Wall, floor, and roof build layers
A little background.
Starting construction on a small house in Texas, west side of Houston,
Climate zone 2A – wet & humid
Conditioned living space is 1280 square feet
House is being built in a FLOOD WAY, Yes I said in a FLOOD WAY
We already have approval to do this, which has taken us quite some time to accomplish.
House will be elevated approximately 6′ off the existing grade on concrete filled
fiberglass pipes with re-bar reinforcing inside. I have designed a steel collar, plate, &
C-channel attachment system that our engineer is comfortable with.
House is being built on a very limited budget, with as much recycled material as possible.
and lots of personal labor.
Have some (many) questions on building design I see most questions for climate zones
north of where iI live, very few for my area.
1st is wall construction.
House to be insulated on exterior with rigid polyiso insulation board, 2×6 studs
Exterior sheeting will be recycled corrugated sheet steel.
What is the best way to construct the wall from the outside moving towards the inside?
Exterior sheet steel
House wrap DuPont Tyvek Commercial Wrap D ( to keep furring strips dry) or tar paper ?
Furring strips
2 layers of 2″ rigid polyiso insulation board ( which facing material ? )
Structal wall support, wrap complete house or just corners with 4 x 8 plywood or other method
2 x 6 studs cavities filled with pink insulation Faced insulatio OK here ?
NO VAPOR BARRIER
1/4″ drywall taped & floated ( This will be air permeability ? to allow drying to the interior ? )
non latex paint, or wood accented walls
This will dry to the interior
2nd is floor
Per our engineer we will be using 1-1/8 x 4 x 8 Tongue-and-Groove OSB Sturd-I-Floor
due to joist decision.
Floor Insulation question.
Will I be safe to use a insulated nail base product over the sub-floor to insulate the floor ?
3rd. is the roof
Simple straight shot gable roof, 36 feet in length, leaving 2 feet overhang on both the east & west ends of house, Non vented roof
New material for roof over conditioned living space.
This will be an open cathedral with rafter over the ridge beams
( 2 @ 6″ X 16″ ) with 3 supports. No can lights, Only openings penetrating the roof will be
a wood stove pipe and a vent pipe.
Interior to be exposed rafters, exposed wood & metal interior ( think rustic )
Should this be the same design as the walls
Outside to inside
Metal roofing
tar paper
air gap
furring strips
2 layers of 2″ rigid polyiso insulation board taped& caulked ( again which facing material ? )
plywood or ? a mix of 1 x 4, 1 x 6, ( think cheap, recycled material )
From this point I am confused on how to end up with the desired interior look, along with the
air barrier without using drywall on the interior.
I will be happy to clear up any question that are sure to come.
Thank you for taking the time to help /point me in the CORRECT approach.
Mark
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Replies
Von,
Your WRB should be interior of the furring strips. The furring strips do not need to be kept dry. You will want to determine where or what is functioning as the WRB in this wall system. Reference Where does the Housewrap Go and other resources on GBA and BuildingScience.org You will be flashing your windows etc to whatever layer is your WRB.
Von Leyser,
Do you have an architect or contractor to help you? Or are you an owner/builder figuring out these details as you go along?
Most building codes require homes in Climate Zone 2 to have roof insulation with a minimum R-value of R-38. Your plan to use 4 inches of polyiso (about R-25 or or R-26) for your roof doesn't meet minimum code requirements.
To answer your question about rigid foam: The type of facing on the rigid foam doesn't matter.
Here are some links to articles to get you started:
How to Install Rigid Foam On Top of Roof Sheathing
Where Does the Housewrap Go?
All About Rainscreens
How to Install Rigid Foam Sheathing
All About Water-Resistive Barriers
Choosing Rigid Foam
The roof may come close to making IRC 2015 code min performance on a U-factor basis, depending on the thickness of the t & g ceiling wood & roof decking. Code max is U0.030, or R33.3 "whole assembly R".
The foam is ~R25 ish if 1lb foil faced, (R23 if 2lb roofing foam)
Interior & exterior air films add about R1, plus another R1 if there's an air gap under the metal roofing and the next layer, so now you're at R27-ish.
With 2x6 t & g you're looking at another R2, now up to R29...
3/4" plywood is another R1, for R30.
If it's 1lb foil faced polyiso it's really close to making it- another half inch would get you there. If 2lb fiber faced roofing iso it'll need at least another inch.
There are probably several square miles of reclaimed 2"- 4" roofing iso in Texas, typically at
https://houston.craigslist.org/search/sss?query=rigid+insulation
https://beaumont.craigslist.org/mad/d/iso-rigid-foam-insulation/6487989392.html
The 2" roofing iso is fine for use on walls too, but you may want to add a 1/2-1" layer of foil faced on the exterior if there's going to be a gap to the insulation. It's easier to air seal foil facers reliably with purpose-made foil tapes too.
Having 4" of exterior wall foam AND 5.5" of fluff in the cavities is extreme overkill for that climate. Code-min could be met with just 2" of reclaimed roofing polyiso over the exterior , and nothing in the wall cavities. Going with a 2x4 / R15 studwall , 2" of roofing foam overlayed with 1/2" of foil-faced foam at the air gap (2.5" of foam total), with typical structural sheathing and wallboard, etc, would be ~R25-ish whole-wall, which would beat code min performance even in Fairbanks AK, and is better than 2x code for Houston TX. For a starting point of where the financially rational limits might be, see the zone 2 row of Table 2, p.10 in this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Note, they're suggesting R15-ish whole wall for zone 2 which would be beaten by 2x6/R20 wall with 16" o.c. studs + 1/2" of exterior polyiso, or a 2x4/R15 + 1" exterior polyiso. But with reclaimed foam stepping it up to R25-ish whole wall isn't a huge cost adder. The R40 compact roof would be 6" of reclaimed roofing polyiso, with typical roof decking and finish ceiling, no fluffy stuff between rafters.
