.

  • It is very clean and has a low embodied energy (impacts the earth lightly in production, installation and recycle).
  • As a residential green architect, I have used it in a few houses and have always had good results.
  • The first house we used it in was in 1999 and those clients are still happy!

Before you decide however, let's look at your situation more closely and then consider options.

Venting your roof

Basically you are insulating a cathedral ceiling (the ceiling is the roof). This brings up the problem of roof venting.

With most insulations, roofs need to be vented. Venting keeps the roof shingles from deteriorating or popping off due to heat build up and also diminishes the chance of moisture build up (mold, rot) within the roof assembly itself. Unless you are using a foam, venting is required by code.

How will you create a vent if you insulate? Placing something like a 'Raft-R-Vent' (a preformed vent piece that sits in each joist bay and runs from eave to ridge) is easy to do if you remove all interior drywall. If you want to retain interior drywall (or plaster, or boards- whatever the current ceiling is) then venting becomes very difficult if not impossible.

How much insulation can you fit into your roof

You have an old house. It is likely that your roof rafters are 2x6's or 2x8's. This means there is only about 6 or 8 inches between roof sheathing and interior ceiling to insulate.

The R-value of your roof is directly related to how much insulation you can fit into it.

You may consider thickening the roof (adding more room for insulation) by removing any sort of ceiling (drywall, or plaster, or boards) and placing 1 or 1.5" furring strips horizontally across the rafters to thicken the roof to the inside. This may not be an option because you may not have a lot of head room inside the attic to begin with. Of course if you are by chance getting a new roof anyway you might consider thickening to the outside.

Insulation options

Many insulations (blown in cellulose, cotton batt, blown in fiberglass) have an R-value of 3.6-3.8 per inch.

  • Airkrete is about R 3.9 per inch.
  • Open cell foam can be R4 or R4.5.
  • Closed cell foam can get as high as R6 or R7 per inch.

If you have about 6" or 8" to work with and Airkrete is R3.9 per inch then you get a total of R23.4 (at 6") or R31.2 (at 8") for your roof

If you used closed cell foam at R6.5 per inch you would get a total of R39 or R52. 

  • Or most likely you would do a combo: flash the roof sheathing with 2-3" of closed cell foam (R20) and then fill the rest of the cavity with blown in cellulose or open cell foam or any other loose fill (R11 - R18.5) for totals of R31 (at 6") or R 38.5 (at 8")
  • Of course if you can build in more thickness to the roof then you can up your insulation levels.

I believe that Massachusetts Code calls for R38 roofs. You probably don't have to comply because you are a retrofit but it shows you a target to shoot for. And usually codes does not strive for as high a goal as 'green building' does. In your climate I would look for something closer to an R50 roof...but that is not always feasible.

Closed cell foam is the 'worst' for the earth and some would say your health, but offers the best R value. The foams and cellulose offer some degree of air barrier. They are basically all made fireproof or are naturally fireproof - like Airkrete. (However burned foam smells forever after- which is very difficult to get rid of if there is a small fire in the home.) Each insulation has its pro's and con's. Expense is also an issue.

Airkrete is a solid choice 

The only downsides to it being that it can be a bit more expensive and hard to source than the others and it does not really have any air-barrier attributes. Air infiltration is a big factor in how well a wall or roof assembly works towards energy efficiency.

Depending on the choices you can make regarding venting and depth of roof assembly you may easily choose to use Airkrete.

  • Airkrete does go in 'like foam' but its properties are different and my Airkrete guy (who has been doing this for 15 years) always vents the roof with something like Raft-R-Vent.
  • Whereas with a foam insulation you do not have to vent the roof (code).

But closed cell has a better R-value

People in your situation often end up using closed cell foam because they can't vent and/or they can't get a good roof thickness. 

I would say that if you can easily solve the venting issue and if you can thicken your roof to at least 10" then the Airkrete is a fine choice (as are many of the other blown in, dense pack or loose fill options out there).

If you have trouble getting either vents or thickness then I would maybe look into closed cell foam. However, a lot of my clients (and I) are veering away from closed cell foams lately because we wonder about the healthiness of them and they do have the worst carbon footprint, however you can't beat their performance.

It is a tough call.

Good luck with your project!

Elizabeth 

 

For more information:

Read "Should I leave an air space between my attic insulation and the wood roofing?" a Q&A answered by Mike Binder.

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