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Question

Are there any negative effects from insulating a basement ceiling?

Story

I have an unfinished basement, with a family member living in a make-shift room.

In my efforts to retain heat in his area I put up some insulation above him (with a few other things). He swears that it has helped drastically.

The unexpected benefit was the floor above him, is warmer (so my wife says). I'm just happy that some of the noise he makes stays in the basement.


Expectations

So my expectations for insulation the basement ceiling, maybe a warmer main floor... and some sound proofing.

Why I'm asking

I started googling around and some articles said insulating the basement ceiling could potentially cause moisture/plumbing problems. Others said that it wouldn't hurt, but don't expect much of a return on the investment. One even said don't insulate both the ceiling and the walls....

My home

Its divided into two different sides, one side is the living room and kitchen, the other part of the house has the bedrooms and bathrooms. The overall shape is an "L" The basement is 100% unfinished, there are two heating vents down there that I leave open... but other than that its just bare concrete walls. I don't know if this makes a difference but figured more information can't hurt.

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    I'm confused by your mission. You can't make the basement and the upstairs more comfortable by insulating between them. In fact, the only way that would have any effect is if you intended to not heat one or the other. – isherwood Feb 2 '17 at 17:39
  • Well the basement gets its own heat through two different vents. But because its unfinished and underground, it is much cooler than the upstairs... – BigElittles Feb 2 '17 at 17:47
  • I sort of agree that the benefit won't be huge. It may help to provide two separate comfort zones. I the person downstairs wants 18C and the person upstairs wants 22 or vice versa. If soundproofing is your actual goal, well there might be more effective solutions for that. – Octopus Feb 2 '17 at 17:49
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    It's unlikely that the basement loses much heat through the ceiling. Most is lost through the concrete walls, the stairwell, and other penetrations. What would help is insulation on the concrete walls. (I think both your wife and your tenant are the victims of "positive thinking". :) ) – isherwood Feb 2 '17 at 17:52
  • @isherwood lol positive thinking. Yea, The walls are the next big project. Building a wall is simple, permits/electric/ductwork is the PITA I've been putting off. Can I just pay someone to do all of the administrative work (design, permits) and I'll do the work and stick to the plan... then have an electrician and hvac guy come in before I insulate and close the walls up? – BigElittles Feb 2 '17 at 18:14
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The insulation in the ceiling will have almost no effect on Uncle Bob's living experience, other than he may not hear as much noise from people upstairs.

What you need to do is insulate the perimeter of the basement. This would be all framing that meets with the outside of the house that is above ground level. By just stuffing fiberglass or roxul into the joist gaps a good foot thick you will achieve 70-80% of your basement insulation needs for maybe $200 for a large basement.

Also this needs to be done all the way around, not in just the area Bob lives. I would certainly do his area first but even a small area not done can sabotage the effectiveness of the rest of the basement. Also note that dirt is a pretty good insulator unless you are in an extremely cold climate so no use in doing the whole wall.

And back to ceiling insulation. In actuality if the basement does not have zoned heating and air, the insulation will probably make the area cooler in the winter. The warm air from the first floor will not be able to infiltrate the lower level as well. So if sound isn't an issue then I would not even think about going there. (Most people would insulate the ceiling in a cold climate because they do not want the cold air from basement moving to first floor - not because they want to make basement warmer)

  • Warm air from the first floor doesn't tend to infiltrate the lower level in any case, as warm air tends to rise not sink. – mmathis Feb 2 '17 at 21:39
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    @mmathis - in general true, however the stack effect should actually make the basement cooler. Not sure if you have ever walked through an attic with leaky walls in the middle of the summer - I have too many times. Attic might be a good 120-130F and a small opening will give you a jet stream of cold air due to the different air pressures. Well the cold air is actually the warmer air from the house (but still much cooler than the attic). Same thing for basement, the "warmer" air will push up via stack effect - albeit not as pressurized. – DMoore Feb 3 '17 at 5:00

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