I have a client who has built a home that is ready for insulation. That said - it is what it is and I am not looking for what should have been done - there will be no reconstruction. We are the insulator. 

House is in Hot Dry climate in Climate Zone 3. Here are the average/mean temps http://www.weather.com/weather/wxclimatology/monthly/85601

The home has a standing seam metal roof over tar paper underlayment and plywood sheathing. No insulating sheathing was applied on this outer assembly - so no control of the condensing surface.

The space we have to insulate is 2x8 cavity with plywood sheathing on one side and tongue and groove wood planking at the bottom chord of 2x8. They planned to ventilate the ceiling assembly - they have a ridge vent and soffit vents(that could be plugged if we decide on unvented assembly. They want me to either downgrade the insulation to allow for the vent space or look into the "eggcrate" spacers or something of the type. Doesn't leave much room for insulation. She is not too keen on spray foam due to "chemicals" but I can try that route.

The question is - How would you do it considering the roof is installed and there is no control of condensing surface and we only have 7.25"?

Thanks for  your input 

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Any ideas?

Hi Craig,

I was watching as your situation is probably more common than we would wish.  Let try a couple of question.

1. Sounds like a new home.  What is the code minimum and are they asking you (?) to authorize something less.

2. Would she object to rigid foam, cut and fit into the cavities?  In your climate it probably wouldn't require a lot of rigid and then fill with something like Roxul.  Essentially a hot roof.

3.  Will there be drywall before the T&G.  T&G will leak a lot and a layer of plastic above it (not mentioned) would also leak.  A layer of drywall would give you a good air barrier.


Very informative and well written post! Quite interesting and nice topic chosen for the post.

  1.  I would cut  1 inch strips of 3/4” pine as spacers  to go between rafters and use 3/8” plywood ripped to 14 ½  to create a ridged air channel from soffit to ridge vent, foamed edges to airseal.
  2. I would add 1.5” of foil faced polyiso, cap stapled to the bottom of the  joists and sealed with siga, zip or good foil tape,  I would then secure the iso with 1x3 through screwed to the joist.   Now just densepack the bays with cellulose and you are good.  Now you have  for the stack up  tongue and grove wood- ¾” airspace- 1.5” polyiso- 6” denspack cellulose-3/8” wood

This stack up has the air sealing handled by the taped polyiso layer.  It also gives you a radiant barrier facing the warm side in winter side(less value due to climate) with a ¾” gap and cuts down on thermal bridging by being continuous over the joists.  You have a 1” gap for a continuous vent from soffit to ridge.  This is a fair bit of effort, but your possible way out of a bad situation.

Ok, now I am going to drop the bomb with this picture. Now is anyone thinking foam? (The lower beams are going to be exposed.

Also, I am trying to find out if this is a permitted job. It is a weird situation, they are already living in a portion of the home, so I am thinking it is not. This is a very remote location 11 miles from the Mexican border. The local code is R38, so they would have to furr out to get that. 

The plot thickens, I will get back with the permit/code status. We probably won't touch it if it is not permitted.


I wouldn't t take responsibility for design if you weren't consulted at the design step.  And I'd avoid attempting to fix something you perceive as fundamentally flawed or you own it.  

I don't know what that white shiny stuff is, but it looks like they've got a wonderful, slippery condensing surface that, if it sees moisture, will nicely drain right into the sheathing.  

What is your ethical obligation here? You were not included in the design phase.  If they aren't asking for building science advice, at what point are you going beyond your obligation?  

Nice eye on the white stuff - it is only over the octagon area, not the whole roof assembly. Just talked to the homeowner and it was a radiant barrier product called Foil Flex and it was put in by the original homeowner/builder in 2005. She is not sure they are even in business any more - he never finished the rest of the roof because he "didn't like the performance" - go figure with no insulation. I will advise them to take it down.

Your statement reflects what I was getting at with the permit issue. They are not taking our advice to get a permit and the cavity is 8" for what should be an R-38. I am going to give them a few options as they are willing to sign a release of liability for any issues arising from the design or reduced amount of insulation. Luckily our buildings get away with a lot here due to the dry climate. What about the option of 2" of closed cell at roof deck and net and blow the rest of cavity full? This will get us damn near R-38 and this approach is outlined in BSD-02 figure 10 and figure 11


Are they paying you for design recommendations?  If not, I'd avoid building a spec like the plague.  You have conflict of interest here which really exposes you later. 

Given Joe's recent studies on Cellulose and how moisture tends to migrate to the top, I'm not sure I'd want to put it in an area where moisture could get blocked.  So if you are going to dense pack, I'd NOT go with foam, seal the bottom and vent the top per his instructions. 

My personal preference would be to simply spray 3.5" of closed cell and call it a day.  

What color is the roof? 

Where do you draw the line between "building a spec" and providing a bid? I don't consider every quote I do a design recommendation as most of my work is retro. I do always recommend what I would consider the best option for each situation I encounter and this situation is no different. How do you offer a bid without making a design recommendation if there is no "designer" per se?

It seems to me that the spray foam is controlling the condensing surface. I remember an equation from one of BSC's papers that allows you to compute the amount of insulation required under roof deck based on the lowest mean temp in the winter months. Have to dig that up.

 "Free Quotes" are expensive enough for contractors that they should try to avoid them, particularly if there is no specification to quote to. In my mind building a spec is what the architect and engineer should have done.  When people start asking what your recommended approach is, that is asking for design advice. 

"Free quotes" leading to "free design advice" end up on the path to "what is best for me and not necessarily best for you".  Often it leads to the corner cutting dance of "how can I make my bid the cheapest so I win the job." 

So Craig, if they are not telling you how to insulate that space, are you not taking on a LOT of responsibility, liability, and headache for no pay, and potentially no job at the other side?  Look how much time you've spent on this already...  

Glen, this is purely emotional bias as I have no concrete experience to base this on, but for some reason I'm uncomfortable with the idea of putting rigid foam against plywood.  I'd want that roof deck sprayed.  

When I have these gut feelings I like having them proven wrong (or right) by others, it's a quicker way to learn than from experience.  So I'm excited to learn if people have experience where this bias is correct or incorrect.



With you quoting CA Title 24 insulation standards I will assume you are here in the Peoples Republic of California. With R-38 you are not a coastal region. The picture shows an eclectic choice of materials and more problems than the roof that are worth mentioning.  Gap above framed door, windows with no coverings for solar gain, a partial dome roof with scabbed in spans,  is that plywood over masonry?

So it looks interesting and definitely not permitted. I am with you as it is more a focus on fitting the customer's needs with the best solution at hand.  There are certainly multiple design issues and a backhoe is the best solution for those.

However given that our main problem main moisture problems will occur during rain and more commonly hitting the dew point and that is outside. I assume the roof is weather tight. Your proposition of foam board and cellulose should work fine in a warm dry climate. While not fixing all the issues in the space given that it is a metal roof with no insulation this remedy will go a long way to increasing the comfort of the occupants in my view.

What a mess but there is one simple rule to always keep in mind - if water might be an issue only closed cell is a viable option. You can try drainage & venting options but you wont know till to late if there will be an issue. If I was going with foam & cellulose where that is a concern, I rather go from above to make sure that the seams are taped & water has no chance of sneaking past

Get a professional CC foam insulator in there to look at it - the radiant will have to go, a primer installed, etc... Thick enough & probably installed you wont have any dew point concerns

As for chemical concerns - not only does the insulation need to be installed properly, but the area ventilated well during & after the work is completed - done right you wont have any issues

If you need to know more about this - http://blog.sls-construction.com/2012/ventilation-strategies-renova... 


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