What are the consequences of a well insulated basement?

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lmei007

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My basement is an old (more than 50 yrs) poured concrete basement, unfinished, no water issue but has lots of effloresces (moisture issue). It is a dry "warm" basement in the winter and dump "cool" basement in the summer (didn't use dehumidifier yet). There is no heating during the winter but the temperature always keeps at the range of 52 - 62 degree all year round.

I am thinking about finish it and have done lots of research about how to do it. I agree the rigid foam for wall and floor is the best way to go for the insulation. A well insulated basement is good to eliminate condensation in the summer but it is NOT good to get the temperature from the ground during the winter. I am worrying that we have to put heating system in the basement to avoid pipe frozen during the long winter after the insulation (in MA). If that is the case, why we need to insulate it so well?
How about the summer, do we need air conditioner down stair after the insulation?
Worry about condensation during the summer for an un-insulated basement? Use dehumidifier to keep a dry environment may be enough.

So if we think 52 - 62 degree is good in basement, we can just finish the basement without any insulation and vapor barrier. There will be no moisture and condensation problem in winter. During the summer if the dehumidifier works as expected, there should be no moisture and condensation issue as well.

What's your experience and opinion about this?
 

Dana

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Subsoil temps in MA are in the 48-52F range. You won't be sucking much heat out of that in winter, quite the contrary- it's a 24/365 heat SINK, never a source. (You get a modest amount of "free" sensible cooling out of it in summer, but not enough to overcome the mold-problems a cooler basement causes.) Putting up an uninsulated studwall and finishing it would be a recipe for mold problems.

Heat loss is all about the difference in temperature and surface area divided by the R value. When you insulate the walls to R15+ (as you should, anywhere in MA) the heat transfer from the basement to the exterior in winter will balance with the heat transfer from the first floor through to the basement at a much lower R value, for a much lower temperature difference between the first floor & basement than from the basement to the exterior. Bottom line, it'll be much warmer there in winter, but also slightly warmer in summer.

If you've made the basement both insulated and air-tight to the exterior, being 68F instead of 62F is a good thing, since it means the relative humidity in the basement will not reach mold potential with or without your dehumidifier running. The condensation risk in the basement is much higher when NOT insulated, since with 62F surfaces around, and outdoor air dew points in the mid to high 60s, any air leaks into the basement condense, and everything down there molds. At 68-70F, not so much. Since when is 52-62F good in a basement? It's a primary cause of basement mold!

But air sealing the basement is still critical- band joists and foundation sills are notoriously large air leaks, and infiltration is driven by everything from heating equipment flues and dryer vents depressurizing the basement. Putting an inch or two of closed cell foam insulation from the band joist over the sill down to the top of your wall-foam as an air seal does a WORLD of good.

With an air-sealed and insulated basement, if you're air-conditioning the first floor, the basement will only be susceptible at dew points of the conditioned space air, which is much lower than the dew point of outdoor air when the air conditioning is running. With a higher basement temp and lower dew point, the mold risk plummets.

Efflorescence on the concrete is an indication of moisture flow from the exterior toward the interior. Clean it up and use a semi-permeable acrylic or silane based masonry sealer on it before insulating. If possible, re-grade the exterior and use channels/pads French drains to direct bulk water away from the foundation.

When insulating, be sure to use something semi-permeable for the interior like unfaced bead-board EPS, nothing with poly or foil facers, or any other type of vapor barrier, or it can raise the moisture in the foundation to levels high enough to put the foundation sill & band joist at risk. (Good deals on used or surplus 3-4" EPS can be had at the Insulation Depot in Framingham, MA, as well as a few other rigid-board recyclers, some of whom advertise on Craigslist.) Don't use XPS at thicknesses any greater than 2" (R10), but 4" of Type-I EPS (used in roof insulation on big flat commercial roofs, and often recycled) runs ~R15, 3" is good for ~ R12. Be careful about using iso too- you need the specs on the facers to know what you're getting into, and it will vary. EPS is a better bet, and pretty cheap.

To meet code you need to put 1/2" wallboard over the foam as an ignition barrier. If you long-screw furring through the foam to the foundation wall with Tapcons, you can then hang the gypsum on the furring. Lateral or vertical, doesn't matter, just so long as you have the wallboard secured to at least 3 of the furring strips. Don't put anything more vapor retardent than latex paint on it, or you can trap ground moisture in the furring. (No poly sheeting, no vinyl or foil wallpaper, not even oil paint- just latex or acrylic latex.)

On the floor you can use either XPS or EPS, but you also need an ignition barrier over than. OSB subflooring or even 7/16 sheathing tap-conned to the floor works from a code point of view, as does a non-structural concrete rat-slab poured over the foam.

