Philip Greenspun's Weblog Sealing a concrete slab to block humidity from entering

Philip Greenspun's Weblog  Sealing a concrete slab to block humidity from entering

Philip Greenspun's Weblog

Sealing a concrete slab to block humidity from entering basement?

philg — October 14, 2008 @ 12:14 am Filed under Uncategorized

Tomorrow is moving day, saying goodbye to Cambridge and hello to Lincoln, Massachusetts.  Our new house requires a little history lesson.  In the bad old days people dug down into the earth to make a basement foundation for a house.  For hundreds if not thousands of years these primitive people stored roots, boxes, and other junk down in this basement while they lived in various floors built well above the ground level.  Above the top floor they built an attic in which to store more junk.  This attic became very handy in the 1970s when people looked for a place to put insulation, a whole-house fan, and other energy-saving devices.

Fortunately for mankind, in the 1960s a group of MIT and Harvard architecture geniuses came along and designed the Deck House kits.  Why have a contractor assemble building materials and the local drunkards on-site to frame a house when the thing could be precision-cut in a factory?  Storing carrots and boxes in your basement?  Why not live in the basement instead?  The typical Deck House, including the one we are moving into, is cut into a hillside so that part of the basement is walk-out level.  Half of the living space is in this first floor, directly on top of a concrete slab.

How about an attic?  Wasteful!  Just make the living room ceiling the underside of the roof.  The roof in this house is about 5 inches thick from the bottom of the living room ceiling to the top of the asphalt shingles.  Im hoping that there is some insulation in there, but am not confident.  In any case, there is no way to add insulation without redesigning the roof to be about twice as thick.  There is also no obvious place for a whole-house fan.  Storage?  Dump it all in the garage.

What happens when you live in your basement?  It tends to be kind of damp.  How about cranking up the central A/C to dehumidify it?  The geniuses behind the original Deck House didnt see the point in running ductwork.  As there are no conventional floors or walls, it is not obvious how to run ducts after the fact.

I am now on a fanatical quest to dehumidify the basement, partly because I want to store some slides and negatives down there.

My first idea is to minimize the amount of moisture coming into the basement.  A landscaping company is coming over to dig away a small portion of earth against the foundation and see if it is moist enough to warrant a French drain (pipe with holes in it).

I know that concrete breathes and am wondering if humidity is coming up through the slab, which was poured in 1968 as part of an outrageously expensive construction project ($38,000 to build 4000 square feet; thank God that the government assures us inflation is minimal or I would be upset by the insurance companys estimate of $800,000 to rebuild the place today).  I have instrumented the house with a Honeywell weather station from Costco (am now a true suburbanite).  The Honeywell is saying that the humidity is about the same throughout the house, at least this evening.  Without the heat being on, it is a dank 57 percent relative humidity upstairs and down.  So we have some evidence that water vapor is NOT coming up through the slab.  A civil engineer friend says that basements are damped simply because they are cooler than the rest of the house and the humidity mostly comes from the outside air.  Nonetheless, I feel compelled to attempt to seal the slab.

The slab is currently covered by some glue, a porous carpet pad, and a medium-thick carpet.  She who has the last word on home decor says carpet is disgusting; I would pay as much as $10,000 to have the carpet replaced by hardwood.  I replied So I guess we wont do it, then, because the quote for engineered hardwood was $15,000.  It seems that we are doing it

The standard way that flooring companies put down hardwood over a basement slab is by not using hardwood.  A solid chunk of wood tends to warp.  The solution for potentially humid conditions is particle board or plywood topped with a thin wood veneer.  It looks like regular hardwood, but is resistant to warping.  The installation starts by taping down a closed-cell foam underlayment, which is more or less impermeable to water vapor, and then floating the floor on top of the foam.  An engineer friend recommends 6 mil polyethylene sheeting underneath the foam, overlapped by at least one foot at the seams.

My question to the suburban heroes reading this is Why not seal the slab with a chemical before putting down the sheeting and foam?  If we dont do it now we will not have another chance for 20 years.