Tyler, I wasn't 100% about keeping the furring strips dry, I have seen some conflicting pictures on the internet so if they do get wet they should dry with an air gap. Got it. Thank you.
Flashing windows. I saw the Thermal Buck product & believe I will be using that to help stop the thermal heat transfer.
My water resistent barrier will be on top of the ridgid insulation then.
Matrin,
Do I have an architect or contractor to help you? No. The one I have was for the structural platform we will need to build to get us a Developmental Permit to build in a Floodway.
Now I am going to be the owner builder figuring out these details BEFORE things get started
so I can hopefully plan this out & locate materials before hand. Will be building this in steps, 1st will be the footings, then uprights extending up from the footings to desired height, collers & plates on top, then steel I beams set in place, metal bar joists on top of beams, tongue & groove on top of the bar joists. Engineer says this will be way over kill, but have 85% of material on site & engineer worked backwards to use what was on hand.
Engineer call out for 18" holes, I am going to be drilling 24" holes, going larger here is not going to cost anymore.
2nd.
Most building codes require homes in Climate Zone 2 to have roof insulation with a minimum R-value of R-38. Your plan to use 4 inches of polyiso (about R-25 or or R-26) for your roof doesn't meet minimum code requirements.
This is why I am here, I have no problem adding additional insulation for the roof to meet code requirements and even going above and beyond it, building better now will be cheaper, in the end, just want no problems with vapor, condensation issues.
The polyiso seems the best for me, & I would prefer to buy used, BUT I question the used insulation how it was stored, covered, concerned about it being WET. I see it on transportation trucks not covered.
I get confused about the different water barriers, air permeability, how much is acceptable and keeping the barriers in what I need for my area.
I need to subscribe and start reading.
Thank you for responding
Dana,.
Is is BAD to go overkill on insulation, say 4" on exterior wall, 1/2" foil face for ease of taping, Tyvek Commercial Wrap over the foil face to act as the WRB / air barrier, furring strips, then metal siding.
Living in a trailer currently and after reading some about insulation, thermal bridging, our crappy single pane windows, I am not seeing an issue with adding more than the required
minimum, I want this small house COMFORTABLE without have excessive electric bills.
What would help me tremendously is having a knowledgeable person suggest the the correct
stack up for me for the ROOF, WALLS ,& FLOOR using the above suggestions
2x6 walls, 4" iso, 1/2 foil face
6" of roof insulation over the rafters, nothing in between
Did my floor insulation sound OK ?
I'm confused, going to read, and will return soon
Thanks for the help.
Building up is good. I would still think about what might happen if things get wet. Two recent articles that you may be interested in reading:
1. Ted Cushman article at JLC, Flood-Hardy Construction.
2. BSI-101 @ BSC, also dealing with floods and how to design such that houses can be cleaned up after suffering from flooding. There are other links from there. Author Joe L. has done a lot of work in hurricane and flood zones.
Good luck. Yours is an unusual project. If you get a chance to document it, please do so for the edification of others.
Andrew,
It would be really interesting to know what percentage of houses damaged during floods had taken even rudimentary design consideration of the possibility they would be affected.
Von,
Q. "What would help me tremendously is having a knowledgeable person suggest the the correct stack up for me for the ROOF, WALLS ,& FLOOR using the above suggestions:
2x6 walls, 4" iso, 1/2 foil face
6" of roof insulation over the rafters, nothing in between."
A. Walls, from the interior to the exterior:
- Presumably, some type of fibrous insulation between the studs (otherwise, you could save money by framing with 2x4s).
- OSB or plywood sheathing
- Two layers of 2-inch polyiso with staggered seams
- Housewrap
- 1x4 strapping to create a rainscreen gap
- Siding
Roof, from the interior to the exterior:
- Exposed rafters
- Sheathing boards, also visible from the interior
- A durable air barrier (for example, a peel-and-stick membrane or a European air barrier membrane)
- Two layers of 3-inch-thick rigid insulation with staggered seams
- Plywood or OSB roof sheathing, screwed through the foam to the rafters
- Optional: A vent channel and another layer of roof sheathing
- Roofing underlayment
- Roofing
While I've tried to answer your questions, you really should read the articles I suggested. There's enough information in the articles to explain what I have just summarized.
Response #5
Andrew C
I read the Ted Cushman article.
I went through one flooded house, a brick sided house, the CORRECT way to fix the
underlying, contaminated, disgusting flood water problem is to remove the brick, cut the
black celotex fiberboard at either 2 or 4 feet, remove the drywall at 2 or 4 feet, along with the
insulation, sanitize as best as possible the saturated 2x4 studs, let dry and begin the rebuild process.
Try running the removal of the brick past your insurance company and see how that flies with them. Long story short I walked away from that house before it was finished, end of story.
GOOD RIDDANCE.
Moved to a single wide trailer on 3.5 acres, Tax flood of 2016 brought water level on property
from 16 inches in driveway to over 6 feet on back of property. I have had enough problems with flood waters to say the very least. Water well was also contaminated, asked FEMA for assistance, they had no clue about what to do with the well, Received $500.00 or $600.00 from them, That went far.
Anyways now, fast forward
I am building more than twice the required height above base flood elevation with an additional 12" on top of that to the bottom of my sub-floor, There is no way I am going to deal with that
nasty water again.
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