New house build in Macon, Nc
Last Post 22 Mar 2020 11:41 PM by A W Warn. 3 Replies.
Printer Friendly
Sort:
PrevPrev NextNext
You are not authorized to post a reply.
Author Messages Resolved
BigdaddygbUser is Offline
New Member
New Member
Send Private Message
Posts:5

--
14 Mar 2020 05:43 PM
Ok so I am building a new home in Macon , NC starting this year. I am planning to build as follows. Basement= CMU with water barrier and drain outside, inside 2 " closed cell spray foam on walls and into floor joist area and r15 rockwool insulation in studded out wall. Slab to have 10" of stone under it to provide and area for water to go and not reach the slab then ( I believe ) its r16 or r25 ips foam underneath in 2 layers covered and sealed with 7mil vapor barrier then 4" of concrete. Vapor barrier will be carried approx 18" up wall and sealed behind spray foam on basement wall. The house is a modest rancher with approx. 3000 sq ft and exterior walls to be 2x6 construction with flash and bat of 2" closed cell foam and R15 rockwool inside. Outside to have zip system 1-1/2" r6.6 sheathing with liquid flashing applied to all nail holes and joints and seams. Next it will have board and batten siding on it and Shaker shingles above trim line. Roof is going to have steel roofing over 7/16" or 5/8" zip roofing ( haven't come to a decision yet) on a 12/12 and 6/12 pitch roofs. My question is this (and some people have had similar situations to mine but not really exactly like mine) So I want to enclose the envelope, my HVAC will be in my basement as most of my ceilings are cathedral or 10 ft ceilings, So I was wondering with either 2x8 or 2x10 rafters ( still waiting on the manufacturers to get back about size and options from my architect) can I do a closed cell spray foam in the rafters without a vented roof? Or do I have to have a vented roof and block off the eves and run baffles up to ridge vent? I planned to use an open grate hidden into the attic from a room somewhere and use a return vent in the attic back to the HVAC from attic to condition the space. This way it would draw either cooled or warm air in to the attic and draw it back to the HVAC to process and run through the house.

Any Suggestions and direction would be great.

Thanks in advance.
BigdaddygbUser is Offline
New Member
New Member
Send Private Message
Posts:5

--
17 Mar 2020 06:20 PM
ok so got my info from advanced energy in Raleigh, NC. thanks anyways.
Dana1User is Offline
Senior Member
Senior Member
Send Private Message
Posts:6991

--
17 Mar 2020 08:24 PM
The 2" continuous closed cell + 2x4/R15 basement wall is considerably higher performance than the above grade R6 ZIP clad 2x6 2"closed cell + R15 flash & batt.

Closed cell foam (at any thickness) between framing is a waste, since it is thermally bridged by R1.2/inch framing for ~25% of the face area of a typical above-grade wall. Do the math:

https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf

It's considerably higher performance and greener to go with continuous 2" of foil faced polyiso (or even 1.5" polyiso) over standard ZIP sheathing and R23 rock wool in the cavities. Board & batten siding works just fine mounted on 1x4 girts 16-14" o.c. through-screwed to the studs with pancake head timber screws, and a foil facer next to the 3/4" air space adds another R1 in "whole wall-R" performance. Foil faced sheathing polyiso has about half the polymer per R of closed cell foam (using a similar but not identical chemistry), and uses very low impact hydrocarbon blowing agents- predominantly pentane variants with a 100 year global warming potential of ~7x CO2, roughly comparable to HFO1234ze used in the more expensive HFO blown closed cell foam, but WAY below the impact of industry standard (and Kigali amendment contraband) HFC245fa (~1000x CO2 @ 100 years).

Slamming 7" of HFO blown 2lb foam to hit the R49 center cavity R on the roof is similarly wasted on the thermal bridging, though not as severely as with wall assemblies due to a typically smaller framing fraction for roofs. A cheaper & better and modeistly higher perfornance solution would be 3.5" polyiso nailbase panels above the structutral roof deck on 2x8 rafters with R30 rock wool in the cavities, or better (greener, cheaper) 3" nailbase panels on 2x10 rafters filled with cellulose. In US climate zone 4 (Raleigh) it only takes R15 above the roof deck out of a total R49 to have sufficient dew point control at the roof deck to mitigate against wintertime moisture accumulation in the roof deck. With the 2x10/cellulose solution you'd have the R15 with a 3" polyiso nailbase panel and about R34-R35 in the cavity. The cellulose would "share" the wintertime moisture burden, wicking moisture out of the structural wood without damage or loss of function. Cellulose has a favorable thermal diffusivity compared to rock wool- lowering and delaying the peak ceiling temperatures during the cooling season. Better yet, cellulose has as NEGATIVE CO2e footprint, sufficient to offset the foam in the nailbase panel.

