rainman
New Member
Posts:37
 |
| 26 Jun 2013 10:51 AM |
|
Hello all and I hope that someone can help me out or at least point me in the right direction.
Having said that this is what I have...
I have designed a square "U" shaped house for my family and have really
had very little trouble with the design until I got to the roof design.
The way I have this pictured is as follows:
The roof will be shed/lean-to/monoslope roof with a pitch ceiling.
The roof will be metal.
Insulation will be 2" of close cell foam.
Ceiling will be wood decking.
Supports will be either open visible trusses or beams.
(This is what I am having issues with)
Most spans will be 20'.
Only exceptions are two 3' sections of wall which will be 23' spans.
Outer perimeter walls of the "U" being 10' tall.
Inner perimeter walls of the "U" being 16' tall to achieve a 4/12 pitch.
ALL perimeter walls will be constructed out of ICFs.
There will a gutter system and 2' of overhang.
How do all of these parts/pieces fit together? How do I put them together? HELP!!!(Please and thank you)
I don't know if I've given too much information or not enough. I can't
stand reading long drawn out posts BUT hope the right person can stand
it and reads mine.
Thanks everyone,
Mike
|
|
|
|
|
|
|
Bob I
Veteran Member
Posts:1435
|
| 26 Jun 2013 11:14 AM |
|
Where are you? 2" foam seems pretty minimalistic, unless the ambient temp is 70o year round. there are also issues with placing foam insulation over decking so you have continuous insulation around the roof and do not have a large thermal bridge and air leakage. We recently worked on a 20 yr old house with foam over decking where the V groove ceiling extended through to the exterior, letting lots of air out of the house every 5" - probably the biggest heat loss of the house.
i don't understand the issue with the roof. If it all pitches to the outside, and the inner walls and outer walls are level, you'll have a "hip" at each of the two roof corners and the facia will be level across both faces. I assume you have a truss company designing the trusses; they'll draw up a roof plan and probably a 3D plan for you to review. |
|
| Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant |
|
|
Dana1
Senior Member
Posts:6991
|
| 26 Jun 2013 03:40 PM |
|
That would be WAY sub-code for R-value at the roof anywhere in the US, and unless the finish of the metal roof was a high solar reflectance "cool roof" type the mid-day solar heat gain through the roof would be pretty high, even in places where the outdoor temp averaged 70F. |
|
|
|
|
rainman
New Member
Posts:37
|
| 27 Jun 2013 11:43 AM |
|
Hi Dana,
Well, I re-checked with 3 of the 5 insulation contractors again and all 3 again said that they only recommend 2" of close cell and that they do not see the benefit of spraying additional foam although they will do it if of course I don't mind paying the additional cost.
I'm having a hard time comprehending why multiple businesses are telling me they advise against purchasing more of their products/services when it can result in more money/profit for their business.
The roof construction portion of this project has been my least favorite to say the least.
Thanks,
Mike
|
|
|
|
|
Alton
Veteran Member
Posts:2164
|
| 27 Jun 2013 12:39 PM |
|
Mike,
Without knowing your zip code, we have no way of determing the correct R-value.
I have found that spray foam companies do not want to quote more than the very minimum R-value because the high price may make them lose the job. Some foam companies have told me that there is a lot of profit in spray foam work. I have had some luck with getting some companies to reduce their quote because of job size or extra thickness. So do not hesitate to ask them to lower their quote. Be prepared to verify the thickness of the finished job before you pay them. Some companies will touch up weak areas where the foam is too thin. |
|
Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
|
|
rainman
New Member
Posts:37
|
| 27 Jun 2013 03:22 PM |
|
Hey Alton,
Great to hear from you again. Great advice on verifying thickness and asking for lower quotes. I've asked for lower quotes but unfortunately it sounds like the same 10 or so companies are all shopping each other so their prices are all within about $300.00 of each other. What I've found is that they charge much different rates for prep/clean-up work, anywhere from $25.00 to $80.00 per truss to wrap them in plastic.
I did check with two additional insulation suppliers, total of 7 suppliers, all of which advised me that 1" gives me 85% benefit, 2" gives me 93%, 3" results in 94% and 4" only gives me 94.5%. All things considered, I believe that I will get my best performance and ROI on using 2" close cell. The general consensus by the suppliers is that using 3 or more inches provides a diminishing return. I can use the $4K per additional inch on upgrading windows, doors and/or appliances.
