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Getting Insulation Out of Your Walls and Ceilings

The PERSIST technique puts insulation where it belongs

Posted on Aug 7 2009 by user-756436

More and more builders have realized the advantages of leaving stud bays empty and putting all of a home’s insulation outside of the wall and 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. . If done correctly, exterior insulation can help produce a building that is almost airtight, very well insulated, and almost immune to water damage.

The construction method was first developed in the early 1960s by the National Research Council of Canada. In its purest form, the method is known as PERSIST — an acronym for Pressure-Equalized Rain-Screen Insulated Structure Technique.

Here’s how you build a PERSIST house:

  • Frame the house with 2x4 walls. Even in a very cold climate, there is no need for 2x6 framing.
  • The building should have no eave or rake overhangs. All walls and roofs meet as simple intersecting planes.
  • After the walls and roof are sheathed with plywood or OSB, the sheathing is covered with a peel-and-stick rubberized membrane like Grace Ice & Water Shield. Because there are no overhangs at the eaves and rakes, the membrane can be folded down at the roof edges onto the walls.
  • The walls and roof are then covered with at least two layers of rigid foam. The thickness of the foam varies depending on climate; while 2 inches of foam might be adequate in Florida, Alaskan builders are likely to install at least 6 or 8 inches of foam. Successive layers of foam should have staggered joints. The foam is mechanically attached to the sheathing.
  • Vertical 1x3 or 1x4 strapping is installed on the walls to create a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. , and siding is installed over the strapping. No housewrap or water-resistant membrane (WRB) is necessary, although one can be used if desired.
  • The roof foam is topped with 2x4 sleepers, installed in the same position as the underlying rafters. The 2x4s are mechanically fastened through the foam to the rafters with long screws. The 2x4s are cantilevered at the eaves to create an overhang. Ladder-framing (outriggers) are used at the rakes to create a gable overhang.
  • A second layer of plywood or OSB is installed on top of the roof sleepers, creating a ventilated cold roof. Roofing is installed normally.

“It going to rot!”

To some builders and building inspectors, PERSIST details seem counterintuitive or dangerous. One typical reaction is, “You can’t install peel-and-stick over your wall sheathing! It’s a wrong-side vapor barrier! The membrane will trap moisture! The walls can’t dry out!”

Actually, the peel-and-stick works perfectly. The membrane acts as a combined air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., vapor barrier, and water-resistant membrane (WRB). Because the membrane completely seals the walls and roof, it produces an unusually airtight envelope.

Since the membrane is on the warm-in-winter side of the insulation, it’s exactly where it belongs. All of the home’s framing and sheathing is on the conditioned side of the membrane, so these wood components are maintained at indoor conditions. That means they aren’t subject to swings in humidity or temperature; the framing stays stable and dry in all seasons, in all climates.

On the exterior side of the membrane, there aren’t any components which are likely to suffer any moisture damage. Since the system includes a rainscreen behind the siding, any water that gets past the siding drains quickly from the walls.

It’s expensive

PERSIST has a few disadvantages. It costs more than conventional construction, because of the cost of the peel-and-stick membrane, the two layers of roof sheathing, and the labor required to build the roof overhangs separately from the main structure. Moreover, integrating windows into the system takes some head-scratching and attention to detail.

But the added cost of PERSIST buys many advantages:

  • Air leakage rates are extremely low.
  • The exterior foam limits thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through studs and rafters.
  • Less expensive 2x4 studs can be used instead of 2x6s.
  • The air barrier and vapor barrier are installed in a single step. Installing the air barrier outside of the sheathing is far simpler than creating an airtight layer of polyethylene or drywall, since there are no problems with intersecting partitions. Complicated roof shapes with multiple dormers, valleys, and cathedral ceilings are easily accommodated without hard-to-detail air-barrier seams.
  • Since the structure and sheathing are insulated, they stay warm. Cold surfaces where condensation might form have been moved to a location exterior to the waterproof air barrier. If there are any imperfections in the membrane installation, resulting condensation simply drains away.
  • Since the rubberized membrane is such an effective barrier, PERSIST works well for high-humidity rooms like indoor swimming pools or greenhouses, where other methods depend on meticulous vapor-barrier detailing.
  • The system easily accommodates cathedral ceilings with exposed framing.
  • The system has the well-known advantages of cold roof construction and rainscreen siding installation. Siding lasts longer, and damage from ice dams never occurs.
  • Polyethylene under the drywall is eliminated, so drywall can be glued to the studs if desired.
  • It’s easy to install wiring and plumbing in exterior walls, since the stud bays are empty of insulation. There is no need to worry about electrical outlets penetrating the air barrier.
  • Plumbing and electrical rough-in can begin as soon as the self-adhering membrane is installed.
  • The system provides usable conditioned attic space.

