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Community and Q&A

Insulating a cinder block lake house in northern NJ

snaholo | Posted in Energy Efficiency and Durability on

My wife & I bought a 750 sq ft cinder block cabin on a lake in Newton, NJ last year. It’s a great spot. The home is not well insulated & we had to shut it down for the winter this year, as the previous owner had done for years.

We had a few things done last year (new roof, new well pump, hot water heater, upgraded electrical service) but are considering making it into a year-round home, including adding on a bedroom. I am not a handyman by any stretch, & have started to do some research, but the options seems limitless.

We will be contacting local professionals for advice/ quotes, but I was wondering what are some good options for insulating & heating/ cooling a home like this. Since it is so small, I would like to be able to do something on the exterior as far as insulation/ siding. We would also replace windows, doors, etc. We would also need to replace the current old boiler (oil) & add some type of AC (either split or window units).

There is a fireplace (needs repair) & radiant heat in a couple of the rooms but it was spotty when we had the home inspected & probably needs to be repaired &/or replaced – I love the idea of radiant but not sure.

Finally there is an attached garage that is totally uninsulated which we would like to convert to usable space (since I’ll never park in there anyway). Humidity was a problem last summer throughout the house, so we’d like to improve the comfort factor.

Other than that, it’s perfect . . . but seriously, any thoughts/ ideas/ tips on how to approach this would be greatly appreciated.

Thanks in advance!!!

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  1. brendanalbano | | #1

    This article might give you a good starting point for thinking about insulating masonry structures on the exterior:

    It's definitely not the only way to do it, but hopefully it gets you moving in the right direction!

  2. Expert Member
    Dana Dorsett | | #2

    A code minimum CMU wall in zone 5 parts of northern NJ can be done with continuous R13 insulation on the exterior, or splitting the difference between exterior and interior as long as at least half the R value is on the exterior. Using 3" of rigid EPS on the exterior with an EIFS finish is just shy of that on an R value basis, 3.5" would have it nailed. Alternatively 2" of foil faced polyisocyanurate board with siding mounted on furring through screwed to the structural wall gets you there. You could also split the difference with 1.5" of EPS on the exterior and 1" of polyiso on the interior if that makes it any easier. Systems such as InSoFast can get it done pretty quickly, but it's more expensive that way. (I believe their exteror solution is R10, so it would need half-inch polyiso on the interior to bring it fully up to code.)

    As a DIY using 2.5" of reclaimed fiber faced roofing polyiso (or 2" of reclaimed roofing iso + half-inch foil-faced goods) can be quite cheap, saving thousands in material cost of the foam.

    A code-min 750' house is going to have a heat load of less than 15,000 BTU/hr @ 0F, maybe less than 12,000 BTU/hr @ 0F, which is WAY less than any oil boiler but within the range of a 1- 1.5 ton mini-split heat pump, which will cost about half as much to run as the oil boiler. (It varies with local electric rates & oil pricing which is volatile.)

    Your actual 99% outside design temperature is higher than that- the generic design temp for Sussex County NJ is +14F, but Newton could could be a few degrees cooler or warmer than that:

    If it's pretty much one big open space a Fujitsu 12RLS3 wall-coil type minisplit delivers over 16,000 BTU/hr @ +5F, 15000BTU/hr @ -5F, which is just one of several possibilities. If there are multiple doored off rooms, the mini-ducted Fujitsu 12RLFCD has a comparable capacity. Unlike the competition that series of ducted mini-split can be mounted vertically against a wall in a tiny shallow "utility closet" of 10 square feet or less, which is probably less space than the boiler, and WAY less space than a boiler + tank. The ducts all need to stay inside of conditioned space- if the attic is insulated at the attic floor the ducts will have to be run below ceiling level. If the one ton version is too big (possible, likely) or too small (unlikely) there are also 3/4 ton and 1.5 ton versions.

    It's almost never worth trying to resurrect an older radiant slab once it has developed leaks. If there is no slab insulation or even slab-edge insulation (likely, for a summer cabin) it would be an extremely inefficient way to heat the place. With an inch of EPS under a wood subfloor for comfort putting down a few low-voltage radiant floor patches where it's most meaningful (like bathroom floors) might make sense, but you're really better off going with a high efficiency heat pump for the bulk of the heating, which will also deliver high efficiency cooling.

    A fireplace is not a heating appliance. For all it's charms, it's a vacuum cleaner sucking the heat out of your house. Installing an air tight EPA rated wood burning insert in the fireplace is "worth it" if you're looking for a similar ambience, and a "Hail Mary" backup heating system for when the next Superstorm Sandy takes out the grid.

    Whatever else you may decide to do for heat, get rid of the oil-burner & tank- it's nothing but a liability with a high operating cost, and there's no "right-sized" solution for a house that size.

    1. snaholo | | #4

      Thanks for the feedback - lots to think about/ consider here but I do appreciate it . . .

  3. Expert Member
    Peter Yost | | #3

    Hi snahalo -

    Keep in mind as you make this building more airtight that whole-house ventilation and assessment of the radon (looks as though you are in Zone 1; see


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