What type of concrete sealant?

Steven Bell · Post by **Steven Bell** » Mon Sep 26, 2005 11:22 am

I am about to begin construction of the shell which will encase my new studio.

I plan to seal the concrete slab to keep the moisture from coming up and was wondering which type was best to use.

A Google search brought up a company called RadonSeal.
Their product seems to do what I need

http://www.radonseal.com/concrete-sealers.htm

Their instructions say NOT to put polyethylene under the slab and the instructions on this forum say TO put it there as a vapor barrier.

They indicate that the vapor barrier prevents moisture inside the concrete from getting out.

Thoughts?

Steven

knightfly · Post by **knightfly** » Mon Sep 26, 2005 5:53 pm

Concrete is porous; it WILL pass water vapor. So if you are building from scratch and plan a facility that will be heated more than cooled, then all vapor barriers need to be on the WARM side of your walls/ceiling/floor, so the radon guys are correct for that application.

Main thing to remember is that you don't EVER want more than ONE vapor barrier in a wall, or you WILL trap moisture between the two barriers and WILL propagate organisms that will either make Alexander Fleming very proud, or that can KILL you...

If you're indeed building from scratch, I would recommend telling whoever pours your slab that you want it on a minimum of 4 to 6 inches of compacted crushed rock; and you also want to provide good drainage around the perimeter. This combination will guarantee (unless you're in New Orleans

) a dry slab, which will minimise the job your vapor barrier has to do.

Not sure how much "lurking" you've done, but this is something you DEFINITELY want to MEMORIZE before you start -

SuperDuperStuff

If you can, please post a link to where you read about vapor barrier on this forum; I may need to make some edits. Thanks... Steve

Steven Bell · Post by **Steven Bell** » Tue Sep 27, 2005 12:46 am

knightfly wrote: Main thing to remember is that you don't EVER want more than ONE vapor barrier in a wall

So if I have polyethylene under the slab then I can't use a sealant on the top? I notice you said "wall" above. I'm just talking about a slab.

If you're indeed building from scratch, I would recommend telling whoever pours your slab that you want it on a minimum of 4 to 6 inches of compacted crushed rock;

Yeah, that's what I thought, too, but my site prep guy only did about an inch of sand over the dirt.

If you can, please post a link to where you read about vapor barrier on this forum; I may need to make some edits. Thanks... Steve

I'll look.

Steven

knightfly · Post by **knightfly** » Tue Sep 27, 2005 2:04 am

Slab, wall, the only difference for this purpose is that one is horizontal; other than that, the migration principle is the same; moisture vapor travels from warm to cold, driven by the heat. Having two barriers will trap said moisture between them.

Other than that, a concrete slab will require 100 times as much trapped moisture as a gypsum partition before trouble with mold - read the chart on page 2 or 3 -

http://www.buildingscience.com/resource ... ldings.pdf

I poured a 5-1/2" slab in my pole barn several years ago - clay soil (generally poor drainange), used 22 yards of COMPACTED crushed rock under 21 yards of concrete. After about a month, I could lay a piece of metal flashing on the slab anywhere, summer or winter, and check it next morning; dry as a bone, no condensation whatever.

Your soil will probably be different, probably better drainage; still, I'd ask for a second/third opinion in your area (different contractors) - ask them how they'd do it if it was THEIR place (and they were planning on STAYING there).

Contractors come and go, but concrete is either FOREVER or very EXPENSIVE to remove; I wouldn't be afraid to step on a few toes if necessary to get it right the FIRST time... Steve

Steven Bell · Post by **Steven Bell** » Tue Sep 27, 2005 2:16 am

So, if we all intend to heat our studios, would we ever want the vapor barrier under the slab? It seems that we'd want to use a sealant instead.

The heat traveling from the room to the slab sends the moisture into the crushed rock under the slab.

If the vapor barrier (polyethylene) is placed below the slab, the moisture (as forced by the heat in the room) is trapped on top of it.

Steven

knightfly · Post by **knightfly** » Tue Sep 27, 2005 2:30 am

True; the general rule is, if you heat for more days than you air condition, vapor barriers go to the inside. If you cool more than you heat (florida, etc) then vapor barriers go to the OUTSIDE.

Reading through those Building Sciences papers a few times should clear all this up for you... Steve

Post by **sharward** » Tue Sep 27, 2005 8:08 am

...Except here in Sacramento (considered a "more air conditioning than heating" place), they put plastic underneath the slabs these days...

knightfly · Post by **knightfly** » Tue Sep 27, 2005 8:25 am

Yeah, but that's just stupid bureaucraps making rules without the encumbrance of knowledge...

