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Community and Q&A

Insulating a 1890s floor from unfinished basement

Joseph Rosen | Posted in Energy Efficiency and Durability on

I am looking to insulate the floor in an 1890s victorian house in Maryland (zone 4) that has an unfinished basement. The floor of the first floor can be quite chilly and its a definite heat loss.

The basement is used for storage and mechanical so we cannot put down insulation on the ground. The other big obstacle is that there is a stream that runs under the house year round (20-40 gallons of water pumped via sump every day). The basement is always cooler than the house and almost always wetter for obvious reasons. I have two major concerns with doing this insulation. The first is that if I insulate I now probably need to control where the moisture goes. I don’t want to insulate and then destroy the insulation with moisture. The second problem is that if I put in a vapor barrier I am concerned that the wood floors in the house will shrink and split; this does happen in the winter when the forced air heating is running.

My though is to use fiberglass batts for fitting in between the floor beams (yes actual hewn beams, its an old place). Put the batts in with the paper or foil face up to the warm side. I am not sure I need any other barrier since I do want the moisture to flow up in to the floor boards. Correct me if I am wrong about this.

If I do need a vapor barrier does this mean I need to add a humidifier? I could test the water from the sump and see if its safe to use that (being green and all that jazz).

I am trying to minimize the amount of change I do because I am concerned about creating unforeseen problems. Let me know what you think or if you need more detail.

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  1. Expert Member
    Dana Dorsett | | #1

    The shrinkage and splitting is from excess air infiltration, unrelated to the R-value of the floor. Quite a bit of that infiltration is usually traceable to the foundation, foundation sill and band joists, typically adding up to more than all window & door crackage combined. There are probably other major leakage aspects to this house to address, but the band joist & foundation would be a good start.

    Rather than insulating the floor, insulate the foundation walls using 2" of HFO-blown closed cell polyurethane from the slab all the way up to the plank subfloor. HFO blown foam runs ~ R7/inch x2" = ~ R14, in a climate zone where the IRC currently calls out R15 continuous insulation for basement walls. That will reduce both the air infiltration and seal the wall from bulk water coming through the wall. Closed cell foam is completely waterproof, and will tolerate the occasional flooding event should your sump pumps fail. In most locations an intumescent paint sprayed on the foam is sufficient to meet fire code, but verify that with the local code officials before taking this approach. In my area 2" of closed cell runs $2-2.25 per square foot of coverage.

    If it's a poured concrete (not likely) or fairly flat brick foundation (possible), there may be rigid foam options, but the amount of air sealing detailing goes up, and it would still be better to seal & insulate the sills & band joists with spray foam.

    Insulating the basement slab/floor isn't really necessary. Once the basement is air tight it will be quite a bit warmer down there and the floors correspondingly warmer, especially if you have a furnace and uninsulated ducts down there with duct losses heating the basement.

    If insulating the floor with batts it doesn't matter which side the vapor barrier goes, but leaving the facers exposed to the basement would usually violate fire codes unless put up a ceiling with sufficient fire ratings (half inch sheet rock would be fine.) The code minimum R value for floors over unconditioned space is R30, or if that's not possible due to shallow joist beams, fill the entire depth (R19 minimum.) With non-standard joists such as yours batts don't usually fit well without spending a lot of time carefully sculpting them for fit with a batt knife.

    Sealing the ducts, duct seams, joints and sealing the duct boots to the subfloor and taping the seams of the air hander will give better control over air-handler driven infiltration. It's also important that the duct design is well balanced, with adequate returns for every supply register so that pressure differences greater than 0.025 water inches never occur between adjacent rooms, doors open or closed. (Energy Star homes can't exceed 0.012 water inches room to room pressure differences, and those house also have less air leakage paths to the outdoors, and are already less susceptible to air handler driven infiltration.) This really matters in a home of that vintage, especially when you have wintertime dryness sufficient to show up in the wood flooring.

    Adding a humidifier is always a mistake. Spend the humidifire money on blower door and infra-red imaging guided air sealing. It takes quite an effort to air seal an antique like that enough to REQUIRE active ventilation. Right now you have too much passive ventilation, which is the whole problem.

