I’ve been playing around with BEopt in order to finalize our retirement home. The home will be built near Waterloo Iowa which is on the border of zone5/6. The home is only 30 by36 built on a slab with R-20 EPS below, R-60 ceiling, double pane windows, ductless minisplit etc. The one variable I compared was wall R-value. My two options were 1) double stud wall cellulose R-33 vs. 2) 2 by 6 wall cellulose R-19. When I ran the scenario the difference in annualized utility bill was only $28.20 for a simple payback of 213 years. I was taken aback at the results. Did I miss something? Does it pass the “reasonable” test? My wife and I are 68 years old and were of the mind set to go the extra mile with the double stud wall but now I’m thinking maybe not. I would save a bit on materials, even more on labor (although it’s family labor) and more still in terms of time/complexity. Does anyone have some insights they would be willing to share? I realize the 2 by 6 wall should have R-10 foam on the outside to prevent moisture condensation. That particular option was not on the BEopt wall pick list. As I think about it a double stud wall would not be that much more complex/time consuming that stud wall with foam.

We build double stud walls, partly because they're relatively simple and straightforward. But it is critical that the house be air tight; a leaky house will have higher moisture content, and moisture in a 10" wall can be a recipe for disaster. Foam on the exterior is a better design - the house frame is kept warmer, so there is less chance for condensation and mold - but it's a little complicated to do. Complicated but not hard. Details at GreenBuildingAdvisor.com.

And upgrade to triple glazed windows! I've upgraded my house over the last decade with excellent double glazed windows; I wish now I'd gone to triple. The interior glass stays closer to room temperature, so is more comfortable to be near.

I just got to Davenport doing consulting work for a lab, hot box, fungi, and structural test. Didn't think I'd survive those tornados last night in my RV. If you can attach your BEOPT model and I'll answer your questions properly. The insualtion sheathing area has the outsultaion. You want to stay focused on BEOPT, DOE can answer your questions far better than any green site or especially GBA sales hype. It's to bad more DIYs are not as smart as you to know better and use the free resources DOE provides rather than look for a quick fast easy answer that is most of the time wrong.

Got to get to the lab I'll check back when I can to see if you attached the model file. Usually double studs make no economic sense you can see in the financial model, another aspect of design green sites fall far short of. BEOPT has a forum too, low traffic again too bad.

BobI, I ran BEopt looking at double vs. triple pane quite sometime ago. As I recall, BEopt spit out a 200 year payback for triple over double pane. Which lead me to consider 2 sets of lower cost double pane windows; one on the outside and one on the inside. It sounds kind of weird and the logistics of opening a window would be double the effort but theoretically one would have more R-value for the buck.

Thanks Agagent3. This is an interesting post, at least related to my own situation. I am 67, and getting ready to construct my house in the CO mountains (between zones 5&6). I have spent a long time looking at SIPs and ICFs as well as many other options.

A double wall has interested me greatly, because foam is ~6Xs the cost of dense packed cellulose. Then the cost for shipping pre-made SIPs and the cost/complications for cement, as well as compressed assembly timeline for the entire house have been concerns preventing me from beginning construction.

Thanks for your comments Bob I. They also are relevant to my situation. Air tightness and minimal thermal bridging have been part of my serious interest in double walls. You say triple glazed windows. However, my window situation is basically DIY. I have 100+ 2'X5'4" double pane safety glass windows; however, they are an old salvage yard purchase (4$ea) and must be separated, cleaned, and new framework built. This gives me the flexibility to make them triple pane, and since making windows adds significant labor, I don't want a tight schedule for the whole house. My question related to this is: do you think nearly all of those future windows could be closed (un-opening)? That would simplify window construction for me.

In a tight, superinsulated house with minisplits, we've seen more people open windows less frequently, since the house is comfortable most of the time. On the cost question for triple pane windows, comfort is not the same as "payback". It's probable that a tent would be a better long term investment than a house, but less comfortable; it's up to you where to draw the line.

GaryO, you must have opening windows in the bedrooms or doors for egress in case of an emergency.

BobI's comment is right on. Once one has achieved a tight, high R-value home we open the windows less and less. Again at 68 I'm not sure triple pane windows are in the mix. (tents no) Our strategy is to cut back on the amount of square feet allocated to glazing. Further, we have used insulated shade before and liked them.

The good thing about BEopt is that it allows one to make more informed decisions. At 68 my planning horizon is a bit shorter than my son's at 35, 38 and 40. Thus to build with ICF's, SIP's etct. makes less sense to me. I'm more open to PV since our typical electrical usage is pretty low.