Thanks again Alton,
Mike
|
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 27 Jun 2013 04:20 PM |
|
Posted By rainman on 27 Jun 2013 11:43 AM
Hi Dana,
Well, I re-checked with 3 of the 5 insulation contractors again and all 3 again said that they only recommend 2" of close cell and that they do not see the benefit of spraying additional foam although they will do it if of course I don't mind paying the additional cost.
I'm having a hard time comprehending why multiple businesses are telling me they advise against purchasing more of their products/services when it can result in more money/profit for their business.
The roof construction portion of this project has been my least favorite to say the least.
Thanks,
Mike
Closed cell foam is the most expensive insulation materials out there at 17-18 cents per R per square foot, which means from a short-term cost effectiveness point of view it falls off a cliff at fairly low (and definitely sub-code) R values. The correct interpretation of."That's all you'll ever need." is: "If installed to code min it would break the bank, take more than 50 years to pay back in energy use savings at current energy prices, and the quote would be so onerous that we would never get a contract."All the stuff about it performing better than the crappy low-density batt straw-man because of better air tightness bla-bla-blah may have kernels of truth in it, but that doesn't mean it meets code-min, or that you can't get something else (or a combination of cc foam and something else) to deliver equal or better performance at a lower cost. Performance builders do that every day, with minimal use of closed cell foam. The "Two inches is all you need" story has been around for at least 25-30 years, and it's never been so wrong as it is today, now that there are more people around who know how to design & build with reasonable air tightness and good moisture control. It's NEVER been "...all you need...", and going R13 when code is R30 or R38 will have considerable comfort and energy use issues. Got a location? That would tell us what code requires, and how to get there for $2/square foot (the typical cost of 2" of ccSPF), or less without running into moisture control or thermal performance issues. EPS and polyiso are bit over half the cost per unit-R of spray polyurethane, and widely used in roofing applications where price/performance matters. [edited to add] Under metal roofing the rigid foam can go between the roof deck and the metal, provided you use the proper underlayments and slip-surface material between the metal & foam (or better yet, purlin-mount roofing.) Double layers with seams staggered and taped is the right way to go, unless you want to go triple-layers. At about R6/inch (R5.5/inch derated for very hot or very cold temps) two layers of 3" goods gets you nicely past an R30 code min, and would blow away 2" of polyurethane on performance, since the heat gains/losses would be about 1/3 that of the spray foam solution. Sure it would cost more, but it would also be code legal, and the payback in comfort would be something you'd feel, even if the payback in dollars was long. Payback is always long, which is why code minimums are necessary. If it needs to be net-present-value-positive on heating & cooling savings in 5 years you couldn't make the financial rationale for even cellulose at code min. (Cellulose is half the cost of iso or EPS per unit-R.) |
|
|
|
|
JimGagnepain
New Member
Posts:61
|
| 27 Jun 2013 05:42 PM |
|
If you use "open visible trusses", isn't your insulation going to show? I guess you could use some furring strips, and put some T&G or drywall over them, and still have the visible trusses. But it would probably be easier to just attach the T&G or drywall to the trusses, and forget about the visibility. |
|
|
|
|
Surfsup
Basic Member
Posts:349
|
| 18 Aug 2013 07:32 PM |
|
I have actually a similar design and am in Chicago. I am considering batt 38 with the zipsheathing system (or just using hard foam on the trusses with the OSB on top). I noticed on all the calculators I've found for BTU loss though, going higher in R value on the roof doesn't really affect the numbers too much. (www.builditsolar.com and www.borstengineeringconstruction.com)
So I was thinking of doing R38 inside, 1" hard foam with roof sheathing on top then with a furring strip then the metal panels (hidden fastener). What do you guys think of this idea? |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 19 Aug 2013 02:58 PM |
|
Posted By Surfsup on 18 Aug 2013 07:32 PM
I have actually a similar design and am in Chicago. I am considering batt 38 with the zipsheathing system (or just using hard foam on the trusses with the OSB on top). I noticed on all the calculators I've found for BTU loss though, going higher in R value on the roof doesn't really affect the numbers too much. (www.builditsolar.com and www.borstengineeringconstruction.com)
So I was thinking of doing R38 inside, 1" hard foam with roof sheathing on top then with a furring strip then the metal panels (hidden fastener). What do you guys think of this idea?
