Replies

1. Expert Member
   Michael Maines | Dec 11, 2020 11:25am | #1

   Crusty, the insulation is doing something but very little. Fiberglass (and other fluffy insulation) needs to be enclosed in an airtight cavity to reach its tested R-value.

2. Expert Member
   Peter Engle | Dec 11, 2020 12:12pm | #2

   Also, insulating and air sealing the basement walls brings the basement into the conditioned envelope, making it warm and dry. That's got its own value.

3. Expert Member
   Dana Dorsett | Dec 11, 2020 12:54pm | #3

   >" I could literally remove it all with no change in the thermal condition of the living space. Am I correct in this?"

   Not really correct. The insulation is doing something, but even when installed perfectly it's insulating against a fairly low temperature difference- the basement is never really THAT cold compared to the wintertime outdoors. It's probably only raising the average floor temp something like 1/4 degree F.

   If you have lossy ducts, furnaces water heaters, boilers, etc in the basement the floor insulation can even be working against you, guaranteeing that the standby and distribution losses are truly lost.

   >" I’m about to undertake a serious insulation endeavor throughout the house, and I’m thinking I’ll spend dollars on insulating the basement walls and sealing the perimeter at the joists rather than re-insulating the 1st floor joists properly… or am I missing something?"

   You're on the right track! Concentrate on robust air-sealing prior to insulating the basement walls & rim joists. The walls can be insulated with a combination of foam & fiber with low moisture risk as long as there isn't a flooding history to the basement, and the ratio of foam-R/total R is comparable to the IRC's prescriptives for above grade walls. For the above grade wall discussion, see:

   https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

4. 
   gusfhb | Dec 11, 2020 05:43pm | #4

   You have to remember that the first 'R' is the most valuable, and the returns diminish from then on.

   a 1000 sq ft R1 surface with a delta of 30 degrees F loses 30000 btu/hr
   increase it to r2 15000 btu
   r3 10000 btu
   r10 3000 btu

   so the first addition 'R' over the original [theoretical sheet of glass airlayer]R1 saved 15k btu, while the next one only saved 5k, the next 7 only 7k more
   Go to R50 and you save 2400 more

   So there is a number at which you will feel the difference on your feet, but increasing that number you only feel it in your wallet, and then another where we only feel it in our collective wallets, or survival or however you want to think about it

5. 
   newenglandwest | Dec 11, 2020 06:13pm | #5

   Thanks for the replies, All. The instructions to the original installers appears to have been "Avoid installing the insulation tight to the warm side, avoid putting the faced side on the warm side, and while you avoid ensuring there are no air gaps make sure you compress the batts as much as possible. If you can compress 3-1/2" batts down to 3/4", you've succeeded!" A little dry humor on my part. Let the insulation adventure begin.

1. Expert Member
   Michael Maines | Dec 12, 2020 09:08am | #7

   Crusty, despite popular advice, compressing fiberglass insulation actually improves its R-per-inch. Compressing t0 3/4" is a bit excessive, though. Also despite the usual advice to put the vapor retarder on the warm side, in floors the vapor drive is minimal so putting the faced side down is probably ok. Those technicalities aside, I've seen plenty of installations just like you're describing and they will not perform as advertised!

6. 
   DCContrarian | Dec 11, 2020 07:53pm | #6

   You have to know where the inside of your house ends and where the outside begins. It seems like a simple concept but many, many older houses have trouble with it. You have to decide, is the basement part of the inside (or conditioned) part of the house, or is it part of the outside (unconditioned)? Generally, you want the basement to be part of the inside of the house.

   The line between the inside of the house is called the "building envelope" and it should consist of four layers: a weather barrier to keep rain out, an air barrier to keep conditioned air from escaping, an insulating layer, and a vapor barrier to keep dampness from crossing the envelope. It should be continuous. Joe Lstirburek has a great description, if you have a plan of the building you should be able to trace the building envelope without ever taking your pencil off of the paper. The entire envelope of the entire house needs to have all four layers. Most houses have a weather barrier, you would notice if rainwater were pouring in. Usually the layer to add first is the air barrier, because neither the insulating layer nor the vapor layer will work if air is leaking out of the house.

   If you decide that the basement is part of the inside of the house that insulation between floors is separating two parts of the inside of the house. There might be reasons to desire that -- soundproofing, or even keeping different temperatures in different parts of the house -- but it's not contributing anything to the building envelope. If that's what you mean by "zero impact" then you are correct.