According to Chris Makepeace, a certified engineering technologist at Alberta Infrastructure in Edmonton, Alberta, and a proponent of PERSIST constructionA method of construction including rainscreen cladding and foam insulation installed on the exterior of the building’s frame. Developed by the National Research Council of Canada in the 1960s, PERSIST is an acronym for Pressure-Equalized Rain-Screen Insulated Structure Technique. (Some PERSIST builders prefer a different acronym: REFORM, for Rigid Exterior Foam Over Rubberized Membrane). Most PERSIST buildings have no insulation in the stud bays or rafter bays. Instead, 4 to 8 inches of rigid foam insulation is installed on the exterior of the wall and roof sheathing. The PERSIST system requires the installation of a rubberized asphalt membrane between the exterior foam and the wall and roof sheathing; this membrane acts as a water-resistant barrier, air barrier, and vapor barrier. , PERSIST homes are often so tight that they are hard to test. “In one of the houses that we tested with a blower door, the place with the most air leakage was between the slab and the grade beam, where the air had to travel through three feet of soil,” said Makepeace.

Will it work In Florida?

Canadian builders aren’t the only ones interested in PERSIST. “Regardless of what climate zone it is in, this system will work,” Makepeace explained. “In Florida, the inside of the building is air conditioned, but you have conditions where at night the humidity is just out of sight. If that warm humid air contacts a cold surface, where will the condensation occur? With typical construction, it occurs in the stud cavity, and you can get mold growth in your buildings. In a PERSIST building, the condensation occurs outside of the membrane, and it won’t hurt the wall. Any condensation will drain down.”

Building scientist Joe Lstiburek agrees with Makepeace on this point. “It will work from Fairbanks to Miami,” he said.

PERSIST in Texas

By now, PERSIST details have spread far beyond Canada. One Texas convert is Austin builder Ray Moore. At a building science conference in 2000, Moore asked Makepeace for tips on air-sealing electrical boxes. “I got an answer that forever changed the way I build houses,” Moore later wrote. “He asked me in return why I was trying to stop the air movement at the level of the drywall when that was such a difficult location to seal with all the complex geometry in the interior of a typical custom home. The intersections of interior walls, electrical outlets, furr-downs, stairs on exterior walls, tubs on exterior walls, wire and pipe penetrations, and upper floor rim joist areas are all examples of detailing that is almost impossible to get airtight. Chris introduced me to PERSIST.”

Moore continued, “Since using this approach we have cut our cooling costs to less than half of our already low-consumption building methods. … Our homes have been blower-door tested at 0.76 ACH50. … The vast majority of structural rot and water damage is due to poor flashing design and poor or nonexistent application of flashings and drainage planes. All windows should be pan-flashed and head-flashed with end dams. All intersections of walls and extending structures should be properly flashed and water managed. I find this to be much easier to accomplish with a modified bitumen (rubberized membrane) than it ever was with felt paper or other housewraps.”

PERSIST with a twist

There are many PERSIST fans in Alaska, where builders have long understood the importance of airtight construction details that prevent condensation in building assemblies. In recent years, the Cold-Climate Housing Research Center (CCHRC) in Fairbanks has been promoting a modified version of PERSIST called REMOTE — an acronym for Residential Exterior Membrane Outside Insulation Technique. The REMOTE method marries PERSIST walls with a conventional unconditioned attic.

It’s almost always cheaper to build a REMOTE house than a PERSIST house. However, for the REMOTE system to work, builders must pay close attention to air barrier details at the top of exterior walls. The CCHRC has produced a useful manual on the technique, “REMOTE: A Manual”.

Alaskan builder Thorston Chlupp wrote a detailed and useful article on the REMOTE technique for the May 2009 issue of the Journal of Light Construction.

Alaskan builders are experimenting with changes to the PERSIST system. To save money, some have switched from peel-and-stick membrane to 6-mil poly or plastic housewrap.

Instead of leaving stud bays empty of insulation, some REMOTE builders fill them with fiberglass or cellulose. This practice is potentially dangerous, however. The more insulation in the wall, the greater the possibility that the wall sheathing will get cold enough to allow condensation to form. That’s why most PERSIST builders prefer to keep framing bays empty, with all of the home’s insulation outside of the rubberized membrane.

Last week’s blog: “High-Solar-Gain Glazing.”

Tags: , , , , , , ,

Image Credits:

  1. Jacques Hébert
  2. Ben Nickerson
  3. Tim Healey - JLC

Aug 7, 2009 8:25 AM ET

I shoulda listened to Ray Moore
by homedesign

When I first heard of Ray Moore and Persist... I thought that it made all the sense in the world.
Here was someone who was actually building Joe Lstiburek's Perfect Wall.
I was so impressed that I went to Austin.... Ray took the time to give me the run down and show me a project in progress.
I went home and started "scratching my head".

Although I had seen much of the process in person.... I still chickened out.
The details around the windows,doors,corners and eaves were still making my head spin.
When Ray explained it and showed me... it looked so simple.. but when I tried to draw it out .. it was not so simple.

I listened again to my Peter Pfeiffer tapes where he said "in our climate..when you go beyond are really reaching a point of diminishing return"
I could easily hit and go beyond R-13 (whole wall not cavity) with more conventional construction.
So...Why go to all that trouble?
So .. I took the easy path and the more conventional construction.
Today ... I have a different perspective/opinion... R-VALUE DOES MATTER.

Ray took the time from his busy day to "show me" as he has done with many others.
Ray, I am sorry that I did not follow through.

I still have a lot of head scratching to do ...and I think that "borrowing" ideas from Thorsten Chlupp's Remote system will help.