Then the question becomes how to seal concrete.  Drylok is the standard product, but Im not sure what value it would be on a 40-year-old slab.  I talked to a company down in Texas, where they should know something about humidity, called Enduroseal.  They recommended

  1. cleaning to remove the adhesive with a product from franmar.com
  2. treating with their own Hydraloc product
  3. waiting 10 days
  4. treating with their NCS-20 product (not sure why they recommended this on the phone; the current slab does not have any flowing water on it and actually feels dry to the touch)

Anyone out there competent to evaluate this companys claims or have personal experience with this stuff?  Is what they are selling better than Drylok from Home Depot?

Stepping back the larger question is whether it makes sense to seal the concrete slab at all.  It is dry to the touch and the carpet feels mostly cool rather than damp.  There is a slight musty smell in the basement and slight mold growth on a couple of beams.

[This is outside the scope of the current blog posting, but the next planned step after the sealing and hardwood is to install some sort of central plumbed-in dehumidifier.  There is a small utility room at the center of the basement.  One idea is to put a standard HVAC air handler in this room.  Run ducts up to the top floor and discharge air up there.  Have the intakes in a couple of the rooms on the ground floor, thus forcing the air to circulate from top to bottom of the house.  Honeywell and Lennox both make whole-house dehumidifiers that plug into air handlers.  Run A/C in the summer and the dehumidifier in fall and spring.  While we’re at it, add an air filter and a humidifier for December through March.]

33 Comments

Ethan

demetri

I have a finished basement with the slab covered by thick plastic sheets, plywood and hardwood strips anchored to the plywood and it is completely dry and warm. The work was done about 20yrs ago by the previous owner and having seen a portion of the slab itself doubt it received any treatment. The fact that the basements walls are covered by sheet rock may also contribute significantly to the feeling of dryness.

Larry

You dont want to solve moisture problems by sealing concrete as they all will fail, according to many guys I had out to look at my basement. We had actual water coming up in several places and moisture for three seasons. I put in a perimeter drain and a sump pump which has solved the water seepage. I am overlaying the floor with dry-core which is a product with a dimpled plastic laminated to 3/4 OSB. This creates an air gap where water vapor can escape into the living space. I still need to run a dehumidifier. The reason for this over the poly directly on the floor is to prevent mold and trapped water if it does come in for some reason. The part of the basement where I have used the dry-core is noticeably warmer in the winter than the office area with only carpet over the concrete. I will be putting dry-core down in the office next.

Ellis Vener

Youll need to keep air moving continuously throug hthe basementto keep it dry. one of the problems is warm air and exhalating + cooler concrete = condensation. Keeping your film archive in the basement is not a good idea. Unfortunately I happen to know this from experience.

Liudvikas Bukys

Considering all the trouble youre already going through, Id consider an underlayment with dimpled plastic that creates an airspace immediately next to the floor (for insulation and air circulation or at least isolation), then put your foam underlayment over that.

Les Jones

October 14, 2008 @ 10:43 am

Moving water away from the house is critical. In most cases the water in your basement comes from your own roof. Keep the gutters clean so they dont overflow onto the foundation. Route the downspout extensions far, far away from the foundation using long extension tubes. You can always run them underground if your wife doesnt want to look at them. Ideally the ends of the downspout extensions will be a foot or more below the foundation so you have positive flow.

You want soil up against the foundation, sloping away from the house.

When you install water heaters, washers, driers, etc. on a concrete slab, be sure to put plastic pans under them. Most people think the appliance pans are to catch water. Theyre actually to keep moisture from the concrete from rusting the equipment.

You got good advice about not using hardwood on a slab. Hardwood on concrete is a recipe for heartache, particularly in a basement.

The usual alternative is Pergo, which is an engineered wood with a plastic top from Sweden. Its installed on top of plastic sheeting and thin foam insulation, pretty much as you described. The stuff in my house is 10 years old and has no problems. (For all other applications, however, I think hardwood is preferable. Pergo eventually gets scuffed up and unlike wood Pergo cant be refinished.) Another option is ceramic tile.

tony

October 14, 2008 @ 11:12 am

also be careful what kinds of adhesive they use to put down your engineered wood productsome of them release some serious VOCs which are not at all good for you!

