Suitability of Rainscreen Wall Design
I am working on a wall design for a small house that we are going to construct this summer in Oklahoma. My planned wall from the inside out goes as follows:
1. 1/2″ gypsum
2. 2×6 stud framing, 24″ O.C.
3. Cellulose insulation in the cavity
4. 1/2″ plywood
5. 1″ XPS
6. 30 # felt
7. 2×4 vertical furring strips attached to FastenMaster specs (3/4″ material seems inadequate for siding fasteners)
8. Foil radiant barrier stapled to a dado cut 3/4″ deep in the furring strips-this also narrows the 3 1/2″ strip down to approximately 2″ to help reduce thermal bridging (sorry that I don’t have a drawing for this detail, but the main idea is to keep a 3/4″ air gap on both sides of the radiant barrier placing it exactly halfway between the felt and Hardiplank)
9. Hardiplank siding.
I am capable of constructing the assembly, but I am an amateur at design. I am a fan of OVE, even though it is hardly ever used here in OK. The extra insulation coupled with reduced thermal bridging seems like a no brainer. I also hope to incorporate the XPS, even though I don’t think that I should get carried away with the thickness here in Zone 3. Although not as common as the ubiquitous housewraps around here, the 30# felt is specified because of its breathability.
I do have a few concerns:
1. Is the felt in the right location? Should it be between the plywood and the XPS? I am even right to use felt?
2. Is there any concern in meeting fire codes with this design? Specifically concerning the proper use of XPS?
3. Are there any concerns with the 1 1/2″ opening created by the 2×4 furring? The cost for 2x material is not much greater than 1x, and when I factor in that they are unlikely to split as well as providing a more suitable fastening surface for the Hardiplank, they seem to be the better option.
4. The radiant barrier is just an idea I have had for a year now. I know that the temperature load on a vertical wall is not nearly what a roof would experience, but we have had over 40 days above 100 degrees for the past couple of years with highs of 109. While Hardiplank obviously does not have the thermal mass of a brick wall, it seems like it will still get hot under such conditions.
By careful assembly, I hope to keep the wall design as airtight as possible, but I have real concerns with any seriously low permeability membranes. The small house design, at only 800 sq. ft., will likely have higher humidity levels and the NE part of the state is often humid to boot. It seems best to allow the wall to move the moisture either way that it needs to.
Sorry for the long post. I would sincerely appreciate any advice that you might have. If I sound way off on this, don’t hesitate to shoot me down. I want to do this right, and it is much better to change a plan now than when I have this all fastened together!
Thanks!!
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Replies
Jason,
Q. "Is the felt in the right location? Should it be between the plywood and the XPS? I am even right to use felt?"
A. Every wall needs a WRB, and asphalt felt is a perfectly acceptable choice. Just make sure you know which material is your WRB, and make sure that your flashings are integrated with your WRB. More information here: All About Water-Resistive Barriers
... and here: Where Does the Housewrap Go?
Q. "Is there any concern in meeting fire codes with this design? Specifically concerning the proper use of XPS?"
A. This is a matter of interpretation by your local code official. Talk to your local building department.
Q. "Are there any concerns with the 1 1/2" opening created by the 2x4 furring?"
A. It's perfectly possible to put Hardie fiber-cement siding over 1x4 furring strips that are only 3/4 inch thick. For more information, see:
http://www.jameshardiecommercial.com/cad-hardiepanel-wood-framing-furring.shtml
"HardiePanel Siding Details - Wood Framing with Wood Furring Strips"
http://www.jameshardie.com/pdf/install/install_panel-canada.pdf
"HardiePanel® siding can be installed over furring strips (in accordance with local building code requirements)."
Can I really use 2" of rigid insulation under fiber cement siding?
Anyone back-vented Hardie lap siding over exterior insulation?
Should you use furring strips under Hardie siding?
Q. "The radiant barrier is just an idea I have had for a year now."
A. I suggest that you use foil-faced polyisocyanurate instead of XPS plus a separate radiant barrier. The installation details will be much simpler.
Your answers are very helpful and greatly appreciated!
I think that I understand the concept of using the foil-faced polyisocyanurate by orienting the foil face towards the air gap in the rainscreen, but I am not sure how that would simplify the installation. Will the polyisocyanurate work as the WRB eliminating the felt?
Did you have a recommendation in mind for the "separate radiant barrier"?
Although I know that these details are going to take me some time to install, my main reason for installing the radiant barrier in the center of the air space was to maximize its performance. I was under the impression that radiant products merely act as conductors when in contact with other materials, so at least one side has to be facing an open airspace. I just assumed that both sides would be best, and that was my reason for favoring the extra trouble over foil faced sheet materials. If the performance improvement of going from one to two sides is not significant, perhaps I should simplify the design as recommended. I'm not really a glutton for punishment!
By the way, I forgot to mention that this will be a ventilated wall assembly, top and bottom.
Thank you again! I sincerely appreciate your response!
Jason,
I recommend that you abandon the idea of installing a radiant barrier suspended between two air spaces. You'll only get a boost of about R-1.5 for each air space -- it's hardly worth the trouble. Stick with a single air space, with foil-faced polyiso on one side of the air space.
I agree with you. Thanks for steering me in the right direction.