For insulating at the roof deck you'd be better off with R20 in rigid polyiso (3.5") above the roof deck, and using 2x purlins through-screwed to the rafters with timber screws on which to mount steel roofing, and doing the rest with either open cell foam or tightly fitted R23 rock-wool batts on the interior side. But it would be cheaper to take a vented attic approach with 25" of cellulose and hit ~2x the R-value. The builditsolar heat load calculator is cruder than crude, and not very useful for assessing high-R buildings. The Borst Engineering calculator isn't a whole lot better, but some. Neither are energy use simulators, and only give you one set of none-too-accurate numbers for peak loads. Modeling the building with all site & climate factors using DOE-2 will tell you a lot more about performance than any dumb (or smart) heat load calculator. BeOpt (which used DOE2 as it's underpinnings) is even better for playing "What if...?" games and cost-optimizing the performance. Both are freebie downloads. |
|
|
|
|
sailawayrb
Veteran Member
Posts:2283
|
| 20 Aug 2013 03:36 AM |
|
Dana, our heat loss analysis and cooling load analysis software is of course specifically intended for peak heating/cooling load analysis. However, you can also take the BTU/Hour/Deg F output from either analysis and then use our integrated cooling/heating performance software which uses local heating/cooling degree day data to estimate monthly/annual heating/cooling requirements. If you have an existing building, you can alternatively use our existing building energy usage analysis software to get the BTU/Hour/Deg F. We also have a suite of passive solar software for designing a passive solar roof overhang, estimating monthly/annual passive solar heat gain, and sizing passive solar thermal mass. The monthly passive solar heat gain estimates can also be used with our integrated heating performance software. Our DIY cooling load analysis uses the older CLTD/CLF methodology so the passive solar is essentially built in but arguably not as accurate as the newer methods we use for our professional design services. Here's the link to our free DIY software and detailed instructions are provided: Borst Heating/Cooling Analysis Software |
|
| Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do! |
|
|
Dana1
Senior Member
Posts:6991
|
| 20 Aug 2013 11:15 AM |
|
Using load tools to refine the design from an energy use perspective is a bit like using a rock for a hammer. In expert hands you can definitely get there, but it's not the clearest, cleanest way to optimize a design for site, climate, and financial factors.
BeOpt/DOE-2 or Hot-2000 etc all have their limitations too, but are probably better suited for the task of tweaking a new design. |
|
|
|
|
Surfsup
Basic Member
Posts:349
|
| 20 Aug 2013 08:26 PM |
|
Posted By Dana1 on 19 Aug 2013 02:58 PM
For insulating at the roof deck you'd be better off with R20 in rigid polyiso (3.5") above the roof deck, and using 2x purlins through-screwed to the rafters with timber screws on which to mount steel roofing, and doing the rest with either open cell foam or tightly fitted R23 rock-wool batts on the interior side.
But it would be cheaper to take a vented attic approach with 25" of cellulose and hit ~2x the R-value.
Dana, the roof is 3:12 and 4:12 and there is no attic - all vaulted. Hence my concern I would like to do this right. Any other suggestions? R20 board with R20 batt inside. Why not just R38 batt inside? The architect I believe specified:
R38 batt inside
ZIPsheathing (R3) on roof
furring strips for the steel roof
Roofing |
|
|
|
|
Dana1
Senior Member
Posts:6991
|
| 21 Aug 2013 11:31 AM |
|
R38 batts inside requires a vented space between the roof deck and the insulation, which makes it harder to air-seal the roof. Any insulation crossed by framing will have significant thermal bridging, and plenty of places to leak air. R38 doesn't even meet IRC 2012 code min (though it meets IRC 2009 and current IL code min.)
With R20 on the exterior you DON'T need the vent gap, and it's easier to make the assembly fully air-tight (taped Zip decking is a good start, taping the exterior foam helps too) and puts R20 of thermal break over the ~R12 rafters. Mind you R20 is a MINIMUM required to be protective of the roof decking from wintertime moisture drives- it's OK to put more, and sometimes it's both easier & better to put 5-6" of polyiso up there and be done with it.
But in high-performance building 5-6" of exterior iso AND some amount of rafter-fill is necessary to meet stringent performance requirements. On a recent deep energy retrofit I was involved with (in climate zone 5, Worcester MA) we went with 6" of exterior polyiso with a full fill of spray foam between the pre-existing 2x6 rafters to better air-seal the plank roof decking. The center-cavity R value was a shade north of R60, but with the thermal bridging of the rafters factored in it came in only in the mid to high 50s, which is still a pretty good roof.
