Pre-construction heat-loss calculations
Last Post 24 Feb 2014 12:53 PM by sailawayrb. 14 Replies.
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jwfrankmanUser is Offline
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17 Feb 2014 09:19 PM

Hey everyone,

I've been reading through these forums for awhile and am impressed enough with the vastness of knowledge and experience that I thought I'd have luck with some radiant heating questions...

I actually have many questions, but I'll try to keep the threads separate so as not to muddy the waters...

I will be starting a 1000sqft slab-on-grade lake cabin in northern MN this spring and, as I have a pretty good idea of size, floor plan, materials, etc, I'd attempt a DIY heat-loss calculation.  I'm planning to install in-floor pex radiant heating myself, and I'm trying to approximate boiler size.

There are a few different resources I've found online to figure this out.  The one I had the least problems with (perhaps because he uses broad generalizations?) is found here:

http://www.greenbuildingadvisor.com...ion-part-1

Since I know my delta-T, cabin dimensions, window/door sizes, and have a pretty good idea about how it'll be insulated (r-19 fiberglass batts in wall, blown cellulose in ceiling--not sure how thick, slab insulated w/ 2 inches EPS on all sides except top ) this method seemed to work, and gave me a total of about 36000btu heat loss.

One thing that I'm having a hard time pinning down is whether his listed u-factor of .04 for a concrete slab is accurate.  I can't seem to find any information confirming this number, and have actually found u-factor (or inverse r-values, anyway) numbers for insulated slabs that, when subbing those numbers into the equation, gave me a much higher heat loss number.  I'm not naïve enough to take these given numbers and calculations as gospel, enough to use to make huge decisions about, anyway.  But what I'd like to know is if using these numbers could maybe get me close?  I've compared this method to others I've found online (which seemed more rudimentary), with wildly different results.

I'm no perfectionist but like I said, I'd like to at least come close.  If needed, I'd consider having a professional look this over and give me a number, but as I like doing things myself (at least up to a point)...  Also, I live a couple hours away from the lot, so I'm not very familiar with professionals in the area.

I suppose I should add that I downloaded Taco's load calculator and found that it was obviously not designed to be used by pharmacists.  Any tips, hints, or help files?

Thanks in advance for any help,

Jason

 

ICFHybridUser is Offline
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18 Feb 2014 09:23 AM
this method seemed to work, and gave me a total of about 36000btu heat loss.
For a thousand square foot dwelling? Have you considered where that ranks in terms of green and/or heating efficiency?

For example; you're using R-19 batts in northern MN?

And only 2" of insulation under a HEATED slab?

That value of U=0.04 is for a below-grade slab such as in a basement and is open to discussion as it is. A slab-on-grade assembly is more properly modelled in terms of perimeter exposed and what insulation is used there.
jwfrankmanUser is Offline
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18 Feb 2014 12:58 PM
Thanks for the reply.

Posted By ICFHybrid on 18 Feb 2014 09:23 AM
this method seemed to work, and gave me a total of about 36000btu heat loss.
For a thousand square foot dwelling? Have you considered where that ranks in terms of green and/or heating efficiency?

Well, no I haven't.  But a quick search tells me a "sanity check" for estimating heat loss is 15-25 btu per square foot for an "efficient" home in Ottawa, Canada.  Is this more reasonable?  I didn't say I considered that number accurate, just that the calculations came up with something that wasn't gibberish.  My questions were centered on the best way to come up with a close approximation, and I'll assume your cryptic "quanswer" implies that I'm overshooting for what I'm intending to be a very efficient structure.


For example; you're using R-19 batts in northern MN?

Again, apparently I'm going to need to guess at your non-answer.  I'm assuming you're implying I should use insulation with a higher r-value, yes?  I had used the Dept of Energy Zone 7 recommendations for wall-cavity insulation found here:

http://www.guardianbp.com/images%5Cgf%5Cdoe_map.pdf

Should I not be using these recommendations?  The stud cavities will be 2x6.



And only 2" of insulation under a HEATED slab?

In reading through other posts on this site for construction in northern climes, I had gotten the impression that 2" all around the slab was adequate.  I can obviously adjust my understanding if the consensus is that I should be using more.  Is 4" more reasonable?

To be honest, I was expecting a bit more help with this.  In reading through this forum for some time now, I've gained a lot of knowledge (thank you!) and have quite a bit of respect for just how much help you professionals have, and give out for free.  I understand you may have multiple posts in multiple forums, and that you're giving advice on your own time.

