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insulate my "sarcophagus" ?

Post in 'The Green Room' started by RustyShackleford, Nov 1, 2011.

  1. RustyShackleford

    RustyShackleford Minister of Fire

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    My stove is nestled in a solid masonry wall, which rests on a big cinderblock foundation (that my builder cleverly dubbed "the sarcophagus"). I'm curious how much heat the thing is radiating into my crawlspace and thus whether or not it's worth insulating the thing - probably by gluing styrofoam blue-board to it.

    The Stefan-Boltzman law tells me that if the thing is about 90 degrees, and the ambient temperature in the crawlspace is about 40, then I'm losing about 5000 btu/hour due to radiation of heat. This is from this calculator:

    http://www.endmemo.com/physics/radenergy.php

    .. you have to plug in 90 and 40 for the temperture and then subtract.

    I'm not sure how to calculate the loss due to convection though.

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  2. snowleopard

    snowleopard Minister of Fire

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    Or you could just be generous and leave someplace for the possums, snakes, stray cats, and spiders to stay warm this winter.
  3. pen

    pen There are some who call me...mod. Staff Member

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    I think I'd get into the crawl space and see how warm the back of that masonry is compared to the rest of the crawl space and the other side of the masonry before making a decision.

    pen
  4. maxed_out

    maxed_out New Member

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    +1 on pens reply. its a good idea to get your actual numbers and we'll give you some more stuff to consider.
  5. turbocruiser

    turbocruiser Feeling the Heat

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    +2 on pen's reply but I guess I'm getting confused by something ... what exactly is 90degrees? Is it the room the stove sits in, or the wall the stove is installed against, or something other than that? Also, is your crawlspace really 40degrees? That seems super cold. How deep into earth is the crawlspace floor and how high above ground is the crawlspace "ceiling".

    Unless I'm ultimately misunderstanding what you wrote, you are not actually isolating anything; IOW, how much heat is lost to the above grade portion, how much heat is gained from the below grade portion (because if the ambient air temp is 40 and the earth temp is 55, the below grade portion is actually adding in some heat from the earth and at the same time subtracting some heat from the stove), what is the frost line level there, etc.

    At least to start my setup I deliberately decided not to insulate the floor/ceiling between the first floor (where the stove sits) and the crawlspace. My thinking was that I could always add it later and that as long as I wasn't getting that area too warm I actually wanted some warmth in the crawlspace for my plumbing (supply and waste). Anyway if you could clarify those things above it would help me have at least an idea about an answer.
  6. RustyShackleford

    RustyShackleford Minister of Fire

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    Sorry, my OP was most confusing. I am talking about heat loss from the masonry foundation, in my crawlspace, that supports the massive masonry wall that is in the living room and is behind the stove.

    This masonry wall is regular-size cinderblocks filled with mortar. It gets QUIITE warm when I've had a fire going for a few days. The backside of it is perceptibly quite warm, perhaps 100 degrees, but just a guess. The back of the wall is still inside the house (it's forms part of a hallway wall), so heat lost from the back of the wall is not wasted at all.

    This wall sits directly on a huge cinderblock structure in the crawlspace (the "sarcophagus"), and there is no insulation at the junction (level with the floor of the house) between the wall and this foundation. The foundation sarcophagus gets pretty warm too, although not as warm as the backside of the wall. It is the loss from the sarcophagus into the surrounding un-heated crawlspace that concerns me.

    The floor of the crawlspace is pretty much at grade (level with the surrounding land) and the floor of the house is about 4 feet above the floor of the crawlspace. The walls of the crawlspace are regular-size (8x8x16") cinderblocks and not insulated at all.

    Obviously I need to get accurate numbers for the temperature of the surface of the sarcophagus and of the crawlspace. The figures of 90- and 40-degrees were simply guesses that I used to run the Stefan-Boltzman calculation to try to get a ballpark number. I guess a regular thermometer will give the crawlspace temperature, and I probably need an IR thermometer to best get the temperature of the sarcophagus wall. Once I know these two temperatures I'll have a pretty accurate figure for how much heat the sarcophagus radiates into the un-heated crawlspace. The question remains, how much heat is lost from convection into the surrounding air ? If there were a insulating barrier with a certain R-value, I'd know how to calculate it, but since there isn't, I'm not quite sure. Whatever the total heat loss is, if I then insulate the sarcophagus, I'll save most of that heat loss.

    It is certainly true that if I insulate the sarcophagus, the crawlspace will be a lot colder, and I may regret it if the pipes freeze. OTOH, it's not all that cold here, and my pipes are Qest. Perhaps I should be contemplating insulating the crawlspace walls rather than the sarcophagus; it's a much bigger job, but might be worth it to go to a heated crawlspace - actually removing the batts on the house floor.
  7. pen

    pen There are some who call me...mod. Staff Member

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    You are already thinking along the lines of what I'd recommend to you to consider.