Philip Greenspun's Weblog

Sealing a concrete slab to block humidity from entering basement?

philg — October 14, 2008 @ 12:14 am Filed under Uncategorized

Tomorrow is moving day, saying goodbye to Cambridge and hello to Lincoln, Massachusetts.  Our new house requires a little history lesson.  In the bad old days people dug down into the earth to make a basement foundation for a house.  For hundreds if not thousands of years these primitive people stored roots, boxes, and other junk down in this basement while they lived in various floors built well above the ground level.  Above the top floor they built an attic in which to store more junk.  This attic became very handy in the 1970s when people looked for a place to put insulation, a whole-house fan, and other energy-saving devices.

Fortunately for mankind, in the 1960s a group of MIT and Harvard architecture geniuses came along and designed the Deck House kits.  Why have a contractor assemble building materials and the local drunkards on-site to frame a house when the thing could be precision-cut in a factory?  Storing carrots and boxes in your basement?  Why not live in the basement instead?  The typical Deck House, including the one we are moving into, is cut into a hillside so that part of the basement is walk-out level.  Half of the living space is in this first floor, directly on top of a concrete slab.

How about an attic?  Wasteful!  Just make the living room ceiling the underside of the roof.  The roof in this house is about 5 inches thick from the bottom of the living room ceiling to the top of the asphalt shingles.  Im hoping that there is some insulation in there, but am not confident.  In any case, there is no way to add insulation without redesigning the roof to be about twice as thick.  There is also no obvious place for a whole-house fan.  Storage?  Dump it all in the garage.

What happens when you live in your basement?  It tends to be kind of damp.  How about cranking up the central A/C to dehumidify it?  The geniuses behind the original Deck House didnt see the point in running ductwork.  As there are no conventional floors or walls, it is not obvious how to run ducts after the fact.

I am now on a fanatical quest to dehumidify the basement, partly because I want to store some slides and negatives down there.

My first idea is to minimize the amount of moisture coming into the basement.  A landscaping company is coming over to dig away a small portion of earth against the foundation and see if it is moist enough to warrant a French drain (pipe with holes in it).

I know that concrete breathes and am wondering if humidity is coming up through the slab, which was poured in 1968 as part of an outrageously expensive construction project ($38,000 to build 4000 square feet; thank God that the government assures us inflation is minimal or I would be upset by the insurance companys estimate of $800,000 to rebuild the place today).  I have instrumented the house with a Honeywell weather station from Costco (am now a true suburbanite).  The Honeywell is saying that the humidity is about the same throughout the house, at least this evening.  Without the heat being on, it is a dank 57 percent relative humidity upstairs and down.  So we have some evidence that water vapor is NOT coming up through the slab.  A civil engineer friend says that basements are damped simply because they are cooler than the rest of the house and the humidity mostly comes from the outside air.  Nonetheless, I feel compelled to attempt to seal the slab.

The slab is currently covered by some glue, a porous carpet pad, and a medium-thick carpet.  She who has the last word on home decor says carpet is disgusting; I would pay as much as $10,000 to have the carpet replaced by hardwood.  I replied So I guess we wont do it, then, because the quote for engineered hardwood was $15,000.  It seems that we are doing it

The standard way that flooring companies put down hardwood over a basement slab is by not using hardwood.  A solid chunk of wood tends to warp.  The solution for potentially humid conditions is particle board or plywood topped with a thin wood veneer.  It looks like regular hardwood, but is resistant to warping.  The installation starts by taping down a closed-cell foam underlayment, which is more or less impermeable to water vapor, and then floating the floor on top of the foam.  An engineer friend recommends 6 mil polyethylene sheeting underneath the foam, overlapped by at least one foot at the seams.

My question to the suburban heroes reading this is Why not seal the slab with a chemical before putting down the sheeting and foam?  If we dont do it now we will not have another chance for 20 years.

