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Building a Foam-Free House

If you plan ahead, it’s not hard to build a house without any rigid foam or spray foam insulation

Posted on Jan 9 2015 by Martin Holladay

UPDATED on May 22, 2015

Many green builders want to build a foam-free house — that is, a house without any rigid foam insulation or spray foam insulation. The reasons behind this desire vary: some builders dislike foam because it is manufactured from petroleum; some because of off-gassing worries; some because of foam’s relatively high embodied energy; some because of the negative environmental effects of the blowing agents used to make foam; and some because they prefer to use natural building materials like straw bales.

I believe that the use of some types of foam insulation is often defensible, and that there are valid counterarguments to many anti-foam positions. However, I’m not going to debate these issues in this article. Instead, I’m going to provide recommendations for those who want to build a foam-free house.

Avoiding foam is usually easy

To build a foam-free house, designers and builders can choose from a variety of foam-free approaches to accomplish the following tasks:

  • To insulate a vented cathedral ceiling;
  • To insulate an unvented cathedral ceiling;
  • To insulate an attic floor;
  • To insulate above-grade walls;
  • To insulate a floor above a ventilated crawlspace;
  • To insulate the walls of a crawl space or basement;
  • To insulate a slab on grade or a basement slab; and
  • To seal air leaks.

Some of these tasks — for example, insulating an attic floor — are easy. Others — for example, insulating a slab on grade — are more challenging.

Vented cathedral ceilings

As long as the rafters are deep enough, vented cathedral ceilings can be insulated with a wide variety of fluffy insulation materials, including cellulose, denim batts, mineral wool, or fiberglass.

For more information on ways to detail vented cathedral ceilings, see How to Build an Insulated Cathedral Ceiling.

Unvented cathedral ceilings

Creating a foam-free unvented cathedral ceiling is challenging. The only approach that I can think of is to install a thick layer of semi-rigid mineral wool insulation above the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , followed by another layer of roof sheathing. The mineral wool insulation would need to be thick enough to keep the lower layer of roof sheathing above the dew point during the winter.

While it’s fairly common to install semi-rigid mineral wool insulation above roof sheathing, the method is usually restricted to low-slope (flat) roofs. Using this method for sloped roofs would be considered experimental, so builders should consult an engineer before proceeding with this technique.

Attic floors

This is an easy one. Foam insulation is rarely used for attic floors, so almost all of the usual materials — including blown-in fiberglass, blown-in cellulose, fiberglass batts, or mineral wool batts — will work in this location.

Above-grade walls

There are lots of ways to build foam-free above-grade walls. Options include:

A floor above a vented crawl space

In dry climates (for example, in areas west of the Rocky Mountains), vented crawl spaces can work well. Even in more humid climates, this approach can work as long as the crawl space openings are large enough to allow wind to blow under the house. (In other words, this approach works better for a house on piers than for a house with an enclosed crawl space.)

If you want to insulate a floor assembly above a vented crawl space, specify deep I-joists or open-web floor trusses, and fill the joist bays with dense-packed cellulose, dense-packed fiberglass, or fluffy batts.

Crawl space walls or basement walls

It’s not a good idea to insulate the interior side of crawl space walls or basement walls with a fluffy insulation material like fiberglass or mineral wool. These air-permeable insulation materials allow warm, humid interior air to contact the cold concrete, and this can lead to condensation and mold.

If you want a foam-free house, the solution is to insulate the walls on the exterior with semi-rigid mineral wool insulation.

Slabs on grade and basement slabs

It’s hard to come up with an affordable foam-free way to insulate slabs on grade or basement slabs. If you are building a foam-free house, you may prefer to avoid this problem by building a house on piers or a house with a crawl space foundation.

While it’s an expensive type of insulation, one solution is to insulate under the slab with Foamglas. For more information on this insulation product, see Foamglas – My New Favorite Insulation Material and On the Jobsite with Foamglas.

Roxul, a Canadian manufacturer of mineral wool, recently announced that it will support the use of horizontal mineral wool insulation under non-load-bearing concrete slabs. Builders who decide to use mineral wool for this purpose will need to use EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. or Foamglas under load-bearing concrete footings. For more information, see Sub-Slab Mineral Wool.