Martin, thanks for bringing up this important topic.
Maybe we can "Nudge" Ray and get him to comment here?

Aug 13, 2009 3:46 PM ET

by Bob Irving

Excellent article. I have three questions:
We are aiming for RValues (in central NH) of 40+ - what if any is the thickness limit for exterior insulation? At some point the screws/nails are going to start to bend. Secondly, if the wall assembly had 3-1/2" of cellulose and 7 or 8" EPS, would that be safe, or do you feel that any amount is dangerous? and lastly,
what are these builders doing about about insects? That can be a problem with 1" or 8", but my experience with stress skin is that it is not rare for insects to be tunneling through & nesting in this material.

By the way I just had a sales visit from the local Tremco rep about their Enviro-Dri spray on coating (on exterior sheathing)which is supposed to be a WRB and air barrier, plus it has a perm rating of 12. An option to the I&WS?

BOb Irving
RH Irving Homebuilders

Aug 13, 2009 6:33 PM ET

Brilliant! I love it.
by T. Inoue

Good building science put to application!
I too am concerned about the length of anchors for the sheathing. I was going to do something like this when I had my roof redone, but I could not convince the roofers to go for it because they couldn't figure out how to deal with the shear forces on the roof sheathing on top of 4"-6" of foam board. I've heard about strapping at the peak so that the opposite sides support the shear forces, but still, I was left in the "scratching my head" phase and went with a conventional roof.

Aug 14, 2009 3:38 AM ET

Screw length and insects
by user-756436

Bob Irving and T. Inoue,
Some of your questions are answered in the resources with links in my blog. In addition to the REMOTE Manual, you should be sure to check out the JLC article by Thorsten Chlupp.

Foam thickness: Many builders have successfully added 6 inches of foam to their walls, as the REMOTE Manual makes clear. If you use polyisocyanurate foam, 6 inches will get you to your R-40 goal.

Screws: Here's what the manual says about screws:
"The standard fastener for attaching furring is a pan-head panel-type roofing screw that is long enough to pass through all layers of foam board and penetrate securely into the framing. As a rule, all furring that supports siding should be fastened to the wall framing members. ... Screw head profiles range from #3 Phillips, Torx, Star, and Square drive among others, to make for more positive driving. Screws should penetrate through the sheathing and a minimum of 1.25 inches into the studs. Longer screws are available, including half sizes in some brands, if significant upsizing is desired. Vertical fastener spacing can vary from one to two feet on center, depending on the siding and wind loads. Driving the screws in at a slight upward angle, such as 0.5 inch in six inches is highly recommended as it creates a mechanical advantage that significantly increases the bearing capacity of the assembly. In addition, a truss clip can be attached to any rafter tails that line up with the furring. In any situations where excessive structural, seismic, or wind loads are an issue, it is recommended that an engineer be consulted to verify that the construction methods are appropriate to the conditions."

The manual further notes that the load capacity of the Oly Log/Timber Lok screw is 136 lb/inch.

Insects: The subject of rigid foam and insects is a contentious one. In some areas of the country, builders have been using exterior foam for 30 years with few, if any, problems. Other builders have reported that ants and other insects tunnel through foam (especially EPS). At least one EPS manufacturer (Team R-Control) makes EPS treated with borates to fight insects. It should be noted that polyisocyanurate foam insulation is installed on thousands of commercial roofs every year, and that the use of XPS on exterior walls has become routine in commercial construction in much of the country.

Concerning whether or not it's wise to install insulation in the stud cavities of a PERSIST building: the topic is discussed in the REMOTE Manual. The old rule of thumb was that no more than 1/3 of a wall's R-value should be on the interior side of the vapor retarder. However, some of the factors that affect this calculation are the indoor humidity levels and the climate. If you install insulation between the studs, danger increases as interior moisture levels rise (for example, in a swimming pool room) or the exterior climate gets colder. If you are unsure about this detail, you should perform a WUFI analysis or hire an energy consultant.

Aug 18, 2009 1:51 PM ET

EZ foam
by Bennh

I am familiar with both techniques. In New England the REMOTE system makes more sense as a heated attic is in my opinion a waste of energy. The CCHRC specs blow in cellulose for the attic. This I can't get behind. I know all the pros and cons for cellulose and would still rather have closed-cell or even open cell. Are you familiar with EZ foam? couldn't you use this to bridge the gap between attic insulation and wall then run an inch of EPS under the trusses as a thermal breay and then continue to layer EPS around the trusses till you reach your required r-rating?

Aug 18, 2009 2:55 PM ET

Layering EPS
by user-756436

I'm not sure what you mean by EZ Foam; it sounds like canned spray foam of some kind. If so, that can certainly be used for air sealing purposes, as you propose.

Your suggestion of insulating an attic floor with many layers of EPS board stacked like poker chips between the trusses is possible but fussy. All of the gaps and seams between EPS sheets and the adjacent truss chords would have to be sealed with spray foam, a time-consuming task. Why not just pile on the cellulose?

Aug 30, 2009 1:10 PM ET

Ice and water shield
by kooldave

The butyl type ice and water shield does the product off gas or have an unpleasant odor; if so, what is the duration?