[rant]However, I give out free advice all day, every day at my job.  If a patient says to me, "I was wondering if I can take this cold medicine with the prescriptions I'm already taking," I won't generally respond with, "Have you considered how this Ex-Lax over here would affect absorption of your prescriptions?".  It may be germane to a different discussion (if she were actually taking the Ex-Lax), but it tells the patient nothing about the question they actually asked.[/rant]

I do appreciate you responding, but all the same, I hope there will be more helpful, informative responses than this...


Thank you,

Jason
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18 Feb 2014 02:38 PM
I hope there will be more helpful, informative responses than this...
I'm sure there will be. I'm just a homeowner. Certainly not a building professional, however I have built a modern home that is coming in under 5 btu/sf. My intent is to guide you to answers rather than just tell you what they are. If you are going to build something better than code, you will have an uphill battle with contractors and it helps to actually understand these things rather than have someone tell you the answers at some place and time. Sorry, but that's just how it is.

Apparently, the DOE doesn't have slab insulation recommendations on that page, but I wouldn't be surprised if code required at least R-15 for insulation for a heated slab in your neck of the woods. Myself, I'd do 4" and I'm pretty sure that includes some depth as well as "wings".
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18 Feb 2014 11:39 PM
Jason - consider that the slab edge is really just a lower extension of the wall. Your R19 wall is really only code minimum at best but then you drop to r-8 to 10 with the 2" of eps.

Most of the contributors on these threads would not consider this in the realm of moderately efficient. My 6000 square feet of heated space near Calgary Alberta comes in at around 8 btu's per foot and we would have a bit lower design day temp and a bit more heat degree days then Ottawa.

If you are floor heating, then the perimeter of the slab will have the highest delta T so yes it would make sense to dramatically increase the insulation at this point. 2" underslab would be okay (sort of) if you were space heating but with slab heat it would be a good idea to go a bit higher (unless your soils are very dry and act as an insulator) because again the floor slab will run around 85º against a probable 45º? ground temp.

But everything is a trade off. Not knowing what your heating fuel costs are, or how efficient your boiler will be, its hard to say what the best course is.

Now I know you only want to know if you are on the right track with your heat loss calc's but tell me, if I came into your pharmacy to fill a 'script for pain killer but I was dragging an obviously broken leg, would you just fill the 'scrip and send me on my way? ;-))

Based on what you have told us, your 36,000 is probably not to far out.
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19 Feb 2014 07:57 AM
Go find Dana1 and get an education on tightening a structure. He tends to offer links and such not just suggestions to go fishing. Try the "General Forum, Residential".
Joe Hardin
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19 Feb 2014 09:44 AM
Go find Dana1
You don't "go find" Dana1. He finds you.
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19 Feb 2014 03:42 PM
IRC 2012 insulation minimums by US climate zone:

http://publicecodes.cyberregs.com/i...sec002.htm

For 2x6 walls in US zone 6 or 7 MN would be R20 cavity-fill minimum, PLUS R5 of continuous insulation on the exterior of the sheathing.  And that's the crummiest assembly you'd legally be allowed to build if MN adopts IRC 2012.

But statewide MN is currently at IRC 2006 levels (with the exceptions of some communities who have pushed ahead) and R19 2x6 construction is still legal, even if it's poor practice.  Check page  41 for the quick reference guide on R-values for current code in MN.

R19s are basically junk- they're the same weight per square foot as R13s, just "fluffed" to a ridiculously low density. Fluffed up a couple of inches makes it FAR less air-retardent- there are even fiberglass air-filters of higher density than R19 batts. That low air-retardency leads to significant loss of performance to convection loops within the batt at the winter temperature extremes, when the siding is more than 70F cooler than the interior air. It also loses performance to air leakage at the sheathing from both convection and wind loads, unless you take great pains to caulk the framing to the sheathing, and foam seal all electrical/plumbing/other penetrations of the sheathing AND the framing (including lateral electric runs penetrating studs.)  All of that air sealing is worthwhile even if you use better insulation than R19s, but absolutely critical for getting any performance out of R19s.  Even when installed perfectly in perfectly air-tight stud bays, when compressed to the 5.5" nominal framing depth the best you can get out of them is R18 (and that's according to the manufacturers' compression charts!)- they are allowed to label them R19 by testing them at the full manufactured loft, which is about 6.25".

