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Full dimensional 2x4s or new 2x6s in roof? And Roxul or closed-cell foam?

I am remodeling a 1910 Bungalow in Oakland. California and will be taking on the roof either later this summer or more likely early next. We are going to slightly change the roof line (going from a hip to a gable to get more head room when entering). The original roof rafters are full dimensional 2x4s and we want to get rid of the comp shingle (4 layers, at least) and put on plywood and a standing seam roof and then solar. The attic will not really be living space although we may have a home office up there. The 2x4's are somewhat bowed but I am hoping that relieving the weight, installing pony walls and maybe some force and sistering will straighten out the rafters. The floor of the attic will become 2x6s and we will use mineral wool insulation there. The questions are a) should I either remove the 2x4 rafters and replace with 2x6s (reducing headroom as well as using new wood), sister 2x6s onto the 2x4's or keep the 2x4 full dimensional?

Secondly, should I try to ventilate the ridge and have an air gap and then use roxul or something similar with the sleeve that allows air passage or have no venting (there will be 5 operable skylights plus operable windows on either end of the roof) and try to use a spray foam. Are there any out there yet that do not have carcinogens and do not deplete the ozone layer?



Asked by rob manheimer
Posted Aug 8, 2017 10:41 PM ET
Edited Aug 9, 2017 5:03 AM ET


12 Answers

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Q. "Should I either remove the 2x4 rafters and replace with 2x6s (reducing headroom as well as using new wood), sister 2x6s onto the 2x4's or keep the 2x4 full dimensional?"

A. To answer your question, you should talk to an engineer. You may be surprised to learn that the engineer recommends 2x8s or 2x10s. It's hard to imagine that your 2x4 rafters are adequate, and it's no surprise that your 2x4 rafters are bowed.

Q. "Should I try to ventilate the ridge and have an air gap and then use roxul or something similar with the sleeve that allows air passage or have no venting (there will be 5 operable skylights plus operable windows on either end of the roof) and try to use a spray foam?"

A. Your goals are unclear. You mention that you intend to install insulation on the attic floor, but you also mention that you "may have a home office up there."

Before you can answer questions about attic venting, you need to answer a basic question: Will this be a vented unconditioned attic or an unvented conditioned attic? If this space will become a home office, then the insulation belongs along the sloping roofline, not on the attic floor.

Q. "Are there any spray foam products out there yet that do not have carcinogens and do not deplete the ozone layer?"

A. The Montreal protocol, signed back in 1987, mandated the phase-out of HCFCs as blowing agents, so you don't have to worry that the blowing agents in spray foam are responsible for depleting the ozone layer.

Your questions about carcinogens is more vague, but it's safe to say that spray foam, properly applied, is not associated with a raised cancer risk for homeowners.

Answered by Martin Holladay
Posted Aug 9, 2017 5:01 AM ET


Thanks Martin
I have had an engineer draw up specs and he is allowing us to maintain the 2x4 rafters, which once the comp shingles are gone can easily maintain the load of a standing seam roof with solar panels. There is no worries of snow here as the Bay Area is a fairly temperate climate. If we were to go to 2x8s or 2x10s, the head room would be too limited for its use as a home office. The home office would only take about a third of the attic inside the new pony walls. We also want the attic to be insulated as we will have a heat pump furnace and its duct work there. With Roxul I can get R15 in the roof and another R23 between the attic and living space. In the winter I can use a space heater in the attic office if needed (not using that much) but having more insulation for the real living space will be a welcome change. If we go closed cell foam, I can probably be in the r20-25 range for the roof. I have seen a lot of criticism in the green building community of foam insulation and wonder about the green trade-off in as temperate a climate as we have.

What I was trying to get at in terms of the vented vs. unvented is the vent channel running behind the batt insulation that you see in cathedral ceilings. It leaves maybe a .5" air gap to keep condensation and moisture problems from the insulation (allows free air movement between the roof and the insulation.) I understood that this is needed on roofs that use more traditional insulation but not if closed cell foam is used.