    With your high water table no matter what you will likely need to run a DE-humidifer in the basement to limit the humidity levels to reasonable levels, but the amount of mechanical dehumidification required goes down if you air seal the foundation and band joist, since summertime outdoor air dew points are higher than your summertime basement temperatures in your area. Insulating the foundation walls increases summertime basement temps a few degrees (and wintertime by several degrees), but more importantly, it blocks humid summertime air from entering the cool basement, raising it's relative humidity.

  2. Joseph Rosen | | #2

    Follow up. There is no slab. The basement was dug in 1890. The foundation walls are brick and rock. There is approximately 12" of above grade wood wall above the foundations and a couple of old windows (that would need to be sealed shut in your plan. I was considering insulating the floor of the first level from the basement so the first floor is warm without causing problems. The bulk water does not tend to come through the walls. It comes up through the floor which is basically at the level of the stream. If I use close celled foam, seal the windows, close up the vents that were installed, etc. are there problems I should look out for since I am changing the internal environment of a house that is 130 years old?

  3. Expert Member
    Dana Dorsett | | #3

    If there is no slab it's not clear how the sump is configured?

    You definitely NEED to "...change the internal environment of a house that is 130 years old..." if you want to lower the mold risk in the basement and stop the seasonal shrinking & expansion of the flooring, etc.!

    If the basement currently has vents to the outdoors, they are bringing more moisture INTO the basement than it's purging in a zone 4A Maryland climate, AND they are adding MASSIVE air infiltration that makes the basement and first floor cold!

    With a dirt floor you're in a good position to dig a perimeter drain that drains into the sump, lay down 3-5 " of washed gravel with an inch or two of sheet EPS on top of that followed by a layer of 6-mil polyethylene sheeting and a 2" concrete non-structural rat slab. The sheeting needs to lap up the sides of the slab to above slab level. The perimeter of the slab can be insulated with 2" EPS to about the high-tide mark of any flooding that has occurred historically, with sprayed closed cell foam above that.

    Replace the windows if the sash & framing are rotting. If they are in good shape, add tight fitting exterior storm windows. Low-E storm windows would be even better, but that's not critical here. Or if it fits with the exterior appearance, yard out the windows and replace them with glass block mortared into place, which is more air tight than windows.

    A handful of years ago I was involved with a deep energy retrofit on a house built in 1890 that had a brick + rubble foundation, and the basement was done with 2" of closed cell on the foam with intumescent paint, 1.5" of XPS under sheet polyethylene under a 2" slab, and the windows were replaced with glass block (non-operable daylighting).

    A handful of years ago I was involved with a deep energy retrofit that had a brick + rubble foundation in zone 5A. It had many other above-grade updates too, but the basement is now completely dry with never even a hint of "musty basement smell". The headroom was pretty low in the original house, so the owner (a sports training coach at a local university) hired a few of his athletes to dig down 6" to accomodate the gravel, foam & rat-slab. The project even got a bit of local press (unfortunately no pictures made the archive):

    The final blower-door test on the house came in at ~600 cfm @ 50 pascals- it's definitely a tight basement and a tight house.

  4. Joseph Rosen | | #4

    Thank you for all your help Dana. The sump is in the deeper of the two wells dug under the house (the other was covered) and in a second lined hole dug in the lowest corner. The deep sump is set out about 5 feet from the foundation wall (because thats where the well was). I will definitely take your suggestions to my father but I am not sure there is a desire to do that much work. I think I can convince him to air seal the whole basement and insulate those walls. Also storms on the windows. I don't think an insulated slab is viable in the space due to it ending up being a lot of work/money but I'll pitch it. There is also the problem that its a historic house and the rules governing what can be done are extremely restrictive.

    I did ask what the temp is in the basement and its hovering around 66F. I think with air sealing and insulating the walls we will get the floor warmth he is looking for and lower energy costs substantially.

  5. GBA Editor
    Martin Holladay | | #5

    As I'm sure you know, and as Dana's answers have made clear, your basement is a mess. If you are ready to fix it once and for all, that's great. It's going to take some money.

    It's essential that you find a way, at a minimum, to install crushed stone over the existing exposed dirt, and to install a durable vapor barrier above the crushed stone. Below that vapor barrier, you need a sump with an airtight lid. The sump needs a sump pump, or a way to drain to the exterior -- but you have that already.

    Above the vapor barrier, you have options. The best job would include continuous rigid foam and a concrete slab above the vapor barrier.