I don't have the headroom to insulate my slab, but when I put R20 on basement walls and insulated & sealed the band joist, tightened up the windows and sealed an abandoned flue in my Worcester home it cut my heating energy use by nearly 20%. The wintertime temp down there never drops below 64F under a ~68F first floor- tends to stay 65-66, despite a 58-60F (cooler in some spots) slab. Being shaded in the afternoons the duty cycle on the AC on the first floor is quite low (less than 20 hours a YEAR), so I do still need to run a dehumidifier down there to keep the RH down to ~ 60%, but it's duty cycle is also reduced (how much, hard to say, since I didn't do a base-line measurement, but it shows up as a somewhat reduced July/August power bills), and running the dehumidifier in the basement keeps the RH on the first floor well bounded too.

What's your zip code? (For more precise weather & subsoil temp data.)
 

Dana

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It's probably worth taking a peek at this thread: https://terrylove.com/forums/showthread.php?38024-flooring-options-for-basement (which ended up being about much more than flooring options.

R10 (2.5") EPS on the wall trapped and sealed to a 2x4 studwall using unfaced batts is a cheap and breathable R20 (after factoring in the thermal bridging of the studs), with zero risk of wintertime condensation anywhere in MA, and is easier to deal with issues like routing your electrical etc. when finishing the basement. It's best to put up the wall EPS first, then put down the floor insulation and subfloor before putting up the studwall. That way you have the least amount of thermal bridging, most-continuous foam, and you can nail the bottom plate of the studwall to the subfloor rather than cold concrete where you'd otherwise have capillary wicking issues to contend with for protecting the studwall plate from ground water issues. The floor & wall foam itself is an excellent capillary break, even if semi-permeable to water vapor.

This thread should really be titled, "What are the consequences of an UNinsulated basement?"

I have some friends in Sterling, MA who finished their basement without insulating, and now have to run an indoor air cleaning system to keep the mold-spore counts low enough, and their (oil) heating bills went up after installing baseboards to keep it warm enough to watch TV in their rec room in winter. Retrofitting it at this point would be expensive, since their stud edges are right up against concrete, and would accumulate moisture & rot if foam or fiber were blown in there- they'd have to rip it out and start over. Don't star in the remake of that movie.
 

lmei007

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Thank you for you valuable input. I will read them carefully again to make sure I understand your instructions and follow them correctly. I have few things to ask right now and hope you can help me figure them out.

As I said our basement is dry but dump. We are in 01803 areas very close to Burlington Mall. We have done two big projects started from last summer: exterior grading and French drain (not down to the foundation) to make sure surface water will have no chance to close to our foundation; interior French drain to make sure no water will come from ground. We have lots of effloresces and they are actively growing and very quick. Few weeks or few months later you will see them again after a simple clean. That means we do have serious moisture issue from the soil because the bad poured concrete. We can see the huge differences between the original 50s’ concrete wall and the 80s’ or 70s’ addition concrete wall. One is flat and smooth with enough cement and water when it was poured; another one is uneven and shows too many gravels and sand were used when it was poured. So I plan to put a layer of fine cement to fill in those small holes to reduce the moisture passages, use something like this Quikrete 50 lb. Non-Shrinking Precision Grout. Do you have any better material to recommend?

I need to clean the concrete surface and remove those effloresce before that. Do you have any recommendation about clean those effloresce? The concrete clean and etch from HD is good enough?

Then I will use semi-permeable acrylic or Silone based masonry sealer as you mentioned to do the final sealing on the concrete wall.
Do you think are they necessary or just use masonry sealer is enough?

After above preparation, I am thinking add one vapor barrier between the concrete wall and the foam. The purpose of that vapor barrier is to trap moisture in between the concrete wall and the barrier. So it will reduce the possible migration from the soil (differential is smaller then) and toward basement space (barrier to stop the movement). The trapped moisture will be absorbed by the concrete wall during the winter, I think. Because there is 2″or 3” foam insulation, the condensation on both sides of the vapor barrier should be not a problem. The barrier will be sealed on the top of the concrete wall and at the bottom of the wall. it should not guide the moisture to the ceiling area because is sealled on the top. What’s your opinion about this extra vapor barrier?

About the XPS and EPS, do you know if the EPS is the same strong as XPS? I know we can walk on top of XPS but not sure about the EPS.

I am in the process cleaning the site and will start the actual preparation soon. Hope we will have a finished basement next winter.

BTW, the used material resources are really good for us to save money.
 
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Dana

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UNDER NO CIRCUMSTANCES should you put a vapor barrier anywhere in the stackup, or you will raise the moisture level of the concrete and rot out the foundation sill supporting the upper floor studwalls! Even kraft facers on batt insulation would be too vapor-retardent, and would result in mold/rot in the basement studwall, and possibly the foundation-sill.

