DIY Chatroom Home Improvement Forum

DIY Chatroom Home Improvement Forum (
-   Building & Construction (
-   -   Installing insulation and vapor barrier to exterior basement walls (

Fernando Sinn 04-07-2008 10:59 PM

Installing insulation and vapor barrier to exterior basement walls
I have heard a recommendation to leave a gap on the bottom of exterior basement walls actually not sealed with vapor barrier to allow air flow from inside of the house to the space between the concrete foundation and the new stud wall constructed to finishe the basement. Is this true? is this a good idea or not?

Ron6519 04-08-2008 11:02 PM

Vapor barrier installation has some regional variation. Where do you live.

Fernando Sinn 04-09-2008 11:40 AM

Ron we are in the Greater Toronto area in Southern Ontario. Always wrap the exterior basement walls fully with the vapour barrier, now an "experts" says wrong, should leave the bottom 3 or 4 inches without vapour barrier is thiS right?

Ron6519 04-09-2008 04:59 PM

Fernando, I would contact the local building department for your local recommended procedures. My only point of reference of Canadian building practices is the Holmes on Homes show and they always have a vapor barrier and tuck tape to seal the seams.

Fernando Sinn 04-09-2008 06:55 PM

Vapour Barrier on exterior basement walls
Ron, thanks I believe the gap on the vapour barrier would be wrong, I searched all applicable web sites, all with the same recommendation to seal the complete wall. Fernando

jason h 04-09-2008 09:41 PM

be careful putting vapor barrier on a concrete foundation wall. water vapor will pass threw a concrete wall, when plastic is covering wall it catches vapor, turns to water droplets puddles at floor , turns to mold which feeds of the lumber and drywall. i say absolutly not. i am in northern michigan.

Fernando Sinn 04-10-2008 07:55 AM

Vapour Barrier on exterior basement walls

You are right on the issue.

There is a poured concrete foundation, 8 inches thick and 9 feet tall with tar coating and a plastic wrap on the outside, then 1 2 inches air gap on the inside, 2x4 stud framing at 16": with vapour barrier on the bottom against the slab and 3 1/2 inches of fiberglass pads snug between studs and then the 6 mil polietylene vapour barrier.

Condensation on the concrete surface is unlike because ther is no temperature differentiel and there is a small caviyi allowing air flow.

Do you still believe that vapor condensation is on the outside and will run on the barrier down to rot the bottom 2x4 plate, or is actually the humidity inside of the house which could for condensation on the inside of the barrier and hence is not going to get to the wood.

hnks your help is appreciated, Fernando

jason h 04-10-2008 09:18 PM

every house in every region has some sort of moisture through out the year.
air move from inside out . this is basic building 101.
if you install a plastic vapor barrier on the inside of your concrete wall you WILL have a moisture problem barried behind the wall.
think about when you boil a large pot of water, you get condensation on the glass in the kitchen and near by windows, water molacules are trying to leave the house. plastic and glass are working in the same manner. both collect water vapors and they turn into water droplets and run down the glass,plastic.
many ,many, people will tell you im wrong. i have been through much education on this , including feild testing. do as you are confortable with , and good luck

Fernando Sinn 04-10-2008 11:07 PM

Vapour Barrier on exterior basement walls
Jason, condensation occurs when warm air with high capacity to retain water is in contact with a colder surface cooling the air and reducing its capacity to retain water it is indeed similar to the rain on a cooler summer day.

Without insulation the vapour barrier cools down on the inside surface as well and will produce condensation by cooling the air in that area. If insulation was perfect it would prevent all temperature differential and the vapour barrier would have the same temperature as the room on both sides.

The reality then is; minimum insulation on the outside, 2 to 3 inches of air gap on the inside surface which is somewhat ineffective as air circulates in this space, 3 and 1/2 inches of fiberglass (R12) and then the vapour barrier, will there be condensation?

If indeed a gap is left on the bottom air will circulate in and out of the space and will be in direct contact with the concrete foundation wall producing condensation there and yet there will be no visible condensation on the vapour barrier because the air ciculation will mostly eliminate the temperature differential that the vapour barrier sees.

Thanks for the comment I am learning every step,


Gencon 04-11-2008 05:59 AM

For Southern Ontario, install an air barrier against the concrete wall( housewrap works well for this), then your insulated stud wall, no gaps anywhere, completely fill the cavity, including behind the studs. Then your 6mil VB, then drywall.Vb goes on warm in winter side of insulation.

Alternatively, 1 1/2" extruded polystyrene (foamboard) adhered to the concrete, then the insulated stud wall, then drywall-no VB to deal with.
Both these methods are acceptable.

You do not want air to circulate within a wall cavity. Even convection currents will carry moisture.

Think of how your upstairs walls are built-no air gaps.

Since this type of work requires a permit and inspections, your local building department actually issues a detailed drawing of what they will accept for finishing a basement.

jason h 04-11-2008 06:45 AM

i am in northern michigan,
i do not agree with gencon about the first senario, how ever his second senario is the best chioce iv heard so far.
1 1/2 foam board ( not the white bead board) then the wall then insulate,drywall no vapor barrier that system will work great.
as i said before many ,many opinoins.
my personal experiance is that the building inspector will only have his belief , which vary widly from inspector to inspector. our group around here has proven them wrong on many subjects .

if you build a house from scratch ,use a wood foundation, you will never ask this question again.:thumbsup:
i to like hearing everyone thoughts, thanks to all

Gencon 04-11-2008 11:11 AM

You're absolutely right about the inspectors, Jason. Thats why I suggest talking to the local building dept first, even the inspector. They will let you know exactly whats acceptable for that particular area.

All times are GMT -5. The time now is 05:51 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