Replies

1. 
   NRTRob | Jul 30, 2007 05:51pm | #1

   it's called a heat load calculation. anything else is just guessing.

   -------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com
2. 
   Tim | Jul 31, 2007 09:26pm | #2

   Rough estimate? 1,000,000 btu/hr or maybe 30,000

   A competent, experienced designer from your climate could make an educated guess and come close.

   Are you a competent builder? First timer? Doing this with a close eye to the budget? Getting help from a friend that worked for acrpenter in college?

   Take the surface area of each envelope element (wall, roof, window, door, etc), multiply that times the temperature difference between inside and outside at the conditions you wish to stay warm, times the inverse of the composite element R-value (the "U" value) and add them all up. Add 15% for elements on the side(s) that are exposed to the prevailing wind, and presto-magico, a load calculation has been performed.

   My bet is 30k.

1. 
   Malades | Jul 31, 2007 11:05pm | #4

   thank you Tim.  This is close enough for me. I am building it myself with a lot of advise from friends, builders and this website. So far we have changed the design at least 30 times but the changes are now limited due to the degree of completion. Finish work now.

    

   Thanks Again

    

   Chuck

1. 
   BillHartmann | Aug 02, 2007 07:00am | #8

   http://www.hvaccomputer.com/
   .
   .
   A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

2. 
   highfigh | Aug 02, 2007 02:38am | #6

   15% is OK for infiltration but a total for the door and window gaps is probably closer. Malades- it's out there- google it and I'm sure you'll find what you need.
   "I cut this piece four times and it's still too short."

1. 
   Tim | Aug 02, 2007 03:48pm | #9

   "15% is OK for infiltration but a total for the door and window gaps is probably closer."

   Closer to what?

   Based on what?

1. 
   highfigh | Aug 02, 2007 04:20pm | #10

   Closer to actual infiltration numbers, based on the fact that using the real length of possible air gaps will be more accurate, but for an estimate, I guess adding 15% is good.
   "I cut this piece four times and it's still too short."

1. 
   Tim | Aug 02, 2007 09:06pm | #11

   My question really was, "Closer to the actual numbers, based on what kind of comparison?". Personal experience? Just guessing? The DanH method? ASHRAE? SMACNA? The Trane Air Conditioning Manual?

   Not saying you're wrong, but I'm curious as to what you base the comparison.

   I have found, based on my limited experience in the field, that crack length estimation of infiltration varies wildly from person to person because of the almost endless amount of variables that can be applied. Do you use a 10 mph wind or 15? Is the crack 1/16" or 1/8"? Is the window taped or DAPed? Or both? Is the weather stripping distorted or perfect? Etc., etc..

   The method I've found to be the most consistent and generated the best (i.e. most comfortable) results for my clients is estimating infiltration based on air changes. Air changes in a space are estimated based on the exposure, tightness of construction, age of the structure, etc. Typical values are used as a base and modifications are applied based on experience. For the purposes of defining the limiting heat loss in a room or zone, depending on the amount of infiltration, 5 - 10-% is added on the north and west exterior spaces, to account for the fact that in most of the lower 48, the winter wind blows from the NW and to bound the calculation.

3. 
   DickRussell | Jul 31, 2007 11:04pm | #3

   In northern Maine? First, don't use "R-19" FG batt insulation. It won't perform at R-19 when you need it most, at very cold outside temps. If you insist on using it, doing the cabin on the cheap, then at least derate it to around R-10, to account for density-induced convective loops within the batt when temperature difference is high. Make sure you take air leakage into account. A poorly sealed exterior shell could cost you more in heat loss than conduction through the insulation.

   Aw, heck, all this has been covered in so many prior posts, and there is so much available out there on the Internet in the way of info on building envelope design. It won't be possible for us to give quick answers on this, because there are so many variables, so many aspects to sealing the shell and insulating it, and all of that has to be considered. Even saying that the windows are "thermopane" isn't enough. You have to go by the "U" numbers on the window stickers.

   Perhaps you could download a copy of the ResCheck software, Maine version if there is one specific to Maine, and enter the details of all parts of the exterior envelope and windows. It will add up all the "U" times area components and give a total (U is 1/R). Then you can multiply by, say, 80 to give you a BTU/hr load when it is 10 below zero F outside.

   
   

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   Edited 7/31/2007 4:07 pm ET by DickRussell

1. 
   Malades | Jul 31, 2007 11:07pm | #5

    

   Thanks Dick,

    

   taking all information into consideration for this project. My "cabin in the woods" is getting a lot of TLC and should be complete in the next 5+ years.

    

   Chuck

4. 
   grpphoto | Aug 02, 2007 06:12am | #7

   http://hearth.com/calc/btucalc.html

   George Patterson