If batts it must be, R21 "cathedral ceiling" batts, or R23 rock wool batts are far denser and far more air retardent, and deliver the true labeled R-value, if installed perfectly.  But it's better to use blown fiber or sprayed foam to get a more perfect fill, no compressions or voids, no thermal-bypassing air channels for unrestricted infiltration.  With a lot of diligence it's possible to get the performance out of batts, but it's far from guaranteed- the "typical" installations have many errors of commssion & omission, readily found with blower doors and infra-red imaging cameras.

But assuming R1 for the gypsum + sheathing + siding at a 25% framing fraction (typical for 16" o.c. framing) a perfectly sealed R19 wall would deliver only about R13.5 "whole-wall" after the thermal bridging of the framing is factored in. With R23 rock wool that bumps up modestly to about R15, but the inherent imperfections in any air sealing & batt installations have less consequence to thermal performance with the higher density goods- it's "worth it". 

An IRC 2012code-min wall comes in at about R20 whole-wall, due to the R5 continuous insulation thermally breaking the framing which roughly doubles the R-value for that 25% of the area, and marginally improving the center-cavity value. Note that 2x4/R13 with R10 continuous insulation is an acceptible alternative to 2x6 R20 + R5, and that too comes in at about R20 whole-wall.

From a resilence point of view it's better to use exterior continuous insulation of sufficient R to keep the average winter temp at the sheathing above the dew point of the conditioned space air, which would allow you to use standard latex paint as the interior vapor retarder. That keeps the structural wood warmer (=drier), and due to the semi-vapor-permeable interior side a HUGE advantage in drying rate for any moisture that does manage to get into the walls.  The IRC has specified minimum R for the exterior foam that are workable for that approach: 

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm


You'll notice that for zones 6 & 7, the 2x4/R20 + R10 c.i. code min is sufficient exterior R to skip the interior vapor retarder, whereas 2x6/R20 +5 is not.  It's a more expensive way to hit code min, but it's a far more resilient assembly.

But again, code-minimums are the crummiest thing you'd be allowed to build, not lifecycle cost-optimized sort of thing.  To get a better handle on what makes sense taking the longer view, read the first chapter of this document, and refer Table 2 on p.10.   Note, those R-values are all "whole assembly", accounting for all thermal bridging, and not center-cavity R values.  An example of an ~R35ish wall in a zone 6 climate would be 2x6 24" o.c. w/R20 cavity fill, plus 2" of exterior rigid polyiso under another 2" of rigid EPS. (The 2"  EPS being necessary to not have to de-rate the polyiso severely for colder temp climates.)  Yes, that's a much more expensive wall than an R19 2x6 shack, but it's literally 3x the performance of that current MN code min wall.  Also note, that even without the radiant heating, they're suggesting R10 as a reasonable starting point for full coverage under the slab. When using the slab as the heating radiation, add R5-R8 to that.  The crummier the R values on the rest of it the higher you'd need to bump the under-slab R, since a higher heat load would mean a higher slab temp during the heating season.  Four inches of Type-II EPS (R16.8) is about right, three inches (R12.6) would be a bit on the skimpy side for a heated slab.


jwfrankmanUser is Offline
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19 Feb 2014 05:11 PM

consider that the slab edge is really just a lower extension of the wall. Your R19 wall is really only code minimum at best but then you drop to r-8 to 10 with the 2" of eps

Thanks for the info.  No, I didn't think of the walls extending downward that way, but it makes sense.  I had intended to ask local contractors about code requirements and what's standard in that area for insulation, and the more info I can get beforehand will make me look like less of an idiot had I not researched at all.  Prior to looking into all this stuff, I had absolutely no knowledge regarding heat loads, R-values, or slab construction.  I have marginally more at this point, and hopefully by the time the weather warms up (!) I'll at least have a working knowledge of what I need to make this project work.  I appreciate the assistance!


if I came into your pharmacy to fill a 'script for pain killer but I was dragging an obviously broken leg, would you just fill the 'scrip and send me on my way? ;-))

Haha, good allegory!  And I suppose a bit deserved...  I realized later it was not exactly the same thing.  Of course, none of you has an obligation to tell me anything.  (Although I have to wonder why anyone would continue to participate in a forum of experiences and ideas if those experiences and ideas are to be guarded jealously)  I'm not looking to be spoon fed:  I've not posted here before, and have in my mind learned quite a bit prior to finding this forum.  I'm looking for differing opinions and project experience from which I can build my own, unique project.  Hopefully, if things work out well, I'd be able to freely offer my own experiences to those who are in my position.  But that's just me...