    If you have to temporarily remove the furnace and other mechanical equipment to install the elements I just described, you can do that. Don't be scared of the work -- it's not helpful to say, "I can't move the furnace." In fact, you can.

    Once you've fixed the floor, it's time to address the walls and windows. It's essential to stop air leakage, and adding R-value to the walls would certainly be a good idea. I'm guessing that you will end up using closed-cell spray polyurethane foam.

    You need to erase the idea from your head that a damp basement is good for your floorboards. It isn't. You want a dry basement. Addressing your air leakage problems will help keep your floorboards from drying out.

    Here is a link to a relevant article: Fixing a Wet Basement.

  6. Stephen Sheehy | | #6

    When we decided to build a new house, we realized that selling our 200+ year old house would require we do something about our basement. It was a lot like Joseph's. Rubble foundation, dirt/mud floor, cold air flowing through. Our stream was intermittent. We bit the bullet and had it insulated, proper drains and sump pump installed, walls insulated, heavy poly on the floor.
    The difference was dramatic. No more drafts coming through the floor, no more mud. The place could be used to store stuff. Other than a concrete slab and insulation on the floor, we did what Dana and Martin suggest. It was well worth it.

  7. Rich Cowen | | #7

    I have a similar basement and have concerns about the foam-on-wall solution proposed by Dana. If your basement walls are not well sealed they are wet and may from time to time need application of new mortar and water proofing material. How are you going to get to those walls if they are covered in foam? The foam will be fine for 10 years but what would it look like if 20 years or 40 years go by and moisture from the outside seeps behind the foam. Would a future owner have to strip and reapply the foam at some point?

    This is a question that was never answered to my complete satisfaction. I think you will have to cover up the foam fairly well to avoid deterioration of the outer surface of the material as well. The paint is relatively expensive as you know and that is why some people recommend putting a real wall in which may be beyond your needs for the space..

    I do however agree that getting rid of the water in your basement is a priority. We dug a 20" trench in the back of the basement which was where water would come from, and run a french drain 70 feet away into a lower part of the yard, a gravity drain. Trench is filled with crushed stone. This eliminated the potential for standing water in wet years but did nothing about the moisture. The excess moisture did go away once we dug down 11 inches, removing the dirt and remnants of slab and put in a new slab on top of plastic and 3 inches of crushed stone.

    Also we had moisture coming through the walls. To reduce that moisture without spending $5000 on foam(it is a fieldstone wall) we chipped away loose mortar, and the masons filled gaps with new mortar and topped everything off with an all masonry based product called BASF Thoroseal. This is inexpensive when purchased in powder form and made the walls a nice bright white color. This was recommended by a mold remediation guy. See the product here:
    [youtube link removed due to spam filter] Now if there is any future deterioration of the wall we can chip the bad spots away and reapply mortar as needed.

    Finally on your insulation issues: I do agree that the application of foam to seal up your rim joists and/or sill is critical. If your basement is in the 60s now it might get down to below 60 in mid February so some ceiling insulation is warranted.

    I wonder if a product like Tyvek Thermawrap applied to your ceiling would work. The cost I saw was $220 for 160 square feet so perhaps. If the ceiling doesn't have a lot of obstacles, maybe this would be easier to apply than fiberglass, considering your timbers are not spaced at exactly 16" and they probably are not 8" high either. I know it is recommended for outside walls, not ceilings.

    rough cost of doing a new slab plus thoroseal plus spray foam in rim joists could be less than $10000.

  8. Jon R | | #8

    In addition to improving the basement, if you want an even warmer floor, you can install radiant heat plates with fiberglass batts below them.

    I'd also air seal the floor - so in warmer weather you can dehumidify the cooler basement independently of the rest of the house.

  9. Expert Member
    Dana Dorsett | | #9

    Rich: Depending on the brick and mortar type, a sacrificial lime mortar parge over the interior side may be worth it. But putting the foam over the mortar PRESEVES it, since it reduces the rate of moisture flow (and the mineral leaching which weakens the mortar.) On the c1890 house with for the deep energy retrofit I was involved with more than one masonry person was consulted, and both concluded that a sacrificial parge would not be necessary or useful, and that it was good for another century (at least), with or without the foam.

    Jon R: Depending on how the plank subflooring and finish flooring were installed, radiant floor heat may make the seasonal crackage issue even worse (but it'll certainly be more comfortable!)

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