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Question DetailsAsked on 8/10/2017

How much should it cost to air seal an attic on a home with 2900 sq ft of main level living space?

I recently had an energy audit done and the auditor gave me a quote for air sealing the attic -- sealing top plates and any wire, plumbing, and lighting penetrations. This $$ number was in additional to the costs to insulate and air seal the knee wall and provide additional blown-in insulation.

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This is almost always done as part of a reinsulate/upgrade insulation job, because it requires pulling up all insulation in the attic to inspect and find the "floor" (aka underlying ceiling) and top plate penetrations. Cost can vary a lot just between contractors (commonly by a fctor of 3-4 between contractors) and also depending on type of insulation (batt is a lot easier to pull up and move around than blown-in for instance), whether your attic has a rafter/rafter girder (easier) or truss (more framing members to mess around) framing and of course on temperature at the time of the job (this is normally a good cool/reasonably cold weather job), and especially on the headroom in the attic - can get quite expensive in a 18-24" headroom flat roof, not so bad in a relatively steep slope gabled roof attic where a major portion of it is at least 3 feet high. Presence of heavy mildew or mold, rodent or bird nests or leavings, insect nests (especially stinging insects), etc can also affect the cost a lot.

You say 2900SF of main level living space - assuming you mean a one-story house with full attic - if different (say your house is actually 2-story so attic is around 1000SF) then roughly prorate the numbers below as appropriate based on actual attic space.

For that space IF all the insulation in there now is coming out and being totally replaced as part of the insulation job, so the attic floor will be completely exposed anyway, for roughly 3000SF of attic normal air sealing cost would probably be in the $250-400 range, depending on whether your attic was used for ducting and wiring runs or not. (In some houses most ducts and wirting run in basement or walls, in others much of it goes into the attic for easy runs, then back down into walls to registers and outlets and such, so there can be a LOT of penetration in that case). If the insulation was not going to come up anyway as part of the insulation job, just being added to - then more likely in the $600-1000 range is what I would expect for the air sealing including pulling the insulation back/out (usually done section-by-section on a rolling basis, taking a sectioon out and vacuuming there, sealing, putting piled up insulation back, then moving to next section) and putting the existing insulation back when the sealing is done, ready for the new insulation overlay.

If your house is in a cooler area (where attic will get below about 45 commonly in the winter) and did NOT have plastic sheet vapor barrier installed on the bottom of the attic joists/trusses before the drywall went up, then putting in fill-in VB strips between the joists or a thin sprayed-in closed cell foam layer to compensate for that would likely add about $500-1000 more. This cuts down on the moisture penetration from the house into the attic, which when the attic gets cold can condense and saturate the insulation, or in extreme cases cause heavy condensation/frosting and mold growth on the roof framing. The need for this on an existing house would be based on whether there are signs of serious moisture accumulation in the attic and how much of that the contractor feels came from penetrations into the attic which he will be sealing, and how much came from diffusion of moisture through the drywall. Adding vapor barrier like this between the joists (nothing can be done at the joists/trusses themselves unless the underlying ceiling is removed during a remodel or such) is only normally done in EXTREMELY damp sticky hot environments where humidity in the house is being increased by moisture from the attic causing ceiling mold as it cools in air conditioned cooler rooms, or in very cold climates where unacceptable moisture from the warm, humid house is causing moisture problems in the attic in the winter. A sometimes used in-between solution for this moisture diffusion situation, especially if a general remodel is being done, is spraying the ceilings with a moisture barrier paint, then overpainting with the desired finish paint.

The insulation - for additional R-value commonly in the ballpark of about $0.60-$1/SF to have an additional R-13 to R-20 put in, or more commonly $1-3/SF for totally new insulation to current recommended R value (commonly in the R30 to R60 range depending on extremes of temperature (hot or cold) in your area.

Here are a couple of Angies List and GreenBuildingAdvisor articles on the subject FYI :

Oh - BTW - you can find a lot of previous questions with answers and opinions on attic insulation issues in the Home > Insulation link under Browse Projects, at lower left.

I also recommend you pay particular attention to the comments on sprayed foam insulation (which can trap moisture where you do not want it) and foil-faced reflective barriers, which lose effectiveness rapidly due to dusting and also can cause problems if improperly placed and are also quite a source of scams, with impossible promises about insulating values and costs which even if the promises were true are commonly many times higher than comparable conventional insulation cost.

Oh - on the basic "option" of air sealing before adding insulation - generally, adding insulation WITHOUT first airsealing (and in extreme environments putting in vapor barrier if not there now) is a bad idea - because the existing insulation (if it does not show a history of condensation) may have allowed excess moisture to escape acceptably, but adding 100-200% more insulation can act as an airflow barrier and also reduce the temperature in the upper part of the insulation in the winter (bottom prt in summer with A/C running below) enough to cause that moisture to condense in the insulation, which can cause mold in the insulation and rot in the framing in the warmer seasons, and icing and loss of insulation in the winter followed by water problems when it thaws.

I have seen extreme cases (in pretty cold winter environments) where adding insulation without first air sealing eliminated the heavy frosting that the insulation was supposed to stop - but the frost built up in the thicker insulation instead, even to the point of causing ceiling collapse ! One multiplex case had several FEET of frosting on the roof underside, they bumped R-12 insulation up to R-60 cellulose insulation, resulting in several feet of ice buildup which caused total ceiling and roof framing collapse. So if adding insulation, particularly if an absorbant airflow-blocking type like blown-in cellulose (as opposed to batt or especially blown-in fiberglass, which lets moisture diffusion through pretty well and also does not absorb moisutre like cellulose does), I would consider airsealing a mandatory part of the job, not an option, and if you had to make a choice betweeen the two based on available funds I would do the air sealing at a minimum, then additional insulation to the extent funds are available.

I presume also that the insulation addition is based on reasonable economics based on your HVAC operating costs and environment - because commonly just the air sealing can contribute a significant percentage of the potential energy savings.

Also, proper attic ventilation has to be considered - because insulating an attic can result in problems, especially in humid or frost condensing winter environments, because the previously poorly insulated attic might have had enough household airflow or heat to dissipate moisture, but after a major insulation upgrade and air sealing can develop moisture issues if there is not adequate ventilation. In normal gable roofs, full open (screened) eaves with appropriate insulation baffles (to prevent the insulation from blocking the incoming airflow) and full-length ridge vents work best at moisture removal as well as removing excess solar heating buildup in the attic, both of which is a good thing not only from a building health standpoint but also for the longevity of the framing and sheathing wood.

Oh - BTW - if using blown-in cellulose insulation be sure it is treated with a borate salt, which acts as a fire preventative (required in most areas) but also inhibits mildew/mold growth and insect infestations.

Answered 3 years ago by LCD

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