As to your script parallel, in my job I actually am obligated to offer advice to all who ask for it, which just makes the information I can glean from you folks all the more appreciated.  But to continue with the parallel game, if the dude with a script came in dragging his broken leg, as long as it seemed he had at least a rudimentary knowledge of the healing process and had plans to further this knowledge, I'd fill his script.  And then I'd take his copay!    Yet another reason I value any info you can give me.  Your two cents is a bargain!


Not knowing what your heating fuel costs are, or how efficient your boiler will be, its hard to say what the best course is

To be honest, this cabin will be mostly seasonal, with the occasional trip up to hunt fish through holes in the ice.  Most of the winter it'd be dialed back to probably somewhere in the 40's or 50's.  But I'd like to get it as tight and efficient as possible for when we are there.


Based on what you have told us, your 36,000 is probably not to far out.

Looks like I have some work to do, then. 

Thanks again for the help.

Jason 

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19 Feb 2014 06:48 PM
If it's a seasonal unit that you have to keep heated to freeze-protect, you may be better off skipping the radiant floor concept and heat it with cold-climate mini-splits.

Unless you go hog-wild on window area, as long as it's air-tight and reasonably insulated the heat load at 45F interior temp and -15F outdoor temp would be less than the output of the Fujitsu AOU-15RLS2-H or Mitsubishi MSZ-FH15NA at that set of temperature conditions. (They'll both put out about 15KBTU/hr @ 70F-inside/-15F outside.)  It'll probably still keep up at 70F-in/0F-out, and any shortfall could be made up with resistance heating, if you're the type who likes to go ice-fishing when it's -20F outside.
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20 Feb 2014 12:20 AM
I had intended to ask local contractors about code requirements and what's standard in that area for insulation
That might work some places, but you couldn't do it here. Based on what I see, you would be LUCKY to get code-minimum while being told it was a real energy saving home. You need to know so you can specify what you want and continue to specify it when they whine and resist.
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20 Feb 2014 04:22 PM
Would have responded sooner Jason, but we just returned from our Maui place…and I always seem to totally forget about work stuff when I am there. Our son was supposed to monitor this forum and provide assistance, but he got overly involved with some water wheel projects and “forgot” to do so…

ICF and Dana have you vectored in right direction as is always the case. I will only add that you might find our free DIY heat load analysis and hydronic radiant floor heating design software to be useful with your project.

Borst DIY Engineering/Construction Software
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21 Feb 2014 12:44 PM
Posted By ICFHybrid on 20 Feb 2014 12:20 AM
I had intended to ask local contractors about code requirements and what's standard in that area for insulation
That might work some places, but you couldn't do it here. Based on what I see, you would be LUCKY to get code-minimum while being told it was a real energy saving home. You need to know so you can specify what you want and continue to specify it when they whine and resist.

Right on cue, today's GBA blog weighs in on "whaddaya MEAN by that", regarding R-values as they relate to code minimums.

Finding a framing contractor/architect that truly understands OVE/AF framing to be able squeak out the last tiny bit of performance with R19 batts can be a search worthy of Diogenes.  Asking the local contractors for "...what's standard in that area for insulation..." will never get you a high-performance building, or anywhere near cost-optimized between up-front costs vs. future energy savings.  That approach will reliably get you is the crummiest thermal performance that is legal to build (or even less, if the inspectors are clueless.)

If the heating fuel is going to be propane or oil you can "afford"  a MUCH higher performance building envelope than if it's going to be heated with mini-splits or natural gas, but even with those going better than code-min is going to be worth it.
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24 Feb 2014 12:24 PM
Thank you very much for the replies.

Dana1, I've been boning up on a couple of the links you provided; much info there to slog through, and I do appreciate the advice from you and others about contractor tolerances.

Sailawayrb, thanks for the software link...I'll take a look and pm you if I have any questions, if that's ok?

Finding a framing contractor/architect that truly understands OVE/AF framing to be able squeak out the last tiny bit of performance with R19 batts can be a search worthy of Diogenes.
    I'll be sure to bring my LED camping lantern to contractor meetings!

Thanks,

Jason
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24 Feb 2014 12:53 PM
Always happy to help Jason and the reason why we participate on this forum…in addition to learning ourselves. There’s no need to PM, you can just use the Contact button on our website to reach our customer support team. Someone will always respond in less than 48 business hours (usually much less time than that) to answer questions…even from DIY folks using our software, etc.
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