If you are willing to try an experimental method, you might want to insulate under your slab with perlite. For more information on perlite, see:

One final point: in a warm climate, a slab on grade may only need vertical insulation at the slab perimeter rather than horizontal insulation under the entire slab. If you are building in a warm climate, you might consider insulating the perimeter of your slab on grade with vertical pieces of semi-rigid mineral wool insulation.

Sealing air leaks

On most job sites, builders seal a wide variety of air leaks with canned spray foam. If you want a foam-free house, however, you’ll have to come up with other ways to seal air leaks.

For thin cracks, use caulk.

For wider cracks, use one of the many types of high-quality European air-sealing tapes sold by Small Planet Workshop, 475 High Performance Building Supply, or Performance Building Supply.

Martin Holladay’s previous blog: “Redefining Passivhaus.”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. Andrew Michler

1.
Jan 9, 2015 10:51 AM ET

I think houses on piers are a
by Bob Irving

I think houses on piers are a terrible idea when & where a foundation of some type directly on the ground is feasible, simply because the base temperature under the floor and insulation is far lower when the house is on piers. (In northern climes) That said, though, my brothers house in Fairbanks and the homes of most (all?) of his friends & neighbors are on piers, floating on the permafrost, and they work.


2.
Jan 9, 2015 10:59 AM ET

Response to Bob Irving
by Martin Holladay

Bob,
Thanks for your comments. Two other things to think about for builders considering a house on piers:

1. If the occupants ever develop a handicap, you have to install a very long wheelchair ramp.

2. In a cold climate, it can be hard to keep the pipes from freezing.


3.
Jan 9, 2015 11:45 AM ET

rock wool on the xterior of the foundation
by Dana Dorsett

While insulating the foundation on the above grade portion of the foundation with rigid rock wool can work, there are issues below grade. Internet scuttlebutt has it that below grade exterior rock wool tends to get thinner over time from both frost-compression and soil pressure, and that it's NOT a subtle issue.

FoamGlas, loose pumice backfill, autoclaved aerated concrete block, AirKrete, etc all have much more compressive strength, making them better suited for the below grade portion. But they all will suffer performance hits from liquid moisture that foam doesn't have, making bulk water management even more critical.


4.
Jan 9, 2015 12:41 PM ET

Edited Jan 9, 2015 12:43 PM ET.

Our unvented, foam-free cathedral solution
by Randy George

I'm not a builder, but the foam-free, unvented cathedral system we used in our house (built last year) was designed by a well-respected Vermont energy consultant and approved by our contractor, who takes a pretty conservative approach when it comes to trying unproven ideas. So at the risk of learning something that might keep me up at night, I'll share it here: From the outside: Standing seam roof, Ice and Water Shield, Advantech, 18" of dense pack cellulose, Siga "smart" vapor barrier, sheetrock. The rafters are 2 x 10's and then we hung 2 x 6's 2 in. below those. The house is in Central Vermont. Could this be added to the list of acceptable foam-free unvented cathedral ceiling methods?


5.
Jan 9, 2015 1:05 PM ET

Edited Jan 9, 2015 1:08 PM ET.

Response to Randy George
by Martin Holladay

Randy,
There are two problems with your roof assembly:

1. Many building scientists point out that the use of air-permeable insulation (like fiberglass or cellulose) in unvented cathedral ceiling assemblies is risky because of the risk of moisture accumulation in the cold roof sheathing.

2. Your roof assembly violates most building codes. (See, for example, the requirements of Section R806.5 of the 2012 IRC.)

For more information on code-approved ways to insulate a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.


6.
Jan 9, 2015 1:39 PM ET

Straw : Cellulose : Wood
by Bronwyn Barry

Thanks for this blog, Martin. If anyone is interested in learning more about this topic, and you live near Oakland, California, you may want to attend this event: http://www.passivehousecal.org/event/feb-phca-mtg-straw-cellulose-wood

Passive House California (PHCA) is collaborating with Architects, Designers and Planners for Social Responsibility (ADPSR), California Straw Builders Association (CASBA), Ecological Building Network (EBN) and Pankow Blue Construction to focus on three practitioners all using natural materials for hight performance assemblies. These cover houses, offices and mid-rise towers. It should be highly informative.


7.
Jan 9, 2015 4:17 PM ET

Edited Jan 9, 2015 4:20 PM ET.