Posted By agagent3 on 06 Mar 2017 09:31 PM
I’ve been playing around with BEopt in order to finalize our retirement home. The home will be built near Waterloo Iowa which is on the border of zone5/6. The home is only 30 by36 built on a slab with R-20 EPS below, R-60 ceiling, double pane windows, ductless minisplit etc. The one variable I compared was wall R-value. My two options were 1) double stud wall cellulose R-33 vs. 2) 2 by 6 wall cellulose R-19. When I ran the scenario the difference in annualized utility bill was only $28.20 for a simple payback of 213 years. I was taken aback at the results. Did I miss something? Does it pass the “reasonable” test? My wife and I are 68 years old and were of the mind set to go the extra mile with the double stud wall but now I’m thinking maybe not. I would save a bit on materials, even more on labor (although it’s family labor) and more still in terms of time/complexity. Does anyone have some insights they would be willing to share? I realize the 2 by 6 wall should have R-10 foam on the outside to prevent moisture condensation. That particular option was not on the BEopt wall pick list. As I think about it a double stud wall would not be that much more complex/time consuming that stud wall with foam.

Is this a rhetorical question. Without a model attachment how do you expect anyone to answer these questions accurately? It is easier to attach .BEOPT model files on their forums where answers are by top DOE NREL Building Scientist and MUCH more accurate than any green site. Discussions on these forms like GBA do not even come remotely close to a holistic design approach these NREL models provide when operated by the right persons. Of course in case you don't know the file can be easily shared using a private drop box link.

So tell us,

1. What weather tower did you use and how far away is it from the site?

Another thing that is VERY easy to do is use the snip tool to copy/paste an image of the two HVAC loads & capacities.

2. How much difference is there be MJ8 & EPLUS for the two designs?

3. What are the two load variances?

4. Did you use a single or multiple CCHPMS zones & what parameters?

5. Speaking of ACH what did you use? What design standard(s) will you use for it?

I wasn't looking for an in depth answer just a "gut reaction" to the difference between the two calculated utility bills, $28. There is the old saying "garbage in, garbage out" so one has to step back and ask does it pass the "reasonable test"? Did I forget something?
BEopt sucks in local weather station data so that is included in my case. I chose an ACH of .5 from the BEopt pick list.
I have attempted attaching files and have not been successful. Any tips?

Posted By agagent3 on 08 Mar 2017 10:34 AM
I wasn't looking for an in depth answer just a "gut reaction" to the difference between the two calculated utility bills, $28. There is the old saying "garbage in, garbage out" so one has to step back and ask does it pass the "reasonable test"? Did I forget something?
BEopt sucks in local weather station data so that is included in my case. I chose an ACH of .5 from the BEopt pick list.
I have attempted attaching files and have not been successful. Any tips?

BEopt sucks in local weather station data so that is included in my case. I chose an ACH of .5 from the BEopt pick list.

I have no idea where you are getting your info from. NREL has developed VERY accurate weather files validated for decades now. The problem is I can tell you don't understand them nor BEOPT. I think that is safe to assume since you did not answer one of my questions above accurately.

I suggest you spend more time on their forums learning Beopt, less on green sites. That and D-View is where you get your sanity checks IF you understand outputs. One does not just wake up one day and decide to be a DIY Building Scientist and dodge all the cost for competent Engineers, even though that is the case with GBA. That is where things are headed and should, the days of builders designing gone with Smart Home tech. If you post your question on Beopts forums and tag a link to it here, you will get an answer from NREL Engineers that actually have been doing design Engineering and decades of international builds BEOPT is calibrated to. There is little to no theory in the model like you'll find on internet sites. One could use steady state IECC code or outdated Glasiers methods that do a poor job of accounting for directionals such as wind-driven rain, solar radiation drying potential, freeze-thaw cyclic damage, etc. I'm getting ready to lab test these properties perhaps I'll share how that works, and how the products you buy are certified by test.

As I said, you can also share a drop box link.

There are two Waterloo filles in Beopt I'm shocked you did not name one since it s/b in your model. TMY2 & TMY3....do you know the difference and how they can affect your results? If your site is more than 50 miles or if altitude varies more than 400-500 feet between site and tower a custom "microclimate' may be required. Adjustments are made 3.6F/1000F. Bob being a "PSIUS Certified Consulatant" should have made you aware of that and PHIUS (Not GBA) has those custom files for purchase. Also, if you are air sealing down to .5 ACH you should have been pointed to the massive very good amount of info on their site since if you design wrong at those levels it can cause major health issues. Most Engineers recommend average DYIs don't go below 1 ACH or seek pro help and get PHIUS certified to "DESIGN" which means you are using WUFI and satisfying 5 basic thermal criteria in WUFI PASSIVE before PLUS hygric analysis which may show foam since it has poor breath-ability may be a BIG costly mistake. I could go on and on... Pretty much everything stated on this thread about air sealing controlling vapor pressures is inaccurate.

Speaking of cost, windows and walls are your last concern. Trying justifying the 5-10% more spent on a high performance home to an appraiser or bank when you or whomever is going to sell. GBA won't discuss that much since they need to push sponsored sales, especially foam. They are known for pushing foam and lacking site supporting data I just described.

Finally, triple panes make no sense at all!! That is replaced by "Dynamic Digital Shading" on the correct windows/climate/design. This is an area BEOPT and all CFDs needs to improve since now there are dynamic SHGC/U/VT ranges it cannot handle yet nor has the mfg made available yet, and it has limited Hygrothermal modeling compared to WUFI. Other needed improvements are noted in the their change log, weather files and definitely not on the list since they are proven.