Aug 30, 2009 2:23 PM ET

by user-756436

I know of some flexible flashings that are butyl. However, they cost more than rubberized asphalt. Grace Ice & Water Shield is a rubberized asphalt product, not a butyl product. There may be some peel-and-stick ice dam products out there that are butyl, but I'm not familiar with them.

Sep 1, 2009 12:02 PM ET

by Bennh

Thank you for your help Martin. EZ foam is just a low expansion canned spray foam. In a perfect world, there should be little to no moisture in a cold roof system like REMOTE but that is not always the case. I have been in water damaged attic with blow in cellulose and can only imagine what the effect on the celluloses r-value was. To be fair the general contractor saw no problem installing the furnace into an uninsulated attic. Hopefully anyone attempting to build a REMOTE home would have the forsight to see how bad an uninsulated furnance within feet of your roof would be!

Sep 4, 2009 10:02 AM ET

PERSIST Insulation Method
by Frank

I am trying to find out more information on the "PERSIST" Insulation Method. The preparation, application, etc. I would appreciate any help in direction. Thank you.

Sep 4, 2009 10:13 AM ET

Follow the links in the article
by user-756436

I suggest you follow the links in the article to access:

1. The Home Energy magazine article of November/December 1999.

2. The CCHRC manual, “REMOTE: A Manual”.

3. Thorston Chlupp's article from May 2009 issue of the Journal of Light Construction, “Installing Exterior Insulation In Cold Climates.”

Nov 23, 2009 8:28 AM ET

Rainscreen and Siding Attachment
by Interested Onlooker

Presumably the rainscreen and siding attach directly to the rigid insulation.

Is there a vented gap between siding and rainscreen?

How is the siding attached?

Nov 23, 2009 11:24 AM ET

Air gap
by user-756436

Dear Interested,
A rainscreen siding application, by definition, requires an air gap between the siding and the sheathing. This air gap is often referred to as a "rainscreen."

There are many methods of creating a rainscreen installation. Perhaps the most common method requires the installation of vertical strapping on top of the sheathing (foam sheathing) or WRB (housewrap or asphalt felt). If the house has foam sheathing, then the vertical strapping is attached with long screws through the foam to the studs. The vertical strapping mimics the stud spacing -- either 16 in. or 24 in. on center. Strapping can consist of 1x3s, 1x4s, or rips of 1/2-in. plywood.

The siding is attached to the vertical strapping. Thicker strapping provides better holding ability for siding fasteners than thinner strapping.

Instead of vertical strapping, it's also possible to install a three-dimensional plastic drainage mat like Cedar Breather. If Cedar Breather is used, the siding has to be attached with long fasteners back to a layer of plywood, OSB, or to the studs.

Dec 3, 2009 10:12 PM ET

by kooldave

What was the brand of tape used in the vidio also, can Zip tape be used such as used in the Zip wall system. Thanks

Dec 4, 2009 4:49 AM ET

Which video?
by user-756436

I'm not sure which video you are talking about.

When it comes to specifying a tape to seal the seams of rigid foam, opinions differ. Many builders have had good success with contractors' tape (for example, from Venture) to seal the seams of polyiso or XPS. However, other builders remain skeptical of the long-term durability of tape, and prefer to seal seams with spray foam or to create an air barrier with another method (sealed housewrap or the airtight drywall approach).

Sep 27, 2010 10:53 AM ET

sorting it out
by Craig

I'm trying to sort out the benefits of this system versus some other systems I'm familiar with like SIPs and ICF's. It seems to me, that the PERSIST system, and SIPs have very similar properties, except you have more convenient chases for all your mechanicals- wiring, and plumbing with the persist system. Is it much cheaper however to use SIPs. . . and build chases where you need one?

One other specific thing I would love clarity on: If it is true that condensation only forms on a solid surface and does not seem to happen within an insulation product (as clearly stated in the GBA article by Martin Holladay 9/17/10) Then, will condensation happen at the point where an insulation material is "bonded to" a substrate? In other words, if foam insulation is "bonded to" (or spayed on) sheathing, can condensation still happen at this junction between the foam and the sheathing?-or does it all begin to act like one unit with varied r-values in which condensation can not occur?? thanks- I am debating which system to use for my next project in North Carolina Mountains- Hot / humid.

Sep 27, 2010 11:10 AM ET

Response to Craig
by user-756436

1. I don't think there is a simple answer to your question -- "Which is cheaper, SIPs or PERSIST?" It depends on the price of the SIPs and the price paid for labor and materials (especially rigid foam and peel-and-stick membrane).

2. If OSB which is bonded to EPS foam (for example, the OSB on one side of a SIP) gets very cold, and if warm humid air can contact the OSB, then condensation (or sorption or moisture accumulation) is possible. However, you have to come up with a mechanism that allows warm humid air (or water vapor) to contact the cold OSB.

If the seams of the SIPs are sealed properly, warm interior air can't reach the cold OSB on the exterior side of the SIP. The foam is thick enough to be a vapor retarder, so vapor isn't going to travel through the foam.

There were SIP homes in Juneau, Alaska that were built with poorly sealed seams. These seams allowed air leakage. When warm, humid interior air escaped through these seams, condensation occurred on the underside of the asphalt felt roof underlayment, which was quite cold. This moisture saturated the cold OSB near the panel seams. The result was rot. However, the moisture didn't travel through the foam -- the problem was air leakage.

