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Insulating basement ceiling - your recommendations


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By James in Toronto, ON on 11/20/2009


Insulating basement ceiling - your recommendations?

Hello:

I am looking for recommendations from the forum regarding insulating an unheated basement. I live in eastern Canada - cold winters! I recently bought a small (approx. 1,400 square feet) two-story home (three-year-old home) that I will live in for a few years while I build my custom home. I would like to make my first floor floors (laminate and vinyl) warmer and increase the overall warmth/efficiency of the first floor (living room, kitchen, hallway). I am considering the following:

-- Insulating the basement ceiling with fiberglass insulation batts. The ceiling is open.

-- NOT insulating the basement walls - currently no insulation on the basement walls (i.e., no sheet Styrofoam)

Remember the basement is currently unheated, and I will not heat it - it is storage space only

I only want to do one insulation technique in the basement due to budget (i.e., insulate the floors with batt OR insulate the walls with sheet Styrofoam)

My questions for you are:

Q1. Is my rationale right that batt in the ceiling will make the first-floor floors warmer/improve heat above MORE THAN doing the basement walls with Styro sheets? My rationale is that batt in the ceilings is directly insulating the first floor floors whereas Styro sheets on the walls is indirect.

Q2. IF I do the basement ceilings should I use R-12 or R-20? I read on a government website that they recommend -12 but I think they assumed that the basement walls were already done with Styro sheets

Q3. If I do batt the ceilings in basement, I know it will make my basement colder than it is right now but I don't care - it is only a storage space. But if I use R-12 or R-20 ceiling batts without the walls having Styro, should I worry about the basement becoming so cold that my water pipes freeze? Or am I overworrying?

Thanks - great forum!


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By Mike in Dover, DE on 11/21/2009


If you insulate the walls rather than the ceiling, you are effectively extending the building envelope into the basement.  If it's airtight, then you'd likely see a reduction in heating bills - you have more insulation between the cold outdoors and the envelope than you had previously.  Also, your floor joists would be entirely within the heated environment, so they should be warm.

If you insulate the floor, the joists will act as thermal bridges - wood has little insulating value.  The overall insulation value will be reduced due to this bridging.

However, batt insulation is much cheaper per R-value than rigid foam.  So spending equal amounts in material, you will get much higher R-valued batt insulation.  This may mitigate the effect of thermal bridging.  I recently ran cost comparisons using local big-box store prices.  Rigid foam prices were about $.10/R-value/sq ft, whereas fiberglass ran about $.025/R-value/sq ft.  So you can get 4x the R-value or coverage with fiberglass for the same cost.  Really, rigid foam makes sense only where you need insulation that is resilient to the presence of moisture, or you need a thin profile.

It's important to consider the airtightness of your basement.  If it's leaky/drafty, it's best to seal it up with either choice.  Also, if you choose batt insulation, I'd recommend getting the kind that is wrapped completely in plastic.  Convection currents are the enemy of batt insulation, and with them not being installed in a sealed cavity, they'll be much more effective in plastic.

Consider, too, if you have HVAC ducting or elements subject to maintenance within your floor joist system.  Ducts fully within the envelope can make your system much more efficient (as duct leakage is common) and with insulation on the walls, it will be out of the way whenever ducts/pipes/wiring/etc. need to be accessed.

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By James in Toronto, ON on 11/23/2009


Mike:

Thanks for your reply. Re your comment "The overall insulation value will be reduced due to this bridging" - how much do you think this will reduce it? I assume some small amount but not enough to cancel out the insulation value gained from all the ceiling/first floor floorboards now being insulated?

Thanks!


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By James in Toronto, ON on 11/23/2009


One small addition to my original post:

Boy, I am confused. I have been researching various owner-builder topics for past few years and this topic of where/how to insulate a basement has the most varied opinions I have seen of any topic!

BTW, an article in the July issue of The Journal of Light Construction that talked about unvented crawlspaces and different insulation methods: foam board on the crawlspace walls versus fiberglass batts between the floor joists. A testing company tested multiple homes in two different climates: Baton Rouge, La., and Flagstaff, Ariz. The study was looking at the energy use and humidity levels. Here was the conclusion: “To save energy, closed crawlspaces in warm climates should have insulation installed on the walls; in cold climate, the insulation should be placed in the floor system, even when ductwork is located in the crawlspace.”

Many conflicting sources it seems! Thoughts from the forum appreciated!

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By Kenneth in Lees Summit, MO on 11/23/2009


Construction techniques do indeed vary depending on environment. I would suggest that JLC is a pretty reputable source.
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By David in Greenville, SC on 11/23/2009


Hi James,

You're very right when you say that this topic brings widely varied opinions! However, looking at your situation, I would suggest insulating the attic ceiling. Why? Because properly insulating the basement walls including over the sill plates and up over the rim joists to the attic ceiling is typically much more difficult and costly than it sounds! And even if done correctly, it won't automatically make the space so warm that your floors will be any more comfortable than they are now. Insulating the attic ceiling on the other hand is generally reported to increase first floor comfort although it doesn't typically net much if any energy savings. Either way, the energy savings will be very small compared to the cost of the process of insulating. Since the energy return on investment will be way too small to recoup your money in the short time you plan to live there, comfort and cost become the primary decision-making factors. Where cost is concerned there is no question that batt insulation in the attic ceiling is the winner. Also, while the evidence is anecdotal, attic ceiling insulation generally gets the nod for making the floors feel warmer. This increases comfort. More comfort for less money would seem to make insulating the attic ceiling the winner and is what I would do in your situation. Now, if I were building from scratch or planned to live there for 30 years then my opinion might change!

At any rate, I saw in another one of your posts that you questioned an assertion that thermal bridging would negate the overall benefit of the insulation. Thermal bridging is often a concern, however, in this case it won't make much difference. While wood doesn't have great R-value, it does have some, to the tune of about R-1.25 per inch. If you have typical 2x10 floor joists, which are 9 1/2 inches deep, then the joist will have an R-value of about 11.8! Add in the values for the subfloor and the minimal amount for the flooring itself, and you have very nearly an R-13 through each joist. Plus, solid wood doesn't leak air, so this R-value is unaffected by convection, which is the most detrimental of all energy-loss transfer methods. On the subject of how much insulation to use, information taken from a Montana state-government site recommended R-19, if that helps any. Also, the most common advice I have found is to use either unfaced (no vapor barrier) batts or to install faced batts with the vapor barrier against the floor side of the cavity.

Hope that helps.


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