Batts are difficult to install perfectly around every framing dimension error and vent anomalies etc. Sprayed or blown goods (whether foam or fiber) tend to work a lot better. As I understand it JM Spider fiberglass can be damp-sprayed at 1.8lbs density at ~R4/inch without needing netting, which makes it a pretty good choice. (1lb Spider is cheaper, but would be lower R and a lousy choice.) Cellulose can also be damp-sprayed under roof decking at about R3.6/inch, but has both thermal mass and hygric buffering benefits, making it somewhat more protective of the roof decking. My personal preference would be cellulose, since after thermal bridging the difference in whole-assembly R is negligible, but I wouldn't pay extra for it if quotes for Spider came in cheaper. But the use of either are predicated on the presumption that you have at LEAST R20 of insulation on the exterior of the roof deck, without which the roof deck is required to be vented from below. |
|
|
|
|
BadgerBoilerMN
Veteran Member
Posts:2010
|
| 21 Aug 2013 08:46 PM |
|
Having done many hundreds heat loads, both long hand and with various computer programs (most of them with at least some element, if not entirely radiant floors) over the last 25 odd years that "DIY cooling load analysis" is wishful thinking at best.
As Dana, in is typical luminous style, correctly points out, the more primitive the tool the more masterful the operator must be to get the same results.
After using most of the available radiant-specific software we settled on Wrightsoft. One only need take a short tour to compare the options and find the truth of it. Or, you could go full AutoCAD with a little training your all set ;-). |
|
| MA<br>www.badgerboilerservice.com |
|
|
Surfsup
Basic Member
Posts:349
|
| 22 Aug 2013 09:03 PM |
|
R20 polyiso. I am wondering what this is going to cost as I am not an endless supply of money. Am I chasing the unicorn here? this plus blow-in insulation sounds great but also seems very expensive.
So you are saying:
metal standing seam roofing
furring strips
R20 polyiso on the TJIs
Blow in insulation between them
Isn't structural sheathing required here? If I use polyiso and be done with it what is stopping roof shear? Again I am a homeowner not a builder/architect. Just trying to figure this out so I get a nicely built home. Agree batt is tough to size properly for all the gaps. |
|
|
|
|
Surfsup
Basic Member
Posts:349
|
| 24 Aug 2013 07:31 AM |
|
Well here is waht i will likely do at this point using TJI 14" rafters starting from the outside going in:
Metal roof
Plywood sheathing
Tji14 rafter
R30+r15 high density batt
Foil sided 1" polyiso on the ceiling
Inside room air
I found r49 batt but it is 15" thick. The double stack of HD batt i propose is 13" and if i need to i can use a plastic rafter vent tacked to the underside of the roof sheathing.
What do you guys think? |
|
|
|
|
arkie6
Veteran Member
Posts:1453
|
| 24 Aug 2013 11:52 AM |
|
You need some roofing felt between the metal roof and wood decking. That metal will sweat when temperature and humidity conditions are right. You don't want that in direct contact with the wood.
Are you going to have soffit and ridge vents?
Don't the bottom of those deep engineered joists need to be restrained some way from rolling? Something more substantial than polyiso and sheetrock (I assume you will be installing sheetrock or some other finished surface on the inside?)
A way to strengthen the roof assembly would be to run 1x4 strapping every 16" to 24" perpendicular to the roof joists on the underside. But first, install vent baffles from soffit to ridge on the underside of the roof decking to allow venting. Then install cellulose insulation netting on the underside of the joists stretched tight and stapled in place. Then install the 1x4 strapping screwed or nailed to the bottom chord of the engineered joists. Then you can have dense pack cellulose installed that will outperform the fiberglass batts and likely at a lower installed cost. |
|
|
|
|
Surfsup
Basic Member
Posts:349
|
| 24 Aug 2013 06:19 PM |
|
Yes I trucated sorry: Metal roof 2x furring strips felt paper with 2-6' of water and ice at the eaves Plywood sheathing Tji14 rafter with possible plastic vent chutes if needed R30+r15 high density batt Foil sided 1" polyiso on the ceiling drywall Inside room air Then install cellulose insulation netting on the underside of the joists stretched tight and stapled in place. Then install the 1x4 strapping screwed or nailed to the bottom chord of the engineered joists. Then you can have dense pack cellulose installed that will outperform the fiberglass batts and likely at a lower installed cost. Interesting suggestion! I didn't think of this. I will investigate. Thanks. |
|
|
|
|
arkie6
Veteran Member
Posts:1453
|
| 25 Aug 2013 01:21 AM |
|
Another thought I had if you like the foil faced polyiso and have sufficient headroom is to replace the cellulose insulation netting with foil faced 1/2" polyiso rigid insulation installed against the engineering joists with the foil facer down. Then screw the 1x4 strapping every 24" perpendicular to the joists. The benefit of this is the foil facer vapor barrier plus ~R2 additional R value over the R3 that the foam provides because you have a radiant barrier facing a 3/4" air space. |
|
|
|
|