July 31, 2015

Question of the Week – July 31, 2015

We hope you all had a productive, safe, and successful July – we continue to thank you for your support, insight, and enthusiasm for the future of high performance, low carbon construction.

The Question of the Week for the last week of July comes from a building inspector in Colorado who was curious about the best retrofit solution for a wall assembly where it’s difficult to remove and re-install siding without damaging it in that process.


The question:

“To Whom it may concern,

I am interested in knowing the recommended assemblies for the Intello Product.

We have a lot of rehab work being done in our town on very old historical buildings. The contractors want to remove the exterior siding to install sheathing and house wrap; however, the siding is difficult to remove and re-install without it falling apart etc.

The idea is to use the Intello product on the inside face of the wall framing and potentially on the inside face of existing rafters. The walls have three layers of 1 x material on the outside and 2 x 4 wall framing. The roof metal will most likely be pulled off, new sheathing applied and Ice and Watershield applied. Will the Intello product work for these situations. Can it be used with closed cell spray foam.

Thanks in advance for your help.”


Our response:

“Dear A.,


Thank you for your question. We have dealt with interior renovations quite a bit, as we believe that historic buildings should and can become more energy efficient, but as you note – their exteriors should be preserved as well. This requires the following focus, in this order:

1 – Water control
2 – Air control
3 – Vapor control
4 – Thermal control

The ProClima INTELLO Plus smart vapor retarder is a great product to use to accomplish the air and vapor control.
It allows you to get very good interior air-tightness, without the risk of trapping moisture. The membrane is vapor retarding in dry conditions (winter), while allowing inward drying (if needed) in summer (see info here of how this works)

It works with practically any insulation material – batts, densepacked fiberglass/cellulose, mineral wool, and spray foam. But as I had mentioned to you over the phone, the type of insulation used does impact how the wall responds if it does experience any unforeseen moisture (in case of a big storm from outside for example, or due to condensation as a result of airleakages).  The best drying to inside can be achieved with mineral wool, cellulose, or fiberglass (all vapor open materials) – foams do not allow easy drying to the inside, for in general they are much more vapor retarding than the aforementioned alternatives.  And as we discussed as well, spray foam is pretty much irreversible, so if something goes wrong during installation or during later renovations, this can unnecessarily contribute to a larger issue (see this blog series for more on this topic).

Here some quick blogpost/sketches of assemblies to could apply to your case in PDF format. It is from our 2×6 series, but you can apply it 2×4 renovations as well.

QOW 8315

2g - Insulated Interior Service Cavity Only


4a - Cathedral Roof


The exterior in your case is not new, so you can ignore those notes regarding WRB etc (but again, if the wall is showing rain leaks or water damage at the exterior one should be careful with any type of insulation and airsealing).

For the roof, there are more and better ways than ice&water under metal to keep icedams at bay and we can actually improve the drying potential quite a bit by not using such vapor closed, expensive peel and stick products. We normally recommend new sheathing – and then SOLITEX MENTO membrane as a roof underlayment (vapor open but very waterproof due to its monolithic membrane), and then vert + hor battens and metal roof. (see the blogpost – A high performance roof should be vented)

Please let us know if you have any other questions, or projects/contractors or architects that are dealing with these kinds of buildings. Happy to help.


You can now check out our 2×6 CAD details in a new eBook, released today, for free right here.  Have more questions on the subject?  Add to the discussion below!


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