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The advice you were given isn’t wrong, you need to be careful when insulating very old basement foundations.
Modern standards of concrete are very different than a century ago, there was very little consistency in the mix, so you just never knew what you would get a century ago. My home growing up was built in the 1920s and the foundation had way too much sand in the mix I remember hearing, and so it was always crumbling. Insulating that and letting it freeze and thaw could have been a very big problem.
Is your foundation poured concrete, cinderblocks or stone? And what condition is it in? When the contractor gave you that advice, was it based on an inspection or just a general opinion? It’s really best decided on a case by case basis with the condition of your specific foundation. And another question – is it finished? If its not a finished basement, you could consider insulating the floor joists above and keeping the foundation at a lower temperature to reduce heat loss. An other solution may be to add exterior insulation if possible, that’s always best if you can, though it means a lot of excavation.
And lastly, if there are really no safe solutions on how to insulate it, you may want to consider a heat pump. A heat pump is a very efficient source of heat, read more here.
It's concrete with parging. According to the home inspection, "no signifcant deficiencies were observed". It's a 2,000 sqft 3-story house built in 1920. The contractor came into the basement but it was just a quick look - I think the advice is simply based on the age of the house. It's an unfinished basement.
Is spray foam the only (or best) option to insulate between the floor joists?
I have an 18-year old high efficiency natural gas furnace. I'd like to replace it with a heat pump for environmental reasons. Is there any data about the up front and operating costs of heat pump vs. gas furnaces? Is a GSHP an economically viable option for such an old, drafty, uninsulated house?
Thanks,
Drew
Hi Drew, We aren’t engineers and certainly can’t make a call for you but if the foundation is in that good shape you may be fine to go ahead with it. it’s for sure worth getting a more experience opinion and based on the actual condition rather than just a blanket statement based on the age of the house. The addition of an exterior skirt insulation may also help, meaning – dig down a foot and lay a 4 foot wide skirt of 2-4 inches of insulation around the house and bury it. That would keep the ground by the foundation wall much warmer. If you decide to do insulate the joists and keep the basement at a lower temperature I wouldn’t say spray foam is the best choice. It’s certainly not the cheapest. You could just use fiberglass or Rockwool. I’m not suggesting you fully insulate it and treat it as if it were exterior, more that you could keep the temperature lower down there since it’s not well insulated, but at least with some insulation in the joists your floors wouldn’t be uncomfortably cold. As for heat pumps – Ground source, as in geothermal heatin (read more here), is not a cheap set up. Figure on your heating bills being about half, but it could take you 30-40k in investment to get there, so if you’re house isn’t very big then the payback period is likely a long way off. Also check out this page, it goes into more detail - Comparing ground source and air source heat pumps.