Can I add rigid foamboard over attic floor?

[97tj-neil]

In my pole barn I have spread web trusses with OSB flooring on the top surface of the bottom chord. From underneath, faced R19 was added prior to steel liner panels with the vapor barrier facing the room below.

I would like to add additional insulation on the top side of the OSB by adding 2 layers of R10 2" foamboard. I would then add another layer of OSB on top where I need flooring for storage.

Are there any issues with this plan?

Is there a concern with moisture? Do I have 2 vapor barriers now with the paper at the bottom of the stackup and the foamboard on top?

 

[pcmeiners]

With 2 layers of foam board moisture should not be an issue. Getting a mortgage for the cost might be. Been checking facebook for insulation boards, some really good buys come up in PA.

 

[Raisedonadeere]

I was concerned about the 10" insulation space under my OSB attic floor. I went ahead and insulated to R38 on the rest of the area over the steel liner panels. I saw no issue with adding a foam board lining but quickly lost interest when my garage heating bill was so low. The layer of OSB over the R19 makes it much more effective than exposed insulation. I am pretty **** over insulation, not just for energy costs, but also for comfort of a draft free, uniform temperature space.

My garage insulation is working so well I would not want the foam panels/OSB sandwich in this situation even if you gave it to me.

 

[FrancisJ]

You have two vapor barriers, and the roofing science people say that's ok (first reference below). However, I'm not sold relative to your design as there's an "undrained" area between your two vapor barriers that could collect, trap moisture. Your interior vapor barrier facing the inside of the barn is done per the codes, per the science. Addressing insulation issues at the roof IS THE WAY TO GO AND SHOULD BE PRIORITY #1, but I'd prefer seeing a pathway for moisture / air to traverse between those vapor barriers.

The other thing most people miss is the addition of a "radiant" barrier -- we do a good job of insulating against "convective" heat/cold movements, to a lesser degree "conductive" heat/cold movements, but most fail to address "radiant" heat movement -- I always add the NASA-developed "radiant" reflective layer to my jobs (third reference below). Without this the Infrared (IR) Energy from the sun passes directly into your working space and there is an entire body of work (e.g. papers / academic studies) showing how that warms up unprotected spaces. The same holds true with "radiant" heat leaving the interior space -- lacking the "radiant" barrier the IR escapes (a good portion of it gets blocked by materials, but not all). Surprisingly, most of the heat coming out of a fireplace is IR as well.

I am prototyping a "Net Zero" (off the grid) 2200sf home and incorporating dual-axis tracked solar and wind power along with geothermal (tapping constant 55 degree aquifer water for heat exchange). I'm not sure if I'll get to "zero energy consumption" (Net Zero), but I'm hoping to get to 80-90% of that goal. I think it's possible in the future to get there --but one of the biggest problems I'm facing is the Bosch Geothermal Sys I'm using --it's quite energy efficient, but a contributor to me not reaching "Net Zero" since it still has issues. https://www.bosch-homecomfort.com/us/en/ocs/residential/water-to-water-heat-pump-systems-1098976-c/

(1)

https://buildingscience.com/sites/default/files/document/bsi092_doubling_down_c.pdf

(2)

Three Ways to Insulate Roofs - GreenBuildingAdvisor

The three best methods and the corresponding materials for insulating roofs and attics with input from Kohta Ueno.

www.greenbuildingadvisor.com

(3)

Radiant Barriers

Radiant barriers are effective for reducing summer heat gain in cooling climates.

www.energy.gov