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Bonzoso 11-16-2012 07:08 PM

More basement insulation
Hi all,
I have a situation that I would appreciate a comment on. We have an old 1950 rancher with poured concrete basement walls that over the years have had cracks and water infiltration. We recently had the basement waterproofed and all the cracks repaired. The waterproofer added a heavy duty fibered poly sheet over the concrete wall as what he referred to as a moisture barrier (not vapor barrier) to drain any new seepage that would develop on the wall face along the inside of the poly and into the drain system. The seams were all taped except for the top near the sole plate and the bottom which fits behind a flange to direct moisture into the drain. This barrier has little if any insulation value. Any thoughts on how to insulate this properly. I have just finished insulation all the rim joists with 2" blueboard and foam. I was thinking originally of just framing against the fibered sheeting and using Roxul insulation in the framing voids with a poly vapor barrier under the drywall since the concrete poly barrier is not completely sealed and therfore not a true vapor barrier. I later thought of using 1″ blueboard against the moisture barrier just held in place by the framing ( for some insulation value but not air tight) and continuing with Roxul and a poly vapor barrier. We live in moderately cold SE PA and I will be using a dehumidifier in the basement as we don't have central air conditioning. In the winter we will heat the basement with a pellet stove. Thanks in advance for setting me straight.

Windows on Wash 11-16-2012 10:25 PM

You got all the bands sealed so that is a great start.

Several other posters have used XPS against their interior weeping system and have taped/foamed/sealed all the seams and framed out there stick wall inside of that.

That keeps the moisture (if sealed up and air tight) out of the frame wall.

Gary in WA 11-22-2012 05:58 PM

Yep, and if you are still around for my belated reply; no foamboard gives condensation above 56%RH at 70*. R-5 f.b. gives safety up to 66%RH in the room, no worries (for your location only-if Philly).

No poly below grade in U.S.!


Bonzoso 11-22-2012 08:37 PM

thanks for the replies. so contrary to the Roxul site info, you're saying, Gary, that I should not use a poly vapor barrier under the sheetrock?
Also a question regarding sealing up my "weep system". Since I likely won't get good adhesion of blueboard to the poly sheet, should I proceed by using adhesive at the top plate and bottom flange(or concrete floor) to the blueboard edges, tape and frame in, say 8' sections(to fully support the bb). then maybe foam and bb the top of the wall framing perpendicular to my rim joist insulation job. hoping for some clarity on sealing up this weep system. Also, would 2" bboard be enough insulation by itself. If roxul is overkill with 2" bb, I'd rather save the money. i really appreciate the info. thanks.:thumbsup:

Gary in WA 11-22-2012 09:18 PM

No vapor barriers below grade; #1:

With an interior drainage system- Fig.1;

The problem with poly:

Any air gap between the foam board and concrete wall will give a condensation area, it needs to be air-tight from the room;

2" f.b. alone is fine for your location, if Philly. Find nearest large city BELOW the map;

Zone 4, "Basement wall" footnote "c";

If you need the space for wiring runs, turn the studs on edge (flat) after troughing a little behind the studs in the foam. Fill the cavities with 1/2 thickness f.g. batt (3-1/2" = R-13) to prevent convective loops or leave it out and just warm your floor joists above, lol.

You should check local AHJ for fire-stopping, per code requirements; "R602.8 Fireblocking required.

Fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space. Fireblocking shall be provided in wood-frame construction in the following locations.

1. In concealed spaces of stud walls and partitions, including furred spaces and parallel rows of studs or staggered studs; as follows: 1.1. Vertically at the ceiling and floor levels. 1.2. Horizontally at intervals not exceeding 10 feet (3048 mm). 2. At all interconnections between concealed vertical and horizontal spaces such as occur at soffits, drop ceilings and cove ceilings." And more areas, from; good guide from another moderator here: #21, 22----
Use a sill sealer under the p.t. bottom plate on the slab for an air/thermal/capillary break from the cold/wet slab/earth;

With R-10, at 7o*, the foam under the drywall/f.g. insulation (1-1/2") is at 64* or up to 84%RH, lol. Better keep some water on the stove for room humidity. Heat savings with f.b.;

That second link in my previous post also has savings for your insulation choice, pp.69.


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