Steven Bell · Post by **Steven Bell** » Tue Sep 27, 2005 1:41 pm

Just when I thought I knew the answer, too.
Since I heat more than I cool, I need to seal the concrete (barrier on heat side).

If, in Sacramento, you cool more than you heat, then use polyethylene under the slab (just like the building code says).

What am I missing here?

Steven

Steven Bell · Post by **Steven Bell** » Tue Sep 27, 2005 1:45 pm

knightfly wrote: Reading through those Building Sciences papers a few times should clear all this up for you... Steve

Sadly, reading it three times did not help me with what I need to do for my project.

Showing vinyl siding (which I am not using) did not help.

Showing graphics depicting things like "while heating" did not help.

They made it sound like I was going to tear off my finished surface and swap the location of the vapor barrier as the seasons change.

(And I'm a fairly scientific guy)

Steven

Post by **sharward** » Tue Sep 27, 2005 1:55 pm

Steven Bell wrote:If, in Sacramento, you cool more than you heat, then use polyethylene under the slab (just like the building code says).

What am I missing here?

You're missing the fact that

I am an idiot

and I got confused (again)!

You're right -- poly under slab in Sacramento is consistent with the "if you cool moe than you heat, put vapor barrier on outside" rule.

I offer my sincere apologies for confusing you (and others).

--Keith

Steven Bell · Post by **Steven Bell** » Tue Sep 27, 2005 2:04 pm

poly under slab in Sacramento is consistent with the "if you cool moe than you heat, put vapor barrier on outside" rule.
--Keith

Whew!

I am so relieved to see this!

Steven

knightfly · Post by **knightfly** » Wed Sep 28, 2005 2:41 am

Sorry Steven (nice name, BTW

) I temporarily forgot you've not been around long enough to realize what a smart-ass I sometimes am (and how much I enjoy taking potshots at bureaucrats, whether they have it coming or not) - I was taking a (rarely) undeserved shot with that comment, sorry it confused you.

The science stands - heat more, barrier INSIDE - cool more, barrier OUTSIDE. You will, however, find local codes in some areas that are (occasionally) so retarded that they want vapor barriers in BOTH locations -

I know the Building Science stuff has more info than you need; I posted it because you're building from scratch, and hadn't mentioned your construction other than the slab. If you have any questions regarding the pertinence of anything to your specific situation, and you're doing something different, feel free to ask and I'll attempt to help. In that case, a clear sketch with everything labeled will be immensely helpful.

BTW, if you're building a free-standing building, and it's close to your house, do NOT, repeat NOT, make the mistake of any solid attachments to the house; if there is something like a breezeway between buildings and you want it covered, build a SEPARATE roof that overlaps or something. Otherwise, flanking noise could compromise isolation between house and studio... Steve

Steven Bell · Post by **Steven Bell** » Wed Sep 28, 2005 3:00 am

Hi Steve,

Thanks for the clarification. I have seen your other posts in which you warn about your smartassidness so I was not completely surprised.

This building will be over 100 feet from the house.

The shell (to be started Monday) is a post building (versus stick frame) with a slab over 4" of crushed rock (per your recommendation) which is over about an inch of sand.

The exterior of the shell will be 5/8" T1-11. The interior of the shell will be insulated (R-19?) (should I caulk first?), framed and sheet rocked.

The studio will be an entirely separate structure within this shell.

The plans call for polyethylene under the slab, but from everything I'm seeing it would be best that I use the sealant mentioned in my original post instead. I am in the pacific northwest corner of the US where it is cool and damp 9/12 of the year.

Thank you very much for your insight and advice!

Steven

knightfly · Post by **knightfly** » Wed Sep 28, 2005 3:36 am

The studio will be an entirely separate structure within this shell

Careful here; before you build a shell, thinking it will have no effect on what you build inside, what (if any) are your isolation requirements? Either FROM or TO the outside world? Asked because multi-leaf effects can make a BIG diff in results.

In my own case, I have one neighbor who's too stupid to train the dogs he shouldn't own to be quiet, another who's a great guy but who is a custom hot rod builder (likes big block MoPar's), and a couple of deputies just over the hill who like playing with automatic weapons, farmers with hay balers, National guard whose flight path for refueling is right above our place, etc. - not to mention rain/wind noise in winter, etc...

I'm in Oregon, about 90 miles south of Portland so our climates probably aren't all that different... Steve