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Insulating basement - questions

jtmann's picture



I am puzzled as to how to proceed with the following project and thought that those here at the forum might have some insight!


I live in a cold climate. I have a two-storey house with a full basement. It is a completely UNfinished basement (i.e., no sheet insulation or wall framing on the basement concrete walls, nor any batt insulation in the open ceiling joists). The basement is unheated and fairly damp. But not a big deal as I use it only for storage.


But I wanted to see if I could make the first floor of the home (above the basement) warmer, especially the floors. Here’s the twist - I will be re-selling within three years and don’t want to spend to level that I won’t get a return on investment when I re-sell. I already spoke to my local building code inspector and he said if I were to put Styrofoam sheets on the basement walls I would then have to go the two extra steps of adding wall framing and then drywall (I assume as a fire retardant). There is no way this will pay out for me as it is an unfinished basement and that’s what most basements in my area are…..not an option.


So then I thought I’d put fiberglass batts in the ceiling joists and cover with vapor barrier on the cold side (facing basement) - clear plastic sheeting to keep the dampness from the basement from getting into the batts. But then I remembered that vapor barriers are typically installed on the warm side, so that might just make the warmth from above/moisture from above collect on the vapor barrier....not sure what to do there...


Any advice from the forum as to how I could do the batts in the joists approach? What about doing two layers of vapor barrier to totally isolate the batts – i.e., one above the batts on the warm side to keep the warmth/moisture from that side and on the other side to keep the dampness of the basement away?


Suggestions or comments on a solution much appreciated!!




P.S. Strangely, my local code does not require drywall on the ceiling joists/batts – go figure.

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ChalmersAbrams's picture

I don't have proper knowledge about insulation of house please provide me some information as I am going to buy a house in my near future ..Thanks.

Nestor_Kelebay's picture

I just thought I'd add several things about insulation that people might want to know.
By definition, an insulation's ability to stop heat loss is called "R Value".
And, by definition, the R value for wood is 1.0 per inch.  So a 1/2 inch thick piece of plywood would have an R value of 0.5 whereas a solid core wooden door 1 3/4 inches thick would have an R value of 1.75.
The R value of fiberglass batt insulation is about 3.5 per inch.  For expanded styrofoam insulation (the "bead" stuff) it's about 4.5 per inch and for extruded styrofoam it's about 5.0 per inch. 
Be very careful about anyone trying to sell you foam insulation for your house that tells you the R value will be any higher than 5 per inch.
The reason why is that it's the blowing gasses that are used to make the foam that increase the R value tremendously when the insulation is first made.  As the blowing gasses inside the insulation is replaced by air in the first year or two, it's R value drops down to a "stabilized R value" which is typically from 4.5 to 5.0
So, if someone tells you their foam insulation will provide an R value of 10, then they're either talking about the R value when the insulation is first manufactured, or they're presuming a thickness of two inches or thereabouts of insulation.
Most people are unaware that the R value of foam insulation diminishes rapidly after it's made, and so there are unscrupulous contractors that will misrepresent their product by quoting "as manufactured" numbers as opposed to stabilized R values.  Any company can hire any testing lab to measure the R value of their insulation, and that way they can provide you with independant laboratory test results that are, nonetheless, misleading.