    If there were no pipes in there, this would be an easy decision. However, if your stove goes cold, would those pipes freeze anyway?

    I'd say if it's easy to insulate that and you aren't going to have that styrofoam insulation anywhere inside the clearance to combustible range of your stove, then go for it and see what happens.

    I'd do it just for the "someday I'll be old and might not be able to run the stove" factor. Might better know now if those pipes will freeze before you are too old to do something about it. (I'd also have fittings on hand to shut them down / repair / bypass if necessary)

    Meanwhile, I wish I could but I can't tell you how much if any noticeable difference you'll have by doing so. However, every little bit counts I suppose, especially in very cold weather when extended burning is taking place just to keep the house up to comfortable temps.

    pen
  8. RustyShackleford

    RustyShackleford Minister of Fire

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    Thanks Pen. I don't think my pipes are prone to freezing, because certainly the stove's been cold plenty of times when it's cold outside - like when I'm off somewhere out of town at Christmas-time.

    Unfortunately this whole conversation has renewed my interest in the ever-popular, and controversial, topic of having a "conditioned" crawlspace.
  9. glenlloyd

    glenlloyd Member

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    I have spent time in my crawlspace and I don't like it. I'm 6'4" tall and it's just a dry dirty mess in there. I've been in there a lot over the years fixing this and adding that and the less time I have to spend in there hitting my head and back on joists the better. I would rather just point my IR thermometer at the masonry under the insert and let that tell the story....which I can do without having to crawl over the dusty dirty space (that needs to be covered because of Radon I'm sure).

    Why didn't they just put a full basement under this house!!

    steve
  10. karl

    karl Minister of Fire

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    When you say cinder blocks filled with mortar, are you talking about solid cinder blocks or hollow concrete blocks? Are you sure they're filled with mortar? Some pics would help. If the blocks are hollow then insulating the crawl space side may not help as much as you think. Even if they are filled. That's still a lot of mass to wick heat away.

    A radiant barrier film may work better than insulation.

    I have a heatalator set in a concrete blocks. I have an insert and I packed rock wool behind, beside, and above my insert. I have seen a big change in the output of the stove. Perhaps you should put some rock wool in the fire place and keep the heat from going in to the block at all.

    I can say from my insulation project, that this is a worth while endeavor. Just think it through so you get the best benefit.
  11. RustyShackleford

    RustyShackleford Minister of Fire

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    Karl, I think maybe you didn't see my clarification of my situation. The whole wall is in the heated space. It's the heat leaking into the foundation BELOW the wall, and then into the crawlspace, that concerns me.

    Yes, it's hollow cinderblocks filled with mortar to make the whole thing solid. The heat loss from the sarcopahgus into the crawlspace is what it is - based on the temperature of the surface of the sarcophagus and the temperature of the crawlspace, and the laws of physics governing the heat transfer from one to the other. My main question is how to compute the convective transfer; the radiant transfer is pretty straightforward.
  12. karl

    karl Minister of Fire

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    Convection is the transfer of heat by air movement. Although concrete is porous , I really doubt your moving air through it. Or not enough to amount to anything , as long as it doesn't have any cracks in it. If you are that worried about convection, put a couple of thick coats of paint on it, attach plastic to it, or just tape the joints on your foam insulation really well.

    I'm still pushing for some sort of radiant barrier. If you don't believe me. Get a roll of bubble foil from Lowes. You can always take it back. Stand in front of your stove with a good fire going and then hold the bubble foil up between you and the stove. It will feel like you turned the stove off.
  13. 48rob

    48rob Feeling the Heat

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    Insulating the crawl space walls will allow any heat "lost" from your chimney to be radiated out, and back up, into your structure.
    An added benefit, besides not worying about frozen pipes, are the cooling effects you will experience in the warmer months.
    You can take the fiberglass currently against the floor, and put it in the attic, where it will do a lot more good.