Thanks for your reply.


Answered by rob manheimer
Posted Aug 9, 2017 10:44 AM ET


If you intend to install Roxul batts between your rafters, building codes require (and building scientists recommend) that you include a ventilation channel between the top of the insulation and the underside of the roof sheathing. This ventilation channel needs to connect with soffit vents near the eaves and a ridge vent near the peak.

For more information, see these two articles:

How to Build an Insulated Cathedral Ceiling

Site-Built Ventilation Baffles for Roofs

Answered by Martin Holladay
Posted Aug 9, 2017 10:57 AM ET


Thanks again Martin. I was aware of the ventilation channel but did not know its name. You are now at the crux of the problem. I have had different roofers give different advice. One advises soffit and ridge vents with fiberglass or mineral wool insulation and the other advice is closed cell foam, which is obviously more expensive and it seems less green in many ways. Which is more advisable in this temperate climate? Especially when considering we can get r-23 between the attic and the majority of the living space.



Answered by rob manheimer
Posted Aug 11, 2017 6:35 PM ET


The building code does not recognize the approach you suggest -- installing half of the insulation along your sloping roofline, and the other half of the insulation on the attic floor. From a code perspective, the insulation in your sloping roofline is the only insulation that counts -- and it will be insufficient.

That said, many retrofit jobs include details that don't quite meet modern code requirements. Installing insufficient insulation in your rafters is probably legal (although you should check with your local code authority, just to be sure).

If you want to maximize the R-value in a space that isn't very deep, you should consider installing closed-cell spray foam without a ventilation channel. Three and a half inches of closed-cell spray foam (closed-cell spray foam installers generally don't fill the entire cavity) will give you about R-22 or R-23 -- not enough to meet code, but better than mineral wool plus a ventilation channel.

To improve the R-value of the roof assembly, you'll need to deepen your rafters somehow (or install a continuous layer of rigid foam on the underside of the rafters).

Answered by Martin Holladay
Posted Aug 12, 2017 6:15 AM ET



I'm confused. You say you are going to change the roof line, add new sheathing, and then install solar panels. This would seem to be a great time to also install R-39 rigid foam on the outside of the sheathing. If you can't install that thickness of foam, perhaps you could install a combination of rigid foam on the outside with air permeable insulation on the inside. See this article for details: http://www.greenbuildingadvisor.com/articles/dept/musings/combining-exte....

Answered by Steve Knapp CZ 3A Georgia
Posted Aug 12, 2017 7:48 AM ET


Rob has asked several times what the environmental concerns are with spray foam insulation, and whether there are alternatives that avoid those concerns, but hasn't gotten a straight answer to that question because he didn't phrase it perfectly, the way someone who already knew the answer would phrase it. Sorry about that, Rob. Here's the scoop.

The main concern with current spray foam formulations is that most use a "blowing agent" (the gas used to make the bubbles) that has a huge global warming impact, 1400X larger than CO2. The good news is that that problem is solvable: there are now spray foams that use "HFO" blowing agents instead of "HFC" blowing agents. For more on this see:

But the other concern with spray foam is that every once in a while it doesn't cure properly. The result is a nightmare for the homeowner. This rarely happens, but I'd want to make really sure I was protected by contracts and or insurance, vetted by a good lawyer, before risking it. See http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/when-spray-fo...

On the overall questions, I think idea of foam above the roof deck is a good one; the other question I'd have is why not build in sufficient head room if you are rebuilding the whole thing anyway.