Responses to comments will be delayed
by Martin Holladay

I'll be on vacation next week (Jan. 12 -16), so responses to any comments posted here (or on other blogs) will be delayed. I'll get around to answering questions, though, when I return to my desk.


8.
Jan 9, 2015 5:10 PM ET

Response to Martin & Randy George
by Dana Dorsett

Read the exceptions in section R806.5, in particular:

"Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:

" 1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling."

The key is what the ACTUAL vapor retardency of the Siga is during the wintertime. If it's 1 perm or less in an ASTM E96 dry cup test it would in fact meet the letter of the code. Other membrane type smart vapor retarders (say, MemBrain http://www.naturalspacesdomes.com/dome_store/dome_insulation_systems/ima... ) need to have the proximate air to the membrane under 35% RH to duck under 1-perm, which means you probably have to be pretty meticulous about not letting the interior humidity rise too much which can be an issue in very tight homes.

The cellulose will buffer quite a bit of moisture, sharing the moisture burden with the sheathing. While that isn't a solution on it's own in climate zone 6 (central VT), in combination with a smart vapor retarder it gives you a bit of margin.

In the spring once the average daily temps start running 40F it might be worth testing the moisture content of the roof deck in a couple places on the north (the shadiest/coolest and therefore dampest section of roof deck) to see just how much moisture accumulation you're getting into the roof sheathing. It's fine to poke through the Ice & Water Shield for the measurement, since the stuff is "self healing", and will seal punctures over time.


9.
Jan 9, 2015 5:50 PM ET

My Roof
by Randy George

Thanks Martin and Dana... I think I can still sleep at night. The critical piece here is the vapor retarder. I'm going to try to find out what the perm rating is for Siga in winter. I'm pretty meticulous about keeping relative humidity between 35 and 40% in the winter. According to what you say, that might be a little too much. I was going over 35 for comfort, but that might not be a good idea with this roof. I've always wondered how much moisture makes it through the sheetrock. If it's 40% outside the sheetrock, maybe the proximate air to the membrane is less? I like the idea of testing the moisture of the sheathing in the spring. That's good to know that I can poke a hole in the Ice and Water shield, but to get to that I have to get through the standing seam.


10.
Jan 9, 2015 10:36 PM ET

not Larsen trusses
by flitch plate

The assembly in the photo is not really a truss, its more of a series of stacked boxes. A truss would not be needed in this application anyway as the building structure is already framed and these are simply to hold the insulation in place. In fact for DP cell install, this setup is preferable since it create lateral pressure boundaries as opposed to Larsen trusses which have a problem getting the resistance for DP cell.


11.
Jan 10, 2015 3:17 AM ET

Foamglas unvented cathedral and basement walls
by Adam Liberman

You mention Foamglas for under slabs, but don't mention it as an alternative to foam for an unvented cathedral ceiling (top half in condensation zone, with fluffy insulation below it), or for basement walls. It seems to me that it would be ideal (in fact the only suitable non-foam method) for those situations, since it is not vapor permeable.


12.
Jan 10, 2015 3:29 PM ET

Exterior below grade mineral wool
by norm farwell

There seems to be some disagreement on whether mineral wool board can replace foam in below grade applications. Martin doesn't mention it it and implicitly dismisses it: "It’s hard to come up with an affordable foam-free way to insulate slabs on grade or basement slabs." Dana cites rumors of compression over the long term.

But BuildingGreen.com and Roxul disagree. A couple of months ago, in an article titled "Move Over, Foam:Sub Slab Mineral Wool is Here," Alex Wilson wrote that Roxul had approved its ComfortBoard IS product for use under slabs, citing engineering data, research, and a number of successful uses of it in those situations.
http://www2.buildinggreen.com/article/move-over-foam-sub-slab-mineral-wo...

Roxul's marketing cites a report from 2009 by the Danish Technological Institute on long term compressive deformation that would seem to contradict Dana's concerns:
http://www.roxul.com/files/RX-NA_EN/pdf/Brochures%20and%20Sell%20Sheets/...

Thoughts?


13.
Jan 10, 2015 8:08 PM ET

Thoughts?
by Malcolm Taylor

I really hope they are right.


