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jtmann 11-17-2010 12:16 PM

Insulating basement - questions

I am puzzled as to how to proceed with the following project and thought that those here at the forum might have some insight!

I live in a cold climate. I have a two-storey house with a full basement. It is a completely UNfinished basement (i.e., no sheet insulation or wall framing on the basement concrete walls, nor any batt insulation in the open ceiling joists). The basement is unheated and fairly damp. But not a big deal as I use it only for storage.

But I wanted to see if I could make the first floor of the home (above the basement) warmer, especially the floors. Here’s the twist - I will be re-selling within three years and don’t want to spend to level that I won’t get a return on investment when I re-sell. I already spoke to my local building code inspector and he said if I were to put Styrofoam sheets on the basement walls I would then have to go the two extra steps of adding wall framing and then drywall (I assume as a fire retardant). There is no way this will pay out for me as it is an unfinished basement and that’s what most basements in my area are…..not an option.

So then I thought I’d put fiberglass batts in the ceiling joists and cover with vapor barrier on the cold side (facing basement) - clear plastic sheeting to keep the dampness from the basement from getting into the batts. But then I remembered that vapor barriers are typically installed on the warm side, so that might just make the warmth from above/moisture from above collect on the vapor barrier....not sure what to do there...

Any advice from the forum as to how I could do the batts in the joists approach? What about doing two layers of vapor barrier to totally isolate the batts – i.e., one above the batts on the warm side to keep the warmth/moisture from that side and on the other side to keep the dampness of the basement away?

Suggestions or comments on a solution much appreciated!!


P.S. Strangely, my local code does not require drywall on the ceiling joists/batts – go figure.

BRO931 11-17-2010 05:35 PM

I'm in a similar situation myself, except that I plan to keep the house indefinitely but don't have a lot of money to spend right now.

My local regs allow me to stick drywall directly onto extruded polystyrene (XPS) panels. No wood frame needed. As long as the XPS panels are fastened well to the concrete block (glue, screws, etc), there are no worries about gluing the drywall to the XPS. Of course you can't run wires or plumbing behind the drywall. And I wonder if the drywall joints might be more liable to cracking. But it can be done. You might want to consult your local authorities with this option.

I've also heard that some places let you stick just a foil layer on the XPS.

Good luck.

SPS-1 11-17-2010 06:03 PM

Do it right, or don't do it at all. The next owner will be on this forum explaining about the mess he has to rip down.

The inspector you were talking to explained how to do it right. XPS is rather expensive, you might want to look at the price of fiberglass. I don't think fiberglass is going to need a barrier over it. But you might as well. It will look so much better. The place for insulation is the basement walls, not the basement ceiling.

But you are not going to get a payback from an isulation job in 3 years just in energy savings. So you either have to do it well enough that it adds value to your home, or just forget about it.

jtmann 11-18-2010 10:36 AM

Thanks for your input Bro931 and SPS-1. I want to did it right.
Bro's suggestion is not applicable for me - can't do it by code.
Do either of you see any problems in doing what I suggested - vapor barrier on both sides of the fibreglass batts in the basement ceiling joists?

BRO931 11-18-2010 03:04 PM


Originally Posted by jtmann (Post 536343)
Thanks for your input Bro931 and SPS-1. I want to did it right.
Bro's suggestion is not applicable for me - can't do it by code.
Do either of you see any problems in doing what I suggested - vapor barrier on both sides of the fibreglass batts in the basement ceiling joists?

I would take your concerns about the vapor barrier seriously. The previous owner of my house had put up frame walls with fiberglass insulation and the kraft paper was a not a sufficient vapor barrier. Water vapor from the inside of the warm basement filtered into the fiberglass-filled wall and condensed onto the cold block walls. This condensed water soaked the fiberglass, which soaked the frame wood and kraft paper. What a moldy, smelly mess! I had to tear out everything down to the bare block walls. If you decide to put fiberglass in the floor joists (ceiling joists for the basement), water vapor might filter through the floor into fiberglass. But you would have no cold surface (like a block wall) to condense water on. It might condense in the cold (lower) part of the floor joists or on the fiberglass insulation.

It is most important to have a vapor barrier on the warm side of the insulation. Such a barrier on the cold side would present a surface for condensation. But if everything is totally sealed, like fiberglass wool in a plastic bag, you have no worries. That's why XPS works so well.

SPS-1 11-18-2010 08:17 PM

Probably somebody who knows more about insulation than me will chime in, but I have never seen a vapor barrier on both sides of insulation, so its probably not a good idea to experiment on your house. One time somebody posted that Tyvek on the outside of a house, and a vapor barrier on the inside was no good because it traps moisture (and he supposedly was a contractor). That made me wonder, so I went to the Tyvek web sight and read that it specifically was vapor permeable... but I digress...
I never heard of insulating between a basement and the living area either. Maybe I just have not been around enough. I have always read that an area is either heated living space, or its treated as outdoor, but not in between. People insulate above a crawlspace, or below an attic, but then they also put ventilation to the outdoors for those areas.
Around here, its code that a new house must have basement walls insulated for a depth of 4' (?) below grade (and covered with a vapor barrier when applicable).

High Gear 11-18-2010 08:57 PM

Posting this over in Construction will get more responses , your in HVAC.

Or doing a search on building science will explain options.

beenthere 11-19-2010 03:20 AM

Just use batt insulation. No vapor barrier.

Don't post this in Construction also. If you want it moved to there, let me know, and I'll move this thread to Construction.

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