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Community and Q&A

What is best insulation process for an addition?

May2016 | Posted in Energy Efficiency and Durability on

We have a mobile home built 2007. We are adding an addition on the north side, 14 feet out, 70 feet long. Do we use 6 inch walls or regular 3 inch walls? With what type of insulation? We live in southern Illinois where it can get below zero. Also, will we need another heating source? Furnace is all electric, Vexar by York. We are trying to keep this affordable to heat and cool efficiently.

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  1. Expert Member
    Dana Dorsett | | #1

    In southern IL / US climate zone 4 a 2x6 framed R20 wall meets IRC 2012 code-minimum. To see all the R-value requrements, see the zone 4 row here:

    That can be done effectively with unfaced or kraft face R20 fiberglass batts if you take the time to make the exterior sheathing air-tight, caulking the framing to the sheathing in every stud bay, but it's worth going with higher-density R21 fiberglass or R23 rock wool.

    If there is a full foundation it needs to be insulated with at least 1.5" of rigid foil faced polyiso on the interior side. If not, R15s between the floor joists (and along the band joists), held up with 1" fire rated polyiso on the underside, secured with 1x4s perpendicular to and through-screwed to the floor joists works.

    Until you have the construction specified clearly enough to run a heat load calc, it's premature to be discussing HVAC options, but the odds are you'd be able to heat & cool it with a 3/4 ton cold-climate mini-split (ducted or non-ducted), very efficiently. If it's a big open space a non-ducted mini-split would work, but if it's 2-3 rooms a ducted version is better, and has to be planned for, since you'll want both the air handler and the ducts to be all on the same side of the insulation as the living space. In your climate a ducted mini-split would use less than 1/3 of the power of an electric furnace, and a non-ducted might only use 1/4. It's a significant cost-adder over what an electric furnace would run, but it delivers high-efficiency air conditioning too.

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