DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Building & Construction (
-   -   Vapor Barrier In Waterproofed Basement (

emagsamurai 11-16-2012 01:43 PM

Vapor Barrier In Waterproofed Basement
Yet another vapor barrier question :)

My poured wall basement has a sprayed on waterproofing membrane, a dimpled drainage mat, plus another drainage board on the exterior. I'm planning to start finishing the basement now, and I'm not sure what the requirements are for a vapor barrier. I called our local inspector, and he said in the central Alabama area, a vapor barrier is not required. However, after reading as much as I can online, there seems to be several different views on this, albeit most seem to be in relation to basements further north.

The room in question will be a soundproofed home theater. The framed wall is approximately 1" from the foundation, and I would like to use fiberglass insulation for sound deadening. drywall will be suspended from isolation clips and hat channel, so there will be an additional 1-1/2" gap between the insulation and the drywall.

I appreciate any feedback!

emagsamurai 11-19-2012 10:06 AM

I thought this topic may have been missed over the weekend. Any advice would be greatly appreciated.

Thanks in advance!

jcarlilesiu 11-19-2012 12:00 PM

Condensation is always a concern, the issue is the dew point of a wall various based on region and composition.

Due to the ground retaining freezing conditions into the spring, while the air may begin to warm and hold moisture, a cold basement wall has a high probability of condensation on the interior.

For that reason, whenever I design a basement build out, I do install a vapor barrier at the interior and frame the studs around the exterior with about a 1" off set from the concrete/block walls.

Weather or not you need that as far south as you are I am unsure.

Give this calculator a shot to determine.

This type of calculation is much more important at above grade exterior walls because an improperly located vapor and/or moisture barrier could actually lead to problems, not prevent them.

I hope this helps.

AGWhitehouse 11-19-2012 12:26 PM

Don't use a vapor barrier at the inside. The waterproofing membrane at the exterior is forcing ALL vapor drive to the interior. If you block its route you'll have issues down the line. Let it all breathe. But as carlisle noted, keep the studs and any insulation off the face of the concrete wall. It may sweat and you don't want the wall to be in contact with any of that direct water...

emagsamurai 11-19-2012 01:37 PM

Awesome! Thanks for the information. That calculator is pretty handy, too. If I use a best guess for the soil temperature, I seem to be in good shape. Everything seems consistent with what our inspector said :)

One last question. Our inspector mentioned that since the condensation is generally not an issue here, I could use Kraft faced batts just for the convenience if I like. Is there a downside? I'm leaning towards unfaced based on AGWhitehouse's comments.

AGWhitehouse 11-20-2012 09:28 AM

the kraft paper facing is considered a class II vapor retarder with an average permeability rating of 1.0 perms...If breathability is your goal, then I wouldn't recommend it.

emagsamurai 11-21-2012 08:30 AM

Thanks again for the information!

Now I need to figure out how to keep the insulation in the wall cavity where it's supposed to be rather than falling back against the wall. Shouldn't be too tough, though.

AGWhitehouse 11-21-2012 09:06 AM

Fiberglass mesh

emagsamurai 11-21-2012 12:37 PM

Thanks yet again. Looks like a trip to the home improvement store is in my future :)

Gary in WA 11-21-2012 08:46 PM

"My poured wall basement has a sprayed on waterproofing membrane, a dimpled drainage mat, plus another drainage board on the exterior."----------------------------

So you have dimpled drainage mat on the interior?

With your low temps averaging 54* a year, the concrete wall would be around that temperature. You would be safe from condensation on bare concrete if the room RH is kept below 57%RH at room temp of 70*. If you have a drainage mat on the interior; air seal it with mastic at the joints: laps, top, bottom, etc. keeping basement air from getting to cold concrete (higher RH wouldn't then be an issue).

Any air gaps will create spaces for convective currents that move your heat and deposit it to higher framing members. 24/7:
The gaps also add speed/distance to an outlet fire; sending flames racing behind the drywall (fire-stopping) unhindered to surface at the other end of the basement or upstairs- including attic, igniting the roof (burning down) while another fire is below (burning up). Be sure to fire-stop every 10' horizontally; the top plates and any opening to the joist cavities at the ceiling level per minimum safety code; "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." from:
Air seal the rim joists with foil-faced PIC;

Air-seal the drywall (ADA):


emagsamurai 11-21-2012 09:31 PM

All three of my waterproofing components are on the exterior. On the interior is concrete, then on the exterior I have the rubber membrane, followed by the dimpled poly membrane, followed by a mineral wool (IIRC) drainage mat. Hopefully that configuration is acceptable from a vapor standpoint. It's supposed to be excellent for waterproofing. Sorry for the confusion.

Excellent point about the fireblocking! I had not considered that. I'll have to research what I'm supposed to do there! Thanks for the tip.

Gary in WA 11-22-2012 12:01 AM

Actually, I was tired when I read it wrong...

IF you want less risk of condensation, add R-5 (1/2") f.b. glued to the concrete, then frame wall with R-13 f.g. touching that. (No convective loops/air spaces). You wouldn't get condensation below 70*and 66%RH in the room. Make sure the foam is air-tight, that is more important than faced/unfaced batts:

You wouldn't need faced for your climate zone, above or below grade.

Use a foam sill-sealer under the code required (p.t.) bottom plate for a thermal/air/capillary break:

Fire-blocking from our forum "How to" by another Moderator:


emagsamurai 02-06-2013 09:07 AM

Sorry for digging this thread up, but I'm planning to start insulating in the next few days. My plan was to install unfaced fiberglass batts in the walls, and use fiberglass mesh to keep it off the concrete wall. However, because my walls are already framed, I can't get the mesh pulled tight enough that I feel like it would be of any benefit.

With that in mind, would I be any better off if I cut 1/4" f.b. to go between the studs and attached it to the concrete, then put unfaced insulation on top of that? My framing varies from about 1" off of the concrete to less than 1/4" in places, so I don't think I can get f.b. behind the wall and sealed as I would otherwise like to do.

brockmiera 02-06-2013 09:50 AM

Put the 1/4" XPS behind the studs where you can. Where it wont fit use a low expanding foam to fill in the air gaps. Again. The most important thing here is to have an airtight barrier from the inside conditioned space to the colder concrete foundation wall. Any wood in contact with concrete needs to be PT Lumber as well.

emagsamurai 02-06-2013 10:40 AM

Great! The 1/4" stuff comes in a 50' fanfold that I might be able to slide behind the wall in long pieces. That would be a huge time saver!

Does anyone see any downside to this? This seems like a good solution, particularly since I'm not supposed to have to worry about condensation in this part of the country anyway. But I worry about the stuff that I don't know enough to worry about :)

All times are GMT -5. The time now is 12:38 PM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

Search Engine Friendly URLs by vBSEO 3.6.1