    Rob
  14. turbocruiser

    turbocruiser Feeling the Heat

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    Okay, that helps to understand some stuff a bit better. The first thing I would do is definitely insulate the crawlspace walls, not the crawlspace ceiling (yet). Then start taking readings and that will tell you much more for the next thing to do. No matter what it will help to have the walls insulated. However, insulating the crawlspace ceiling might make the crawlspace prone to problems from freezing if it really is 40 degrees in there. BTW, Qest really is not intended to freeze; Qest and almost all the polybutylene manufacturers advertised that because the piping could sustain some swelling from slightly freezing water. The problem though is the fittings; they will get loose with any swelling and then start to leak and of course if the water that's inside really fully freezes then the pipe is popped open. Anyway, I'd do the walls and then take more temp measurements. Hope that helps.
  15. begreen

    begreen Mooderator Staff Member

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    We are fortunate to have dry sandy soil where the house is and have a fully insulated crawlspace. It has worked out well as a conditioned space. The temp never gets below 60F down there now. I open up the vents in late spring and just closed them off last weekend.
  16. RustyShackleford

    RustyShackleford Minister of Fire

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    I wasn't really thinking that air would blow *through* the concrete. But there's also convective (and conductive too, I suppose) loss from air that's moving (and just in contact with, moving or dead still) the side of the sarcophagus.
    Certainly whatever I do to stop radiant loss is also going to help against convective/conductive loss. I'd just like to ball-park how much it is, as part of figuring out if this is worth doing.

    Good point. Maybe instead of blueboard, I would use that rigid foam that has a foil barrier on it.
  17. RustyShackleford

    RustyShackleford Minister of Fire

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    The crawlspace ceiling already IS insulated, since the house was constructed in the conventional fashion of having an uninsulated and ventilated crawlspace.
    Oh really ? Thanks for the heads-up on that.
  18. RustyShackleford

    RustyShackleford Minister of Fire

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    Yeah, that's the argument for conditioned crawlspace. I want to do a little more research before I commit though. Although, as previously pointed out, there's no harm in insulating the crawlspace walls, even if I don't decide to remove the floor batts.

    I guess insulating the sarcophagus doesn't really do much harm either, if I decide to go with the conditioned crawlspace. Because then I'd remove the floor batts and the crawlspace would get plenty of heat through the floor. It'd be wasted work, but wouldn't be counter-productive.
  19. karl

    karl Minister of Fire

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    I don't know what part of NC you're in. I know the Mountains can get pretty cold there, but if you don't have frozen pipe problems now, I'm betting you won't by adding 20-30 square feet of insulation. I'm further north than you, and when I had natural gas only, I kept the house really cold. My crawlspace is uninsulated and so is the floor. The only frozen pipe I ever had was with a frost free garden spigot. I had forgotten to disconnect the hose.

    As for cost benefit. When heating with wood, you can't do a strict dollars and cents analysis. I heat my house with wood for about $300.00 a year. Even if I cut my wood consumption in half with an improvement, I could only justify spending about $750.00 dollars on the improvement. There's no way I could manage that type of gain. So I looked at it as, I'm spending $40.00 bucks for a roll of rock wool. If does nothing, I've still waste more than $40.00 on other things.
  20. dwillistein

    dwillistein New Member

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    if you want to calculate the convection heat loss, it's not that hard to do. Estimate the area of the concrete that is exposed to the cold air and use the following formula. The only unknown is the convection coefficient, but I would use the number that represents natural convection.

    Q (heat transfer) = h (convection coefficient) * A (area)* deltaT(Temp difference).

    h (natural convection) = 1.7 (Btus/hr ft2 F)

    Use feet squared for you area, degrees F for you temps, and the equation will give you Btus/hr.

    If you use 90 degrees for the concrete and 40 for the air your Btu loss per square foot is 85 Btu/ft2/hr.
  21. RustyShackleford

    RustyShackleford Minister of Fire

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    That's what I was looking for, thanks. So it's the same as the calculation for conductive loss through insulation with a certain U-value (the reciprocal of the R-value), except there's this 'h' instead of the 'U'. Where does the value for 1.7 come from ? Seems like this factor would depend on how much air movement there is. Of course, in my crawlspace I hope it's not very windy ! Are you saying this 1.7 for "natural" convection is the value to use when it's dead still, and the only movement of air is from heat rising ?
  22. dwillistein

    dwillistein New Member

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    Yeah, it's very similar to conduction heat loss. The value for h can vary depending on air flow, but that value is for no forced air flow, and like you said, just heat rising. It can easily vary 50% either way depending on the geometry, air currents, etc. The 1.7 is really just an estimate and is equal to 10 W/m2 K which is what I'm familiar with.
  23. dwillistein

    dwillistein New Member

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    This value h can vary a lot based on wind. A little breeze and the number could go up by a factor of 5 to 10 pretty easily.
  24. RustyShackleford

    RustyShackleford Minister of Fire

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    Thanks. I expect air movement to be very low in a crawlspace with the vents closed. Looks like it can be as low as half the number you quoted, or 5 watts per m^2 per degreeC, as per:

    http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

    Anyhow, it's looking like the convective transfer may be more significant than radiative. Depending on the relative temperatures down there. I need some measurements ...

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