Polyiso, EPS, and rock wool have roughly comparable CO2e footprint per R at about half that of HFO blown closed cell foam, or 1/3 that of HFC blown foam (and an order of magnitude lower than XPS), but cellulose is "sequestered carbon". While denim batts have similar hygric buffering & thermal diffusivity to cellulose, they are roughly carbon neutral rather than negative, due to the comparative agricultural & processing impacts of cotton vs. paper.

https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

With 14" I-joist rafters and a vented roof it's possible to go even greener at reasonably low cost with an all cellulose solution using 1/2" asphalted fiberboard MDF tacked to the underside of the I-joist flanges as the vent baffle, which leaves a 1.5" vent channel, and 12" of cellulose. That's only R45 of cellulose, and R1.3 of fibeboard, for a total of R46-ish at center cavity, but the web of the I-joist has only ~1/3 the thermal bridge of 2x dimensioned lumber- it still beats code performance on a U-factor basis. (= U-0.026 max, or R38.5 "whole assembly").
A W WarnUser is Offline
New Member
New Member
Send Private Message
Posts:15

--
22 Mar 2020 11:41 PM
Posted By Bigdaddygb on 14 Mar 2020 05:43 PM
Ok so I am building a new home in Macon , NC starting this year. I am planning to build as follows. Basement= CMU with water barrier and drain outside, inside 2 " closed cell spray foam on walls and into floor joist area and r15 rockwool insulation in studded out wall. Slab to have 10" of stone under it to provide and area for water to go and not reach the slab then ( I believe ) its r16 or r25 ips foam underneath in 2 layers covered and sealed with 7mil vapor barrier then 4" of concrete. Vapor barrier will be carried approx 18" up wall and sealed behind spray foam on basement wall. The house is a modest rancher with approx. 3000 sq ft and exterior walls to be 2x6 construction with flash and bat of 2" closed cell foam and R15 rockwool inside. Outside to have zip system 1-1/2" r6.6 sheathing with liquid flashing applied to all nail holes and joints and seams. Next it will have board and batten siding on it and Shaker shingles above trim line. Roof is going to have steel roofing over 7/16" or 5/8" zip roofing ( haven't come to a decision yet) on a 12/12 and 6/12 pitch roofs. My question is this (and some people have had similar situations to mine but not really exactly like mine) So I want to enclose the envelope, my HVAC will be in my basement as most of my ceilings are cathedral or 10 ft ceilings, So I was wondering with either 2x8 or 2x10 rafters ( still waiting on the manufacturers to get back about size and options from my architect) can I do a closed cell spray foam in the rafters without a vented roof? Or do I have to have a vented roof and block off the eves and run baffles up to ridge vent? I planned to use an open grate hidden into the attic from a room somewhere and use a return vent in the attic back to the HVAC from attic to condition the space. This way it would draw either cooled or warm air in to the attic and draw it back to the HVAC to process and run through the house.

Any Suggestions and direction would be great.

Thanks in advance.


... I've been pondering this same issue while considering how I'm going to construct the roof. The minimum size of the rafter is dictated by the building code, see the span chart in the code book. ... In your area (high wind/hurricane prone, low snow load, low earthquake, not extremely cold winter, hot humid summer) I think I would go at least one size larger rafter than minimum.(this is my opinion, not required) I think that is a 2 x 10 @ 16" o.c. ... There are some very specific code requirements about how to construct a vented or non-vented attic. Read here> https://codes.iccsafe.org/content/NCRC2018/chapter-8-roof-ceiling-construction#NCRC2018_Pt03_Ch08_SecR806 …... I'm still thinking about it but this is what I'm leaning toward:> (1.) Standing seam roof (2.) Grace Ice Water barrier (because of high wind driven rain) (3.) 5/8" OSB sheathing (4a.) Air vent space between rafter, above insulation (4b.) 6" rockwool batt between rafter (4c.) 4" rockwool batt beneath rafter, diagonal to rafter ... The reason I want that air space is to remove moisture at the roof peak. If the rafter space is unvented and done like building code requires, the wood sheathing is enclosed between water and air barrier so it can't breathe if it gets a leak. That is a sure way to encourage mold/rot. ... I do not like foam insulation unless it is on the outside/exterior of the building envelope. How will you inspect for termites if the insulation on the interior of your basement is covered with spray foam (there is a code requirement) ... I am a retired general contractor in central NC, 45+ years of building experience. IMO, based on your local conditions, 1-Whatever insulation you choose, lean toward materials that will not absorb and hold moisture. 2-Consider a whole house dehumidifier in addition to the AC
You are not authorized to post a reply.

Active Forums 4.1
Membership Membership: Latest New User Latest: janvin New Today New Today: 0 New Yesterday New Yesterday: 0 User Count Overall: 34705
People Online People Online: Visitors Visitors: 164 Members Members: 3 Total Total: 167
Copyright 2011 by BuildCentral, Inc.   Terms Of Use  Privacy Statement