Dec 6, 2010 12:55 AM ET

Response to Martin re: sealing foam seams
by mzwkTaMXQx


I'll be installing 4" of polyiso on a remodel, and plan to seal both the sheathing and the foam.

Would a foil tape be the best option for sealing foil-faced polyiso boards if tape is to be used? Does one need to investigate the specific adhesive used or find a specific tape known to work with these boards?

When you said that people are using spray foam to seal rigid foam boards, can you explain exactly what you mean? My initial take on what you mean is that one would spray a thick bead of foam in the center of the edges of each panel before you butt another panel up against it, spreading that bead of foam relatively uniformly across the thickness of the edge of both boards. Then one would attach them as usual with 1x4s and screws. Is that about what you mean?

Since we also plan to seal the sheathing before installing the foam, what tapes are recommended for sealing OSB sheathing?

I'm considering using the Zip System Walls from Huber. Would you think that the Zip panels combined with their special Zip tape is equivalent in performance (i.e. air sealing) to using standard OSB and a peal-and-stick membrane as in the PERSIST / REMOTE systems.

Thanks for all of your great advice, both here and in so many other column, blogs, etc.

Dec 6, 2010 4:11 AM ET

Response to Robert Dickenson
by user-756436

Robert Dickenson,
Q. "Would a foil tape be the best option for sealing foil-faced polyiso boards if tape is to be used?"

A. Yes. If you have any doubt, you can always call up the tech hot line for the manufacturer of the foil-faced polyiso you are using and ask for tape recommendations. I have written a detailed article on selecting tapes for sealing rigid foam and OSB; you can read the article by clicking here: Air-Sealing Tapes and Gaskets.

Q. "When you said that people are using spray foam to seal rigid foam boards, can you explain exactly what you mean?"

A. Although most builders find it easier to seal the seams of rigid foam with tape, it's also possible to seal the gaps with spray foam. The usual (least messy) method is to install the rigid foam first, leaving a 1/4-inch gap between panels, and then to seal the seams after installation with spray foam.

Q. "Since we also plan to seal the sheathing before installing the foam, what tapes are recommended for sealing OSB sheathing?"

A. See the article I mentioned -- Air-Sealing Tapes and Gaskets.

Q. "I'm considering using the Zip System Walls from Huber. Would you think that the Zip panels combined with their special Zip tape is equivalent in performance (i.e. air sealing) to using standard OSB and a peal-and-stick membrane as in the PERSIST / REMOTE systems."

A. That's a hard question to answer. It depends on the longevity of the ZIP tape, which (for now) is unknown.

Jan 3, 2011 3:27 PM ET

Perfect wall and Concrete
by pbschulz

Mr. Holladay
I really am a fan of the perfect wall/Persist method. I am trending away completely from wood construction (I live in Maine) and would like to move completely to concrete. I would like to take the Persist method a step further and enclose the foam in concrete; much like the method. What are your thoughts on the design? It seems like a lot if not all of these issues are answered by the thermomass system.

Jan 3, 2011 4:33 PM ET

Response to PB Schulz
by user-756436

PB Schulz,
If you can afford it, the Thermomass system is a very durable one, with high levels of airtightness. Be sure to order the walls to be extra thick -- don't skimp on the foam layer.

This is a popular construction system for prisons. But if you include enough windows, your home won't feel prison-like.

Jan 10, 2011 11:19 PM ET

at the top
by brian carter

I found this thread in my GBA e-mail today but it seems to be pretty old. ,I hope you still see new additions,Martin.I am wondering why there seems to be the need in "green building" to devote so much effort to reworking the past instead of a taking a deeper look at the basics.For instance,one way to save a lot of money with the systems under discussion would be to just build a flat roof ! Commercial roofs have been built with membrane over foam by the millions(as was pointed out in a question about foam)-all flat and in all climates.The labor and material savings more than compensate for a beefier rafter system.But skip the peel and stick and go for rubber.It's currently 38 bucks per square in my area with a choice of systems to secure it-no glue and no roof pitch to fight.
What can you do to sell a flat roof? Make it a real green roof and grow something on it. If you want to take this a step further there are many greenhouses available in any size.A roof will always lose some heat which will contribute to heating the space if it's desired.A greenhouse roof can contribute excess heat to the house below in turn.
Even if a conventional pitched roof is planned it would not seem that hard to build and wrap a box shape and add the roof framing afterwards,with precautions and details to protect the membrane.This would spare the need for a second layer of sheathing to create a "cold roof".

Jan 11, 2011 4:37 AM ET

Response to Brian Carter
by user-756436

I suspect there will always be some designers, like you, who are tired of "reworking the past" -- designers who are modernist flat-roof fans have, after all, been with us since the 1930s -- and there will be other designers who remain in love with traditional shapes and forms, including the beloved gable roof. These two design philosophies both exist, and can't be resolved by debate.

If you are interested in vegetated roofs, you'll find plenty of information on the topic here on the GBA Web site. For example:
Lessons Learned on a Living Roof
Design a green or living roof system
How do you include an eco-roof into a home energy audit?
Green Roof Plants
Trailblazing Solar Home Made of Composite ICFs
Roofing Material Choices

One of the virtues of a rubber roof, you wrote, is that there is "no glue and no roof pitch to fight." The lack of a roof pitch may be a virtue for the roofer -- but it is often a drawback for the homeowner, who needs to worry about ponding and leaks. In most cases, a roof pitch is advantageous, since the pitch helps get the water off your roof.