Ground moisture (as water-vapor) to the interior so as not to saturate the concrete. The efflorescence indicates an already high moisture content, and putting a vapor-permeable sealer on it will slow that down some, but the the true waterproofing really needs to be on the exterior (and below ground only.) If excavating for a French drain, use a waterproofing sealer on any foundation that you expose in the process- every little bit of bulk-water waterproofing you can do on the exterior is a HUGE benefit. Read the specs on any sealer you use, a few are under 1-perm at 2 applications and should be avoided. Many won't specify a perm rating, but those always have higher permeability.

I couldn't get the link to work. But your problem isn't holes in the concrete from a lousy pour, it's the inherent capillary-wicking of moisture through the concrete, which is inherent in any standard concrete material. The moisture dissolves some of the mineral content of the cement, and deposits it on the surface as it dries. A masonry sealer reduces the capillary draw by wicking into those porosities keeping the mineral content in the wall, not the surface, but it still allows water to move through the wall as vapor, which is what we want. There's no need to etch the foundation wall prior to sealing. Silane/silex sealers are available through masonry supply houses, but usually not at box-store/home-center type outlets.

The whole point of using semi-permeable EPS and un-faced batting as insulation is to allow the foundation to continue to dry toward the interior. The EPS still needs to be air-tight (foam seal the seams & edges) to prevent convective losses and wintertime transport of moisture to the now-colder foundation, but the R value of the EPS keeps the average temp of the cold edge of the basement studwall above the dew point of the interior winter air, preventing wintertime condensation.

In zip code 01803 with R11 batting in the studwall you can get away with R6 (1.5", if EPS, 1.25" if XPS), but bump that to R7.5-8 (2" EPS, 1.5" XPS) if using R13 batts. (My office is in 01803- hi neighbor! :) ) If XPS, don't go more than 2" (R10) or it will be too vapor retardent. Given that you have known high-moisture issues (as indicated by the efflorescence), I'd recommend unfaced EPS (and not XPS), even at 2" to maximize drying toward the interior. EPS sheeting sold at home-centers usually has a low-permeance facer (foil, poly, or vinyl) on it- don't use that. But roofing EPS is usually just raw bead-board (like cheap coolers or coffee cup) without facers and will breathe sufficiently, but not so much that you'd end up developing a frost layer between the foam & concrete in a Burlington MA climate. (In Burlington VT that could be a problem, but not here.)

With even 7/16 OSB as a subfloor with a concrete slab below the compressive strength of EPS is sufficient even for parking a truck on it, and is just fine for residential floor applications. Putting a poly vapor retarder between the slab and the EPS is a good idea. If you have the headroom, 2" of EPS or 1.5" of XPS (R7.5-8) has a long-term cost effectiveness argument in this climate as well.
 
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lmei007

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A masonry sealer reduces the capillary draw by wicking into those porosities keeping the mineral content in the wall, not the surface, but it still allows water to move through the wall as vapor, which is what we want. There's no need to etch the foundation wall prior to sealing. Silane/silex sealers are available through masonry supply houses, but usually not at box-store/home-center type outlets.


Could you please recommend few Silane/silex sealers to me? I have contacted two Silane/silex sealer producers but their product cannot be used on undergrade vertical wall. how about Xypex Concentrate (http://www.xypex.com/products/product_types.php?pageID=14)? is it one of the Silane/silex sealers you mentioned above?
 

Dana

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I hadn't heard of silane products being inappropriate for walls- it's news to me!

The Xypex is a waterproofer & crack repairer, more than just a sealer, but has a similar chemistry (I think). I think it would do the trick.
 

JohnfrWhipple

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[video=youtube_share;1Zn2g9sWvsQ]http://youtu.be/1Zn2g9sWvsQ[/video]

While building my home we received a ton of rain and the framing and foundation got soaked. This "Fuzz" grew shortly after. The Fuzz is efflorecesence and once the home was sided and heated it went away.

I cleaned it off of course and in two and half years not on patch has returned.

If you are getting this in certain areas of a basement chances are that you have water migration behind your foundation.

JW
 

lmei007

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If you are getting this in certain areas of a basement chances are that you have water migration behind your foundation.

JW

we don't have "water" issue but moisture issue. There is no outstanding water around the house even under the heavy reain because we have a French drain which takes all surface water out from the fundation. We want to reduce/minimize this issue before we finish the basement.
 

Dana

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If you have efflorescence on your foundation, you have a water problem, even if you don't have saturated soil up against the exterior due to the drain. Odds are the water is wicking up into the foundation wall through the footing, which is very common.
 
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