Then the question becomes how to seal concrete.  Drylok is the standard product, but Im not sure what value it would be on a 40-year-old slab.  I talked to a company down in Texas, where they should know something about humidity, called Enduroseal.  They recommended

  1. cleaning to remove the adhesive with a product from franmar.com
  2. treating with their own Hydraloc product
  3. waiting 10 days
  4. treating with their NCS-20 product (not sure why they recommended this on the phone; the current slab does not have any flowing water on it and actually feels dry to the touch)

Anyone out there competent to evaluate this companys claims or have personal experience with this stuff?  Is what they are selling better than Drylok from Home Depot?

Stepping back the larger question is whether it makes sense to seal the concrete slab at all.  It is dry to the touch and the carpet feels mostly cool rather than damp.  There is a slight musty smell in the basement and slight mold growth on a couple of beams.

[This is outside the scope of the current blog posting, but the next planned step after the sealing and hardwood is to install some sort of central plumbed-in dehumidifier.  There is a small utility room at the center of the basement.  One idea is to put a standard HVAC air handler in this room.  Run ducts up to the top floor and discharge air up there.  Have the intakes in a couple of the rooms on the ground floor, thus forcing the air to circulate from top to bottom of the house.  Honeywell and Lennox both make whole-house dehumidifiers that plug into air handlers.  Run A/C in the summer and the dehumidifier in fall and spring.  While we’re at it, add an air filter and a humidifier for December through March.]

33 Comments

Ethan

demetri

I have a finished basement with the slab covered by thick plastic sheets, plywood and hardwood strips anchored to the plywood and it is completely dry and warm. The work was done about 20yrs ago by the previous owner and having seen a portion of the slab itself doubt it received any treatment. The fact that the basements walls are covered by sheet rock may also contribute significantly to the feeling of dryness.

Larry

You dont want to solve moisture problems by sealing concrete as they all will fail, according to many guys I had out to look at my basement. We had actual water coming up in several places and moisture for three seasons. I put in a perimeter drain and a sump pump which has solved the water seepage. I am overlaying the floor with dry-core which is a product with a dimpled plastic laminated to 3/4 OSB. This creates an air gap where water vapor can escape into the living space. I still need to run a dehumidifier. The reason for this over the poly directly on the floor is to prevent mold and trapped water if it does come in for some reason. The part of the basement where I have used the dry-core is noticeably warmer in the winter than the office area with only carpet over the concrete. I will be putting dry-core down in the office next.

Ellis Vener

Youll need to keep air moving continuously throug hthe basementto keep it dry. one of the problems is warm air and exhalating + cooler concrete = condensation. Keeping your film archive in the basement is not a good idea. Unfortunately I happen to know this from experience.

Liudvikas Bukys

Considering all the trouble youre already going through, Id consider an underlayment with dimpled plastic that creates an airspace immediately next to the floor (for insulation and air circulation or at least isolation), then put your foam underlayment over that.

Les Jones

October 14, 2008 @ 10:43 am

Moving water away from the house is critical. In most cases the water in your basement comes from your own roof. Keep the gutters clean so they dont overflow onto the foundation. Route the downspout extensions far, far away from the foundation using long extension tubes. You can always run them underground if your wife doesnt want to look at them. Ideally the ends of the downspout extensions will be a foot or more below the foundation so you have positive flow.

You want soil up against the foundation, sloping away from the house.

When you install water heaters, washers, driers, etc. on a concrete slab, be sure to put plastic pans under them. Most people think the appliance pans are to catch water. Theyre actually to keep moisture from the concrete from rusting the equipment.

You got good advice about not using hardwood on a slab. Hardwood on concrete is a recipe for heartache, particularly in a basement.

The usual alternative is Pergo, which is an engineered wood with a plastic top from Sweden. Its installed on top of plastic sheeting and thin foam insulation, pretty much as you described. The stuff in my house is 10 years old and has no problems. (For all other applications, however, I think hardwood is preferable. Pergo eventually gets scuffed up and unlike wood Pergo cant be refinished.) Another option is ceramic tile.

tony

October 14, 2008 @ 11:12 am

also be careful what kinds of adhesive they use to put down your engineered wood productsome of them release some serious VOCs which are not at all good for you!

