Spray Foam Attic Gas Furnace

We re getting proposals to install a new central ac and gas furnace in the attic of our weekend home.

Spray foam attic gas furnace.

Yes we re going to do math but it s just simple arithmetic. Most often the attic is sealed by installing spray foam insulation at the roofline thus bringing the attic inside the building enclosure and turning it into conditioned space directly or indirectly. Let s say you bring the attic inside the building enclosure by putting spray foam insulation at the roofline the case with the two furnaces in my previous article on this topic. The attic is a sealed space by polyurethane foam and will need a combustion air source.

The spray foam contractor installs louvered grilles between your attic and your living space. Nothing unusual about that. Note that this isn t a good idea if you have a very airtight high performance home or if your home simply isn t large enough to provide the air needed for combustion. A while back i wrote about the incompatibility of putting an atmospheric combustion furnace in a sealed attic most often the attic is sealed by installing spray foam insulation at the roofline thus bringing the attic inside the building enclosure and turning it into conditioned space directly or indirectly.

A lot of builders and homeowners are going with spray foam insulation because of the airtightness benefits but then the questioner mentioned that the spray foam contractor had intentionally left big holes to the outside by not sealing the gable vents. The good news is that some installers understand this problem and seek to address. A while back i wrote about the incompatibility of putting an atmospheric combustion furnace in a sealed attic. A furnace in the attic is a design that works for all homes.

Sealed combustion appliances fueled with natural gas cost less to operate than electric ones. Spray polyurethane foam insulation can achieve air tightness of 0 10 to 0 20 air changes per hour. If a high efficiency unit can hit 90 with a typical installation then you can achieve a rating of at least 65 with a standard installation. Now let s put some numbers to it.

In the south we generally have these hvac systems installed in the attic because we don t have basements. Earlier this year i got a question about a home that had spray foam insulation in the attic. These allow the furnace to use air from your home for the combustion process.