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Retrofitting an Uninsulated Slab

Paul H | Posted in General Questions on

My question: Is it worth investing in exterior slab insulation even if I abandon in-slab radiant heat in the future?

I’ve got a single story home built in 1997 located in Northern Arizona were we get cold weather and a decent amount of snow.

– The slab is almost 50/50 on grade/above grade. Almost the entire living area of the house has some exposed exterior slab/stem wall that is between 4′ and 6′ above grade. The only areas of the house that are on grade are the master bath (abuts garage slab) , half of the master bedroom, and an interior 2nd bedroom.

– No basement anywhere. No crawlspace. All the plumbing is under the slab.

– The slab, to my knowledge (I’ve been through it, talked with the concrete contractor who likely poured it) is not insulated at all, with the exception of a 4′ perimeter horizontal piece of insulation. Basically the entire floor space of the 1600sqft home has no insulation under the slab. I’ve got basalt cinders under the slab.

– My hydronic system is 4 zone, single pump, heated with a commercial gas hot water heater from 1997.

– During the winter, when I point my FLIR at the house, it’s very apparent it’s absolutely BLEEDING heat from the slab. The exterior framed walls (2×6 with batts) are dark green, then down at the sill plates they just light up blood red.

Our plan is to be here for awhile. The house is a little smaller than we’d like, and we could potentially add a second story above the garage.

Question:

– Is it worth investing in “wrapping” the entire exterior masonry walls with 2″ rigid insulation to hold in some of the heat I’m putting into the slab if I may not stick with radiant heat?

This will be a retrofit install over finished stucco masonry, so I’ll likely use adhesive to secure the insulation, and likely use Styro’s Prefinished Panels to cover.

I’ve already got a 2″ drip rail that is fastened to the sills at the bottom of the siding where it meets the masonry, so that should save me some effort in trimming out the joint between insulation and siding.

I’ve talked with some folks who have some experience with this (although nobody yet has ever done a retrofit, all new build installs) – they suggested that I don’t even need to go more than a single sheet of insulation down the masonry wall, but that it’ll be easier to hide the insulation if I bury the bottom.

I would assume that insulating the slab would be a good thing no matter what. Right?

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