Philip Greenspun's Weblog

Sealing a concrete slab to block humidity from entering basement?

philg — October 14, 2008 @ 12:14 am Filed under Uncategorized

Tomorrow is moving day, saying goodbye to Cambridge and hello to Lincoln, Massachusetts.  Our new house requires a little history lesson.  In the bad old days people dug down into the earth to make a basement foundation for a house.  For hundreds if not thousands of years these primitive people stored roots, boxes, and other junk down in this basement while they lived in various floors built well above the ground level.  Above the top floor they built an attic in which to store more junk.  This attic became very handy in the 1970s when people looked for a place to put insulation, a whole-house fan, and other energy-saving devices.

Fortunately for mankind, in the 1960s a group of MIT and Harvard architecture geniuses came along and designed the Deck House kits.  Why have a contractor assemble building materials and the local drunkards on-site to frame a house when the thing could be precision-cut in a factory?  Storing carrots and boxes in your basement?  Why not live in the basement instead?  The typical Deck House, including the one we are moving into, is cut into a hillside so that part of the basement is walk-out level.  Half of the living space is in this first floor, directly on top of a concrete slab.

How about an attic?  Wasteful!  Just make the living room ceiling the underside of the roof.  The roof in this house is about 5 inches thick from the bottom of the living room ceiling to the top of the asphalt shingles.  Im hoping that there is some insulation in there, but am not confident.  In any case, there is no way to add insulation without redesigning the roof to be about twice as thick.  There is also no obvious place for a whole-house fan.  Storage?  Dump it all in the garage.

What happens when you live in your basement?  It tends to be kind of damp.  How about cranking up the central A/C to dehumidify it?  The geniuses behind the original Deck House didnt see the point in running ductwork.  As there are no conventional floors or walls, it is not obvious how to run ducts after the fact.

I am now on a fanatical quest to dehumidify the basement, partly because I want to store some slides and negatives down there.

My first idea is to minimize the amount of moisture coming into the basement.  A landscaping company is coming over to dig away a small portion of earth against the foundation and see if it is moist enough to warrant a French drain (pipe with holes in it).

I know that concrete breathes and am wondering if humidity is coming up through the slab, which was poured in 1968 as part of an outrageously expensive construction project ($38,000 to build 4000 square feet; thank God that the government assures us inflation is minimal or I would be upset by the insurance companys estimate of $800,000 to rebuild the place today).  I have instrumented the house with a Honeywell weather station from Costco (am now a true suburbanite).  The Honeywell is saying that the humidity is about the same throughout the house, at least this evening.  Without the heat being on, it is a dank 57 percent relative humidity upstairs and down.  So we have some evidence that water vapor is NOT coming up through the slab.  A civil engineer friend says that basements are damped simply because they are cooler than the rest of the house and the humidity mostly comes from the outside air.  Nonetheless, I feel compelled to attempt to seal the slab.

The slab is currently covered by some glue, a porous carpet pad, and a medium-thick carpet.  She who has the last word on home decor says carpet is disgusting; I would pay as much as $10,000 to have the carpet replaced by hardwood.  I replied So I guess we wont do it, then, because the quote for engineered hardwood was $15,000.  It seems that we are doing it

The standard way that flooring companies put down hardwood over a basement slab is by not using hardwood.  A solid chunk of wood tends to warp.  The solution for potentially humid conditions is particle board or plywood topped with a thin wood veneer.  It looks like regular hardwood, but is resistant to warping.  The installation starts by taping down a closed-cell foam underlayment, which is more or less impermeable to water vapor, and then floating the floor on top of the foam.  An engineer friend recommends 6 mil polyethylene sheeting underneath the foam, overlapped by at least one foot at the seams.

My question to the suburban heroes reading this is Why not seal the slab with a chemical before putting down the sheeting and foam?  If we dont do it now we will not have another chance for 20 years.