14.
Jan 11, 2015 1:23 PM ET

Vapor closed exterior - foam free roof w smart vapor retarders
by floris keverling buisman

Since the foam free roof proposed by Randy, has a vapor closed roof underlayment (ice & water), the airbarrier on it's interior should indeed be smart/vapor variable. Siga majpell has a fixed perm rating of 0.68perms and while being a class II vapor retarder in winter. It will also be a class II vapor retarder at 0.68 perms in summer - so not variable or smart retarder, a fixed one like siga's specs show - Any construction humidity or moisture infiltration by unforseen airleaks, won't easily diffuse back inward in summer - and will ping-pong there - which can cause rot/mold.

Under a metal roof, which can be easily vented, a vapor permeable roof underlayment like ProClima's SOLITEX membrane would allow the roof also to dry outward and prevent these issues (and be code compliant). However many contractors like ice&water or asphalt shingles, but still would like a foam free roof and a vapor variable retarder can accomodate that safely when installed properly/airtight.

membrain is as Dana points out is a vapor variable retarder, but it does open up rather early - which could lead to humidity diffusion into the assembly in winter. ProClima's INTELLO that my firm www.foursevenfive.com distributes stays below 1 perm longer - it surpasses 1 perm after 60%RH and at that time rapidly opens up - see image) - to allow inward vapor drive to dry out any unforseen humidity in summer. Leading to durable, foam free roofs and walls with drying reserves that are larger than the moisture stresses from construction.

INTELLO smart vapor variable retarder by ProClima - graph.jpg


15.
Jan 13, 2015 1:12 PM ET

Rainscreen Roof?
by bryan shephard

How would this cathedral roof work? Insulated 2"x10" rafters, taped plywood, 2"x2"s that create a 11/2" vented air space, then another layer of plywood, topped with felt and roofing. The sheathing would get cold but it would have a good chance to dry, similar to rainscreen walls.
I'm in a moderate maritime climate by the way. Thanks.


16.
Jan 13, 2015 3:07 PM ET

Edited Jan 13, 2015 3:07 PM ET.

GUTEX for unvented roof
by Mark Fredericks

Another product that supposedly works well for an unvented roof is a wood fiberboard insulation material called GUTEX that 475 sells. It's a highly vapor permeable insulating sheathing, and appears to reduce a lot of the risk associated with dense packing roof rafters.
They illustrate how it can work in a roof here:
http://foursevenfive.com/alford-ma-gutex-roof-makes-sustainable-high-per...


17.
Jan 13, 2015 7:12 PM ET

Dana's comment
by Jeff Stern

Dana - You seem to suggest that randy's unvented roof would meet code with the correct vapor retarder, but the exception you note is for a vented roof (it just reduces the amount of ventilation area). Am i missing something in the code?


18.
Jan 13, 2015 9:41 PM ET

moisture checking methods
by Randy George

O.k., I think the various comments that have been made about my roof have made me concerned enough to do a moisture check this coming spring. I'd like to hear about any methods people have of doing this. It has been mentioned that I can pierce the Ice and Water Shield without damaging it, but I still have to get through the standing seam roofing. And once I'm into the Advantech I have to get all the way through that to the inside. I've heard of people piercing the sheetrock and the vapor barrier with a very long moisture probe and going all the way to the sheathing that way. That sounds pretty tough and risky too. Anyone care to comment on how they would check he moisture level on my assembly?


19.
Jan 14, 2015 11:42 AM ET

Roof insulation
by Ed Dunn

The platinum standard here in Flagstaff, AZ (Zone 5) is Willow Bend Environmental Education Center. They use ⅛ cord of wood per year and now AC. R50 blown-in fiberglass with energy leg scissor trusses in the vented attic. This performance has held for 14 years. It is a building that is occupied 6 days a week. Temps have never fallen below 55 and normally are at 65 since it is a passive solar building with ⅓ of the south in trombe wall and the rest in glass. Straw bales in the N, E and W walls. We did use rigid foam under slab (4' in from stem) and down the exterior of the stem to top of footing.


20.
Jan 14, 2015 8:53 PM ET

Been there, done it, have the award
by Alec Shalinsky

Excellent thoughts Martin. When we built our Italian CasaClima Gold house, we used many of the techniques that you mention. Have a look (www.vettabuilding.com). We are also starting a new project in Stratford, Ontario that will use many of these systems.