Yes, PARAHOMES, I did in fact use the Waterloo weather data files. The "foam pushers" have me fearful of a double stud wall because of moisture migration. Therefore, what generally accepted practices may I employ in a double stud wall stack up? I was thinking from outside in: vinyl siding, water protective barrier (15 lb felt), plywood (latex+silicone caulk taped joints), exterior wall stud filled with cellulose, plywood (caulk & taped joints, 2" space, interior wall stud filled with cellulose, 5/8" sheetrock, latex paint. Thoughts? Is there a better stackup? Do I need a smart membrane beneath the drywall?

"Foam Pushers" I like it, lol!! Like drug pushers needing a detox and 10 step program. I call them Foamaholics.

You know there use to be some real sharp chemist and Engineers on GBA that stated all the issues with foam until Martin Holladay whom BTW is some x-framer hippie from Vermont with no nationally proven design-builds nor inventory to view silly people follow, kicked them off. I'm convinced those followers are trolling over here.

I get over there once in while for entertainment, this one is completely ridiculous where his foam advice is being challenged: http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/state-our-union#post_comments

The "State of our Union" is what team to join "Team Foamaholic Martin or Team Nature Boy Jacob" ????

Is that hilarious or what? LOL!!

I think the whole blog is staged to brain wash ppl into thinking GBA is some "data acquisition" site like one would get from NREL Beopt, WUFI, etc, which NO WAY, anyone qualified professional would laugh at!

I like when one poster asked for life cycle foam data under slabs and nobody could deliver, just more jibberish! There is none, nor mating material chem reactions that has little to nothing to do with stand alone specs derived from a lab like I currently work in, and most on the site including Martin have ever been a part of, nor on design teams which in High Performance design it takes several engineering disciplines to design cost effectively. Cost as I said, initial, resale, there is no data nor tied tech data like Beopt does both financial/tech modeling to optimize design choices to tested standards and schedules.

Foam does not care if it is in a double or single wall, the reactions with other materials are the same.

I'm not going to sell you down the river with bad advice I don't have enough holistic info to properly advise on a stand alone wall, nobody can. You can guess as good as anyone that is looking for fast cheap advise which is why ppl come to these sites rather than pay qualified pros until the day comes when they are experience mold, rot, etc, negative equity or cash flows from bad initial design decisions.

If it's a High Performance Passive home you want find some pro's to get it PHIUS certified and designed right within your budget. That may help resale since you'd be one of the 1300 in the nation IIRC, if you use AGREEA from AI, follow DOE sales advice since when/if the time comes agents have to be trained to sell them...Get with some local appraisers see if they know how to value it unless you are one of the many that have no idea or don't care if they loose money.

WUFI also has a forum or ask PHIUS for Designers to model. I'd offer but I'm booked right now and very expensive.

Good luck!

BTW: Use the TMY3 file.

Posted By PARAHOMES on 08 Mar 2017 08:04 AM
>Without a model attachment how do you expect anyone to answer these questions accurately?

As a long-time lurker on this board for many years, as well as the old board, (having built several houses using it, and never a sign of a model attachment) I can tell you that the personality of this board is one of politeness. I hesitate to suggest, but will, that your manners need improvement. No knowledge that could possibly be gleaned from your incoherent posts is worth having to listen to your barrage of insults.

.

Thank you Robertson!

Yes, well said.

Robertson -I agee. That needed to be said and you said it well. I have not been active on this board for a while but still monitor it. I have recommended it often as a collegial and cooperative place where ideas and knowledge are shared in a civilized way and someone learning or exploring possibilities is welcome. We seem to be losing that....

Robertson, thank you!

Agagent3, what you are seeing is the “diminishing return of insulation R value”, which is something you can Google for a very detailed explanation and better understanding. You have to double the insulation R value in order to cut the heat loss via building assembly heat conduction in half. So going from R value 0 to 19 makes a huge difference. Going from 19 to 33, not so much. Furthermore, heat loss via building assembly heat conduction is only part of the total building heat loss. Heat loss via air infiltration is often the larger building heat loss component, especially as building assembly R value increases above code minimums.

We also have a Building Assembly Moisture Analysis Calculator on our website that you may find useful for evaluating moisture issues in unconventional building assemblies. This calculator is based on the 2013 American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Fundamentals Handbook historical steady-state dew-point or Glaser methodology for evaluating moisture accumulation and drying within building assemblies.

First, comfort. It is the main reason we do not live in tents.

There is no conceivable reason to build a double-wall for comfort since a true R-19 wall can be achieved with 2x4 construction and the appropriate exterior insulation.

Double pain windows will also afford perfect comfort in such a mild climate.

With slab-on-ground construction with double the sub-slab R-value and nearly double the recommended ceiling insulation once the walls and windows are addressed you are all set.

You will need an ERV for fresh are, not as much as is generally recommended and a heating and cooling appliance. Multi-head heat pumps can be perfect if you don't mind the noise generated by fans during peak load conditions and you know that each conditioned space must have it's own "head".

Finally, and perhaps initially you will need an ACCA Manual 'J' heat load to properly model the HVAC loads you will experience after the paint dries.