Then the question becomes how to seal concrete.  Drylok is the standard product, but Im not sure what value it would be on a 40-year-old slab.  I talked to a company down in Texas, where they should know something about humidity, called Enduroseal.  They recommended

  1. cleaning to remove the adhesive with a product from franmar.com
  2. treating with their own Hydraloc product
  3. waiting 10 days
  4. treating with their NCS-20 product (not sure why they recommended this on the phone; the current slab does not have any flowing water on it and actually feels dry to the touch)

Anyone out there competent to evaluate this companys claims or have personal experience with this stuff?  Is what they are selling better than Drylok from Home Depot?

Stepping back the larger question is whether it makes sense to seal the concrete slab at all.  It is dry to the touch and the carpet feels mostly cool rather than damp.  There is a slight musty smell in the basement and slight mold growth on a couple of beams.

[This is outside the scope of the current blog posting, but the next planned step after the sealing and hardwood is to install some sort of central plumbed-in dehumidifier.  There is a small utility room at the center of the basement.  One idea is to put a standard HVAC air handler in this room.  Run ducts up to the top floor and discharge air up there.  Have the intakes in a couple of the rooms on the ground floor, thus forcing the air to circulate from top to bottom of the house.  Honeywell and Lennox both make whole-house dehumidifiers that plug into air handlers.  Run A/C in the summer and the dehumidifier in fall and spring.  While we’re at it, add an air filter and a humidifier for December through March.]

33 Comments

Ethan

demetri

I have a finished basement with the slab covered by thick plastic sheets, plywood and hardwood strips anchored to the plywood and it is completely dry and warm. The work was done about 20yrs ago by the previous owner and having seen a portion of the slab itself doubt it received any treatment. The fact that the basements walls are covered by sheet rock may also contribute significantly to the feeling of dryness.

Larry

You dont want to solve moisture problems by sealing concrete as they all will fail, according to many guys I had out to look at my basement. We had actual water coming up in several places and moisture for three seasons. I put in a perimeter drain and a sump pump which has solved the water seepage. I am overlaying the floor with dry-core which is a product with a dimpled plastic laminated to 3/4 OSB. This creates an air gap where water vapor can escape into the living space. I still need to run a dehumidifier. The reason for this over the poly directly on the floor is to prevent mold and trapped water if it does come in for some reason. The part of the basement where I have used the dry-core is noticeably warmer in the winter than the office area with only carpet over the concrete. I will be putting dry-core down in the office next.

Ellis Vener

Youll need to keep air moving continuously throug hthe basementto keep it dry. one of the problems is warm air and exhalating + cooler concrete = condensation. Keeping your film archive in the basement is not a good idea. Unfortunately I happen to know this from experience.

Liudvikas Bukys

Considering all the trouble youre already going through, Id consider an underlayment with dimpled plastic that creates an airspace immediately next to the floor (for insulation and air circulation or at least isolation), then put your foam underlayment over that.

Les Jones

October 14, 2008 @ 10:43 am

Moving water away from the house is critical. In most cases the water in your basement comes from your own roof. Keep the gutters clean so they dont overflow onto the foundation. Route the downspout extensions far, far away from the foundation using long extension tubes. You can always run them underground if your wife doesnt want to look at them. Ideally the ends of the downspout extensions will be a foot or more below the foundation so you have positive flow.

You want soil up against the foundation, sloping away from the house.

When you install water heaters, washers, driers, etc. on a concrete slab, be sure to put plastic pans under them. Most people think the appliance pans are to catch water. Theyre actually to keep moisture from the concrete from rusting the equipment.

You got good advice about not using hardwood on a slab. Hardwood on concrete is a recipe for heartache, particularly in a basement.

The usual alternative is Pergo, which is an engineered wood with a plastic top from Sweden. Its installed on top of plastic sheeting and thin foam insulation, pretty much as you described. The stuff in my house is 10 years old and has no problems. (For all other applications, however, I think hardwood is preferable. Pergo eventually gets scuffed up and unlike wood Pergo cant be refinished.) Another option is ceramic tile.

tony

October 14, 2008 @ 11:12 am

also be careful what kinds of adhesive they use to put down your engineered wood productsome of them release some serious VOCs which are not at all good for you!

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