Your basic point, however, is certainly true: flat roofs (or, more accurately, commercial-style low-slope roofs) are tried-and-true and have been with us for years. For homeowners who don't mind the look of a flat-roofed house, your suggestion will work.

Jan 11, 2011 10:39 PM ET

flat roof
by brian carter

Martin- thanks for the good links to Green roof info. I actually like a pitched roof as much as the nexy guy if the design works. But I like to keep reminding others of ALL the options.I'm a bit disappointed that you have accepted the idea that flat roofs leak more than pitched ones.It seems inuitive that water has less chance to find an entry point if it is running off,but water is still feeling the gravity even on a steep roof.Straight down is the path it wants.On a square foot basis I doubt if there is as much advantage as people imagine-speaking as one who has tracked down plenty of leaks in roofs.It really comes down to proper materials,properly installed.The long tradition of pitched roofs is at least partly a product of materials available.
I should say that I am really intrigued by the idea you started this post with and I thank you for explaining it and providing the links .

Jan 12, 2011 4:14 AM ET

Tracking down roof leaks
by user-756436

Like you, I have spent a fair amount of time tracking down roof leaks. I used to work as a roofer.

I disagree with your apparent view that roof pitch doesn't affect leakiness. It does. In my opinion, the steeper the roof pitch, the more forgiving the roof. A small hole in a flat roof can mean disaster, while a small hole in a steep roof will probably never leak.

When I re-roofed my 25-year-old cedar shingle roof on my 12-in-12 roof, my roof was full of holes. Standing in the attic, I could see daylight through the shingles, which were installed over skip sheathing. However, the roof didn't leak. If that roof was laid flat, my house would be soaked.

More observations:
1. Roofs don't leak at the ridge; they leak near the eaves.
2. Roofs don't leak at the hips; they leak at the valleys.

Jan 15, 2011 12:28 PM ET

roof leaks
by brian carter

Martin, I have a 14/12 roof on an old barn and I can confirm that daylight does not mean leaks.I agree that these would mean trouble on a lower pitch roof.And I agree that the pitch makes some difference in that the water will be more likely to deflect onto an adjoining surface rather than drip through.But I stand by my observation that the incidence of leaks is about the same because the installation process for modern rubber roof systems,plus the large area covered by continuous membrane,leaves much fewer locations for error than a roof composed of many small units,often put on by people with little or no training.You can say the pitch gives you a margin of error.I say that as a roofing system both perform equally.Some suppliers of commercial roofing systems require inspections of any roof using their products to qualify for any liability coverage in case of leaks.Its a good idea,since flat roofs are the norm for large commercial buildings where very sensitive and expensive equipment is often at risk. As a homeowner I would trust this accumulation of expertise if offered by a reputable company . Now if you want a good roof ,use rubber on a pitch-there is nothing that says a green planted roof has to be flat.

Jan 15, 2011 12:37 PM ET

OK Brian -- we agree
by user-756436

I think we've come around to agreement. Thanks for your comments.

Jan 15, 2011 12:38 PM ET

by brian carter

I forgot to point out that daylight does not necessarily mean an opportunity for rain to enter unless it's wind-driven.The idea of a pitched roof is to overlap layers so water is shed from layer to layer. If you can see daylight between layers it doesn't mean the water will find that gap if it has to go back uphill to do it. Thus there are roofs composed of irregular overlapping stones that work just fine.

Feb 18, 2011 9:48 PM ET

Thermal Conductivity of Fasteners
by user-950613


In a discussion today about exterior foam, the point was made that most energy modeling for exterior foam projects don't take into account the large number of large screws used to hang the foam, and that these screws are likely conducting a significant amount of heat right out of the conditioned space.

What do you think?

Any ideas on alternative methods for hanging foam?

Feb 19, 2011 5:40 AM ET

Response to Devan Anthony
by user-756436

This topic has been debated several times at GBA, although I don't have the time to track down a link right now.

Although some people claim that the heat loss from fasteners matters, most reputable energy experts who have run the numbers have concluded that the net benefits of the foam layers far outweigh the heat flow through the fasteners. It's not that big a problem. If the idea of heat flow through fasteners bothers you that much, you can always add another inch of foam.

Feb 19, 2011 9:46 AM ET

Link to debate about heat loss through fasteners on outsulation
by user-950613

Feb 19, 2011 11:35 AM ET

by user-756436

Thanks for tracking down the link.

Feb 24, 2011 3:11 AM ET

screw heads
by user-939142

you could always put a little patch of aerogel over each screw head ;)

the stoneguard stucco system has a glue component than you run in small vertical beads over the stonguard on the outside sheeting and attach the foam to that. of course you have no way of attaching siding unless its stucco over the foam

you could hang your vertical strapping from the roof rafters and attach it at the foundation base too. with some proper bracing and siding attached no through foam screws should be needed - well maybe a few on the corners and around windows/doors

logically if you add up all the screw area and its Rvalue, i'm sure it is less than that lost around a single typical window frame

Nov 12, 2012 7:13 PM ET

Edited Nov 12, 2012 8:22 PM ET.