Nestor_Kelebay's picture

J. T. Mann:
The question you're asking is a perfectly reasonable one if one understands insulation.
The whole point of putting a vapour barrier on the WARM side of the insulation is to prevent warm moist air from getting into fiberglass insulation.  That's because there will be a temperature drop through the thickness of the insulation that equals the temperature drop from the indoors to the outdoors, and at some point any warm, moist air passing through that insulation will condense to form water and frost inside the insulation.
The danger in water and frost forming inside the insulation is not that neither one is a good insulator and that there will be more heat loss.  Rather it is in the fact that insulation works by keeping air stagnant and preventing convective air currents that would otherwise form if the insulation weren't there.  Consequently, when fiberglass insulation gets wet, the lack of air flow through the insulation hinders the drying out of that moisture, and the insulation stays wet for a long long time.  And, any wood studs or joists that are in contact with that wet insulation will absorb water from it, and also stay wet for a long time.  That's the kind of situation that lends itself to wood rotting.  When wood stays really wet for a long time, then the wood rot fungus (latin name: Serpula Lacrymans) starts feeding on that wood, and the result is that all of the cellulose in the wood is removed, and what's left behind is a material called "lignin" which is best understood as the "skeleton" of the wood that forms a thin film around all the wood cells.  That's why rotted wood looks similar, but is very much lighter in weight and weaker than unrotted wood.
Consequently, it is important to put the vapour barrier on the WARM side of the insulation to prevent moist air from getting into the insulation and condensing to form condensation and frost.
But, what happens when the moisture can get in from either side of the insulation, such as in the case of an insulated FLAT roof on a commercial building?  In that case, the warm side of the insulation is clearly under the ceiling drywall on the top floor.  But, in the case of a roof leak, then rain water or snow melt can get into the insulatoin wet by dripping onto it from above.
In any case like that, then the WORST option is to use TWO layers of vapour barrier; one on each side of the insulation.  The reason for that is that when (not if) water gets into the insulation between those vapour barriers, it'll never dry out, and that's guaranteed to cause problems with wood rot and the growth of mold and mildew inside the insulation.
In those cases, the best option is to use an insulation that doesn't need a vapour barrier.
Polystyrene insulation comes in two forms; expanded and extruded.  Expanded polystyrene is the "beaded polystyrene foam" that is typically white in colour and looks like it was made by compressing polystyrene "beads" into the correct shape.  Extruded polystyrene is typically blue or pink in colour and doesn't look like it has any "beads" in it.  Dow Corning's "RoofMate" that's made for insulating flat roofs is an example of extruded polystyrene insulation.
The difference between expanded and extruded polystyrene insulation is that, although both are "relatively" impermeable to both air and water, the beads in expanded polystyrene insulation are for the most part, interconnected, whereas when they make extruded polystyrene insulation they use less blowing gas, and the bubbles in it don't expand to the point whee they interconnect.  Consequently, extruded polystyrene insulation is both heavier and less permeable to air and water than expanded polystyrene insulation.
So, while both make for effective insulations, expanded polystyrene has a greater likelihood of having condensation and frost forming inside it than extruded polystyrene insulation that has ZERO, NO, NADA, ZILCH chance of having any condensation or frost form inside it (because it's quite impermeable to air and water).
So, in a case like your basement floor, instead of using fiberglass, it would make much more sense to use either expanded or extruded polystyrene insulation between the floor joists as the extruded absolutely doesn't, and the expanded seldom does, need a vapour barrier.
     The way you would physically install the insulation is with something called "stick clips" which look like steel pins sticking out of the middle of a flat steel base.  You would glue those flat steel bases to the underside of the wooden subfloor between the floor joists, and then literally impale the insulation onto those steel pins, and then push "stick clip washers" onto the short length of pin sticking out below the bottom of the insulation.  Those washers push on, but won't pull off, so they hold the foam snugly against the underside of the floor.  And, if you wanted to do a Cadillac job, you would also use expanding foam between the insulation and the floor joists.
However, truth be told, it makes more sense to insulate your basement walls than your basement ceiling.  That's because when someone does refinish that basement and insulate the walls, then the money spent on insulating the basement floor becomes "wasted" money.
You are correct in that drywall IS an effective fire barrier.  However, the purpose in installing drywall in a basement is only to finish the basement walls so they look "civilized".  The wood strapping behind the drywall in a basement wall does not support the house, and so little would happen to the occupants or firefighters inside the house if that wood strapping were to burn.  However, the exterior walls of a house are often built with 2X4 and 2X6 lumber studs, and if those studs were to burn, then the house would collapse on anyone inside it.  Drywall has a fire rating of half an hour, meaning that in the event of a fire, drywall will prevent any wood behind it from starting to burn within a half hour, thereby giving any occupants ample opportunity to escape from the house.
So, your best bet is to save your pennies and eventually finish the basement walls with either fiberglass or even polystyrene insulation.  That will increase the market value of your house very much more than insulating the basement ceiling ever would, especially to someone who intends on finishing that basement.
Post again if you want to know how drywall protects wood studs from fire, and therefore why drywall has a much better fire rating than other wall coatings, such as wood paneling.

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