21.
Jan 15, 2015 6:54 PM ET

Our cozy cottage
by John Holscher

Thanks for this article Martin. It answers many if not all the questions that arise when going down this path. My wife Joyce and I were very uneasy about the use of foam to insulate our new home as had been recommended by other folks we talked to. Thanks to lots of good information from you and other forum participants we came close to the foam-free house.
The only foam we used was for the basement wall and slab insulation, slab on grade insulation (the house has a basement 1/3 and slab on grade 2/3) and spray foam to seal around window and door openings.

I'm happy to say that we are snug as a bug in a rug and are very pleased with the choices we made. Here they are...

Basement slab: Tu-Tuff poly vapor barrier, 2" XPS foam insulation, 4" concrete
Basement walls: 10" thick using Thermomass 4/2/4 insulation system...4" concrete/2" XPS foam insulation/4" concrete.
Slab-on-grade: Tu-Tuff poly vapor barrier, 4" XPS foam insulation (2 alternating layers of 2"), 4" concrete
Slab-on-grade walls: 2" XPS foam insulation inside walls
Above-grade walls: Double 2x4 walls with dense pack cellulose insulation, 10" total thickness
Vented cathedral ceiling: 2x12 rafters with strips of 1" XPS foam insulation on inside rafter edge, 1 1/2" vent space under sheathing and dense pack cellulose insulation.


22.
Jan 18, 2015 4:55 AM ET

Response to Dana Dorsett (Comment #8)
by Martin Holladay

Dana,
I agree with Jeff Stern (Comment #17) about code requirements for a ventilated air gap above air-permeable insulation when air-permeable insulation is the only type of insulation used.

Although your comment cites an "exception in section R806.5," the exception you quote is actually found in section R806.2, not R806.5.

The way I read R806.5, there is no wiggle room on this issue, other than the usual (blanket) wiggle room provided by the fact that a local inspector can approve whatever he or she wants.


23.
Jan 18, 2015 5:02 AM ET

Response to Randy George (Comment #18)
by Martin Holladay

Randy,
I think that the easiest way to check the moisture content of your roof sheathing (at this point, since you didn't embed monitors in the sheathing when the house was under construction) is to use a moisture meter with very long probes. This method (and caveats about false readings) is discussed in the following two articles:

Dense-Packed Cellulose and a Wrong-Side Vapor Barrier

An Affordable Zero-Energy House in Massachusetts


24.
Jan 18, 2015 5:07 AM ET

Edited Jan 18, 2015 5:08 AM ET.

Response to Bryan Shephard (Comment #15)
by Martin Holladay

Bryan,
Your suggested roof assembly will work, as long as you pay attention to airtightness -- especially at the level of the finish ceiling material (usually drywall). No recessed can lights allowed!

Your method is one of the methods I discuss in my article, How to Build an Insulated Cathedral Ceiling.


25.
Jan 19, 2015 9:25 PM ET

Edited Jan 19, 2015 9:43 PM ET.

Sturdy and insulated
by Brian Godfrey

In the coastal areas of the PNW, moisture is a huge problem. Sometimes the rain is blasted at the house by 100+ MPH winds. The older houses framed with notched in diagonals and sheathed in Celutex have far fewer problems with rot than the older ones with more conventional construction. They do tend to sway in the wind.
But newer houses must withstand an earthquake of 9.2 or something like that. And winds of 130 MPH.
Making a house dry, bomber strong, and highly insulated is a lot to ask.
I looked at some of the links including the Larson Truss one. I wonder what those wiser than I would think of the following: frame and sheathe the house to earthquake and wind load standards, cover with a vapor barrier, and install Larson trusses. But then sheathe that exterior surface with Celutex for its breathability. That might be a good way to do it in any case where you are concerned about condensation on the inside of that outer layer of sheathing.


26.
Jan 20, 2015 7:48 AM ET

Response to Brian Godfrey
by Martin Holladay

Brian,
You aren't the only builder who prefers to specify a vapor-permeable exterior sheathing like fiberboard. The main disadvantage of fiberboard sheathing is its tendency to belly outward when cavities are filled with blown-in insulation.

For more information on vapor-permeable wall sheathing, see these two articles:

How Risky Is Cold OSB Wall Sheathing?

