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slownsteady 06-05-2009 10:28 PM

Sealing both sides of a block wall
I read somewhere that cinder block needs to breathe because it it needs to vent moisture.

So now that I waterproofed the outside of my basement wall (excavated and installed sheet drain and pipe) should I be using Thoroseal (or something like it) on the inside?? Will this prevent the block from venting natural moisture? If I don't seal the inside, will I be able to insulate the wall?

When I stripped off the old drywall I found that the previous owner had installed a sheet of plastic between the block and the framing. I had to take this down so my wall would dry out. I imagine the Thoroseal will take the place of the plastic.

slownsteady 06-07-2009 11:08 AM

should I move or copy this question to another forum? Maybe floor, walls, etc.

inspectorD 06-07-2009 06:39 PM

It's okay just where it is.:)
Sorry for the lack of a response.
I try not to answer all the questions that come by, gives others a chance to help out and gets folks involved.
Well it all depends on what was existing.
Did you have water while the plastic was there before you fixed the outside?
Do you have water now after your drains have been put in?

Sealing the foundation from the inside is usually a waste of time and money. The water comes in from the exterior, from the walls, floors and cracks and seams, this you already know I am sure. Sounds like you want to try to stop the migration of vapor, this is tough to control at best.
What this means are there is no guarantee what you want to do will work.
My advice?
Seal the basement walls,plastic is easier to control if there is spalling of the walls or issues in the future. And water can be controlled to drain to a wall drain if this becomes an issue, and you do not have to do it again in 10 years.
Thoroseal can cause water to get trapped and spall(deteriorate) the block if there is too much moisture.

Hope this helps, try to remember fiberglass against a basement wall can support mold. Fiberglass traps air to create the insulation barrier. Styro or dow blueboard does not. Insulate as a continuous sheet then build your walls.
I have applied a few steel studs across the concrete and screwed the sheets to it. Then built a wall.

Hopefully otheres will direct you to a site or two. Search basement finishing to get advice.

slownsteady 06-08-2009 07:14 AM

Thanks. The basement has been dry since we sealed the exterior (so far, knock on wood). I just would have normally gone ahead with the Thoroseal for extra measure - because it's standard ops. But that piece of info about the blocks collecting moisture made me doubt. It sounds like you agree that the blocks may retain moisture if they are Thorosealed.

I wasn't planning on putting up plastic in any case. The old framing is already in place and I think it's in good-enough shape to keep.

handyguys 06-08-2009 08:06 AM

My opinion is to do only one side and that that side should be the exterior. Thats how my poured concrete foundations is done and my basement is bone dry.

slownsteady 06-10-2009 10:55 AM


Originally Posted by inspectorD (Post 31164)
My advice?
Seal the basement walls,plastic is easier to control if there is spalling of the walls or issues in the future.


Originally Posted by handyguys (Post 31187)
My opinion is to do only one side and that that side should be the exterior. Thats how my poured concrete foundations is done and my basement is bone dry.

OK. That's one vote for and one against. Anyone care to tip the scales?

Also, how do you determine what "too much moisture" is?

handyguys 06-10-2009 11:14 AM

Hmm - InspectorD and I usually agree.

Take a read of this article and see if it tips the scales for you.
RR-0202: Basement Insulation Systems —
(download and read the .pdf)

As for moisture - there is a simple test.
First visual - If yu have efflorescence (white powder) then you have moisture. Its minerals left behind from evaporation of water.

The test though - You can tape a small sheet of plastic on the wall, let it sit for a few days. Look at it, if moisture under the plastic then its permeating through the wall, if on top of plastic then its humidity moisture in the air.

inspectorD 06-10-2009 06:46 PM

Well Handyguy's we are agreeing.
He already sealed the exterior which we both wanted.:rofl:

The issue with the block foundations can be very different than a poured foundation. To many times water seapes into crackes in the mortar joints later on and fills the hollows of the blocks (IF) they are hollow blocks.
The plastic will drain the vapor to the interior perimeter drains if directed to do so, whereas any applied sealant will only spall and cause further damage.

And the link is great, it just has to be applied to each different homes characteristics.
Basements are a very real and large problem if they are finished incorrectly, and they cost lots of money to fix when you finally realize there is a problem.

Good luck on your project, and do lots of homework to find out what applies to your situation.

handyguys 06-11-2009 08:14 AM

we certainly are agreeing. That paint stuff is a waste of money for controlling moisture. I think it looks nice over block or even stone foundations though.

slownsteady 06-19-2009 11:32 AM

Well, I think I'm going to split the difference and do the lower half of the wall. It's already partially done (previous owner) and the wall seems to be pretty dry despite the recent rainy spell we're having here. It would make me a little more confident and the wall would still have some room to breathe.

Thanks to both of you.

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