Pardon my enthusiasm, but I've found a new religion...
by user-1119494

PERSIST seems impressive for all the reasons you've mentioned, but also because the nominal R value of your insulation = your assembly R value. This is very impressive! According to my numbers, spray icynene at R6/inch in a standard wall would need to be about 4.5" thick to equal 2" outsulation.

What's more, since outsulation foam is more effective than the same foam installed between studs, we can add more R with the same payoff time! My calculation show that in my climate, an additional mm of polyiso will pay off in 33 years at 2" outsulation (R12), but reaches 33 years with 3" of insulation (assembly value=R8).

On a lesser note, after years of chasing decay in houses, the concept of keeping nearly all the house structure inside the conditioned space is terribly attractive.

Finally, as long as the screws are covered by a little insulation (like sheetrock or plywood) the losses they create should increase wall heat loss by less than a percent.

My only concern is that each cold screw could chill a few inches of a stud to the point that it would absorb enough moisture to create decay. I have a free freezer I am using for experiments: I should run a screw through it and into piece of 2x4 on the outside...

Nov 13, 2012 7:29 AM ET

Response to Dustin Harris
by user-756436

The only way that rot can occur in a piece of framing lumber is if the wetting rate exceeds the drying rate.

Wood is hygrosopic. Even if there was slight moisture accumulation at the area of a screw penetration, the moisture would be redistributed through the lumber and would dry very quickly. Remember, the studs are at room temperature and are always dry.

Nov 21, 2012 9:05 AM ET

are there 2 WRBs in PERSIST buildings?
by mfredericks

I'm trying to wrap my head around using the PERSIST approach to retrofit my own home and keep getting caught up with the flashing details. I wonder whether the membrane applied to the sheathing is technically the WRB or if the outer face of the taped foam is considered the WRB. Or do both of these surfaces act as WRBs?

Nov 21, 2012 9:23 AM ET

Response to Mark Fredericks
by user-756436

In a PERSIST building, the membrane applied to the sheathing is the WRB, and all window flashings should be integrated with that membrane. If you follow the traditional approach to PERSIST, and use a rubberized asphalt membrane like Grace Ice & Water Shield, you'll find that the WRB is bulletproof.

The rigid foam is not harmed by moisture, and in any case dries readily due to the presence of a ventilated rainscreen gap between the siding and the rigid foam.

For more information on WRBs, see:

All About Water-Resistive Barriers

Where Does the Housewrap Go?

Apr 1, 2013 6:12 PM ET

Grace Ice and Water
by Steven Knapp

Grace notes that this product has a slight asphalt odor and should not be installed with exposure to the interior. Does this mean you would need to air seal the interior walls to prevent a problem?

Apr 2, 2013 4:54 AM ET

Response to Steven Knapp
by user-756436

You raise an interesting point. If you are worried about the odor of the peel-and-stick, there are other options. For example, you could use taped Zip System sheathing to create an air barrier, and then install a layer of polyethylene or synthetic roofing underlayment on the exterior side of the Zip sheathing as your vapor barrier.

Jul 11, 2013 10:04 AM ET

PERSIST/REMOTE in a masonry construction
by christinlai

I'm planning to re-frame my addition (approximately 5' x 15', 3 side wall) that has a second level attached to it. The rest of my house is of masonry/stucco construction and I would like to maintain the look. How would I apply the PERSIST/REMOTE principle using cinder blocks/bricks with stucco? The cinder blocks/bricks would be load-bearing. For reference, I live in climate zone 4 in the Northeastern US.

A extension of this question: If I were to retrofit the rest of the house with PERSIST/REMOTE, would it be feasible?

Jul 11, 2013 10:34 AM ET

Edited Jul 11, 2013 10:36 AM ET.

Response to Christin Lai
by user-756436

You could insulate the exterior of your concrete-block wall with several inches of rigid foam, and then install brick veneer if you wanted. This is a standard method of commercial construction which is often used for retail buildings, schools, and hospitals.

You could also cover your concrete-block wall with EIFS (rigid foam and synthetic stucco).

Retrofitting an existing house according to PERSIST principles is certainly possible. This type of work is usually referred to as a "deep energy retrofit." The work isn't cheap, but if your checkbook can handle the cost, it's done all the time.

Jul 11, 2013 12:04 PM ET

Hi Martin, Thanks for the
by christinlai

Hi Martin,

Thanks for the response. EIFS is an interesting concept. There're several concerns I have with respect to using EIFS:

- Would it be a fire hazard? Insurance companies don't seem too keen on the idea of wrapping a house with non-fire retarding material.
- Does it require specially skilled labor to do the job?
- How much does it cost approximately?

I do like the idea of the concrete block-rigid foam-brick veneer. It doesn't seem to require people with heavy experience in PERSIST/REMOTE to build. If I understand the concept correctly, I would be placing the waterproof membrane between the outside of the concrete block and the rigid foam in this construction? How does the cost compare to EIFS?

How do you feel about foam-in-place insulation compare to PERSIST/REMOTE retrofit for the rest of the house?