Answered by Charlie Sullivan
Posted Aug 12, 2017 4:02 PM ET


HI Charlie, Steve, and Martin
Thanks for your insightful answers. Charlie, I think you hit the nail on the head--I knew there were huge environmental concerns with foam insulations but was not explaining myself well. I will look into the new HFO foams--I read about one the other day, lapolla, which as a Spanish instructor made me laugh (horrible name for Spanish). I was not really aware that I could put foam on top of the new roof sheathing. Is it stable enough that someone would still be able to walk on the roof once the standing seam roof is on or does it require a second layer of sheathing? One of the reasons I am using standing seam is its light weight, which is beneficial in earthquake country (I am less than a mile from the Hayward fault). And the city and engineers would not allow me to raise our roofline. The attic is seen as storage, not living space. Thanks especially for the advice on the bad insulation jobs and having a lawyer look over the contract. I will read the links in the next day or two. One more question--Is the foam in the rigid insulation, the one I could possibly put over the new sheathing, manufactured in a way that it is not ozone depleting?

Thanks again


Answered by rob manheimer
Posted Aug 12, 2017 10:58 PM ET


Here is a link to an article that tells you everything you need to know about installing rigid foam above your roof sheathing: How to Install Rigid Foam On Top of Roof Sheathing.

Some types of metal roofing can be installed on 1x4 or 2x4 purlins, 24 inches on center, instead of solid sheathing. If you choose the right kind of roofing, you will be able to avoid the cost of a second layer of OSB or plywood.

Here is a link to an article about choosing rigid foam with environmental responsibility in mind: Choosing Rigid Foam.

Note that when it comes to rigid foam, environmental concerns are similar to those for spray foam. These environmental concerns have nothing to do with injury to the ozone layer; they have to do with the global warming potential of the blowing agents.

Answered by Martin Holladay
Posted Aug 13, 2017 5:40 AM ET



You can do the environmentally responsible thing by using reclaimed rigid foam. Reclaimed foam is also much cheaper than new material (often half to a third the cost).

Answered by Steve Knapp CZ 3A Georgia
Posted Aug 13, 2017 10:08 AM ET


I own a solar company, and to be honest your situation troubles me.
1. Your insulation is low, and I think for slightly more money you will for sure be happy if you spring for 2x6 atleast if not more to allow you more insulation.
2. Did your engineer factor in solar panels ? They are around 40lbs a piece, and given your history, I would think again atleast 2x6.
3. Closed Cell and Roxul are both great products. Roxul has some sound and fire benefits, but if your not changing the rafters, if it were me I would do closed cell.
4. Be very picky about your solar mounts given what you describe. I would be looking for something like Quickmount PV, or one of the quality flashings. Snap and Rack, unirac, etc, but the key things you need is a good flashing to protect that new roof, and if you go with 2x4 I would over engineer the penetrations. You don't want to lose an array off your roof, and there is no such thing as too much strength in solar mounting. 2x6 I would feel ok about, 2x4 you would want to hit every span, with no non structural spans. Most tools will tell you to go ever other one, I would recommend a mount in every rafter if it is 2x4, and I would double check the spec with your solar rack manufacturer. Also most mounts come in different grades, I would go with a heavier strength rail to help backup your roof with the limited rafters.

Just my opinion.

Answered by SLEaton
Posted Aug 15, 2017 4:15 PM ET
Edited Aug 15, 2017 4:18 PM ET.


Hi Sleaton
Thanks for replying. I am located in Oakland, CA which is a very temperate climate. The attic is, more than anything else for storage and we'll have r-23 between the attic and the living space below. If we use roxul in the roof, that gives us another r-15. If we go closed cell it would probably be closer to another r-23 or 24. That is a ton of insulation by Bay Area standards. (I know of almost nobody on this side of the coastal hills who has air conditioner for the summer and the temps go below 32 degrees maybe once or twice a winter.)
I am more confused by your solar comments. We are planning on installing a standing seam roof and my understanding of one of the huge benefits of standing seam is that the panels are attached to the roofing via clips screwed to the seams. In other words there are no roof penetrations. I hadn't gotten as far yet as specifying what type of clips to use but I will look into that and ask the solar companies wha they use and what guarantees they offer that I will not loose a panel.



Answered by rob manheimer
Posted Aug 15, 2017 5:08 PM ET

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