The Klingenberg Wall


27.
Jan 21, 2015 8:56 PM ET

Brian
by Malcolm Taylor

Faced with the same problems I have been thinking along the same lines as you propose. Build the house to meet the BC seismic code with the required braced wall panels or engineered shear walls. Then add some variation of exterior framing: A second wall to make make a double wall, Larsen trusses or exterior foam. I'm not sure fibreboard is a great choice as it is hard to work with and offers a poor nailing base for siding. Perhaps eliminating the exterior sheathing is the way to go. Although I've still to do a project that goes that route, some of the projects featured here seem to have successfully done so.


28.
Dec 18, 2016 10:37 PM ET

Foam-Free Unvented Cathedral Ceiling/Roof
by Fred Williams

Could Roxul Comfortboard IS be substituted for rigid foam above the roof sheathing in an un-vented cathedral roof assembly? I live in zone 5 so 5" of Roxul (R-20) would be sufficient to keep the sheathihg warm enough to prevent condensation. 2 x 12 joist bays below the sheathing (11.5" actual with no vent space) would be filled with dense pack celluose for an additoinal R-40. Interior would be finished with drywall.


29.
Dec 19, 2016 6:14 AM ET

Response to Fred Williams
by Martin Holladay

Fred,
From a building science standpoint, installing mineral wool on the exterior side of the roof sheathing of a sloped roof makes sense. It should work.

That said, it's almost never done in the U.S., so builders who attempt it can expect resistance and skepticism from roofers and building officials.

Here is a link to a Q&A thread where this approach was discussed: Mineral wool roof. On that thread, I posted these comments:

"While exterior mineral wool insulation is routinely used for flat (low-slope) roofs, especially in Europe, its use on steep roofs is unusual. You will be experimenting. ... I recently spoke to [building scientist] John Straube about his building project using mineral wool on the exterior side of a sloped roof assembly, and he said everything went fine, and that the roof didn't feel squishy once the second layer of plywood was installed. (Of course, John wasn't doing the actual installation -- he was directing a contractor.)"


30.
Dec 19, 2016 7:24 PM ET

Response to Martin Holladay
by Fred Williams

Martin,

Thanks for your response and the link to the Mineral Wool Roof thread. That thread led to another thread titled Using Open-Web Trusses as Rafters for Superinsulated Roofs. I'm wondering if an open-web truss would substantially reduce thermal bridging through the structure and mitigate the need for any foam or mineral wool whatsoever? Do you forsee any problems with using a 20" deep open web parallel chord truss with a baffle at the top chord (1.5") for ventilation, filled with 18.5" of dense pack cellulose (R-60), and finished with sheathing, underlayment, and asphalt shingles on the top, and drywall on the bottom? The total thickness of the assembly would be slightly more than the 2x12 plus mineral wool combination, but I assume much easier to build and cheaper.


31.
Dec 20, 2016 6:07 AM ET

Edited Dec 20, 2016 6:10 AM ET.

Response to Fred Williams
by Martin Holladay

Fred,
Q. "I'm wondering if an open-web truss would substantially reduce thermal bridging through the structure and mitigate the need for any foam or mineral wool whatsoever?"

A. The use of a deep open-web truss will substantially reduce thermal bridging through the structure, as Alex Wilson explained in the article you are talking about (Using Open-Web Trusses as Rafters for Superinsulated Roofs).

There never was a "need" for exterior mineral wool. That was simply a choice you considered. Every type of roof assembly you can think of has advantages and disadvantages.

Here's how I would analyze what an insulated roof assembly "needs": If the roof is unvented, it needs exterior insulation (which is usually rigid foam). If the roof is vented, it doesn't need exterior insulation. Remember, however, that not every roof can be vented. If your roof has valleys, hips, dormers, or skylights, it's not a good candidate for the vented approach.

Q. "Do you foresee any problems with using a 20" deep open web parallel chord truss with a baffle at the top chord (1.5") for ventilation, filled with 18.5" of dense pack cellulose (R-60), and finished with sheathing, underlayment, and asphalt shingles on the top, and drywall on the bottom?"

A. The assembly you are talking about (similar to or identical to the assembly described by Alex Wilson in his article) will work fine. Do I foresee any problems? Well, your framers may be unhappy about using open-web trusses as rafters, because they will probably be unfamiliar with the methods required to attach these trusses at the ridge, and they will probably be unfamiliar with blocking details. As I said, every approach has advantages and disadvantages.


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