Jul 11, 2013 12:32 PM ET

Response to Christin Lai
by user-756436

Q. "Would EIFS be a fire hazard?"

A. EIFS is code-approved everywhere in the U.S., as far as I know. But if you have any concerns, you should contact your local building department or fire marshal, as well as your insurance company.

Q. "Does it require specially skilled labor to do the job?"

A. All construction labor is skilled labor. (My advice is: don't let unskilled workers work on your house.) EIFS is usually installed by an EIFS contractor.

Q. "How much does it cost approximately?"

A. You'll have to get bids from local contractors to learn your local prices.

Q. "I do like the idea of the concrete block-rigid foam-brick veneer. If I understand the concept correctly, I would be placing the waterproof membrane between the outside of the concrete block and the rigid foam in this construction?"

A. Yes.

Q. "How does the cost compare to EIFS?"

A. Brick veneer over concrete-block walls is a very expensive system. I imagine it costs more than EIFS.

Q. "How do you feel about foam-in-place insulation compare to PERSIST/REMOTE retrofit for the rest of the house?"

A. Spray foam insulation can be installed on the exterior of your wall sheathing if you like spray foam. It doesn't have to be installed between the studs. It's always better to place your insulation on the exterior side of your wall sheathing rather than between the studs.

Aug 6, 2013 11:56 PM ET

Housewrap penetrations a problem?
by jchwang

I think it was mentioned somewhere that Grace I&W seals itself around penetrations from fasteners. However, in the REMOTE version using Tyvek DrainWrap, will the penetrations be an issue? My understanding is that Tyvek does not self seal, and is more like paper which would shred/tear.

Aug 7, 2013 6:16 AM ET

Response to Jerry Chwang
by user-756436

For a PERSIST or REMOTE wall, a rubberized asphalt membrane like Grace Ice & Water Shield will be more robust and dependable as a WRB and air barrier than Tyvek DrainWrap.

Only you can decide whether you feel that Tyvek will hold up for the long term. Of course, any penetration through a WRB requires proper flashing or sealing, and that requirement certainly applies to Tyvek. Some types of penetrations (for example, a window or a pipe) require fairly complicated flashings, while others (for example, an electrical cable) may only require sealing with a high-quality tape.

Aug 9, 2013 12:32 AM ET

Housewrap penetrations a problem? Solution?
by jchwang

My builder and probably someone else on this site suggested strips of Grace I&W on top of house wrap around windows and doors.

I also thought the Grace I&W could be cut into small squares and stuck on the Tyvek where there will be penetrations from fasteners, utilities, etc.

What do you think of this potential solution?

Aug 9, 2013 3:20 AM ET

Response to Jerry Chwang
by user-756436

Every window rough opening needs to be flashed. Peel-and-stick flashing can certainly be used as part of a window-flashing system. The idea is to flash the window rough opening so that any window leaks are collected at the rough sill and directed towards the exterior.

It's also possible to use pieces of peel-and-stick membrane to flash penetrations through housewrap, as you propose.

Aug 10, 2013 11:53 PM ET

Interior VB needed?
by jchwang

Thanks Martin.
I discussed this wall system with my builder and architect, and we were wondering if the interior drywall and vapour barrier are needed if we decide to completely eliminate insulation in the stud bays. Several reasons: interesting look for interior (architect); cost savings by putting in finishing later (me); easier to assess house performance in all seasons before deciding whether I want to add some interior batt (me) and I'm a bit worried that any moisture needs to dry to the inside because outside of my sheathing is Tyvek with 2" mineral wool and then impermeable insulated steel panel ((I asked about solar vapour drive in Q&A).

If a VB is still needed, can you help me understand why? How much of a barrier is needed (perms)? Would a selective membrane be needed, or is this overkill?

Also, I'm going with a low-slope flat roof, so was thinking of designing it same as the wall with an additional membrane as final top layer (any 'green' suggestions for this layer). Anything risky with this line of thinking?

Aug 12, 2013 9:20 AM ET

Edited Aug 12, 2013 9:26 AM ET.

Response to Jerry Chwang
by user-756436

You are asking this question on the page with my PERSIST article, so I assume that you want to follow the PERSIST approach.

The PERSIST method is based on installing a vapor-impermeable membrane on the exterior side of your plywood or OSB wall sheathing. The best material for this purpose is a peel-and-stick rubberized asphalt membrane. If you follow the PERSIST approach, this membrane acts as your vapor barrier as well as your water-resistive barrier (WRB). You don't need a second interior vapor barrier (like polyethylene), because the wall already has one.

Because peel-and-stick membrane is expensive, some builders substitute 6-mil polyethylene for the peel-and-stick membrane. I don't recommend this approach, however, since any holes in the polyethylene can leak. Peel-and-stick membrane is self-sealing around fastener penetrations.

Concerning your question about your roof assembly: the final top layer has a name. It is called roofing.

Aug 13, 2013 12:04 AM ET

Response to Response to Jerry Chwang
by jchwang

Actually, I was going to follow the REMOTE approach because of the cost of Peel and Stick. Using Tyvek Drainwrap with small squares of Peel & Stick where the screws go in.

So my questions remain above. Appreciate any additional advice.

[Editor's note: Comments continue on Page 2. Click the number "2" below to continue reading comments.]

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