in North Texas, I plan to build a home (conventional wood framing)
spray foam in the stud and rafter cavities(conditioned attic)wrap the walls with OSB,housewrap,rigid foam(maybe), AIRspace,Cement(Hardi)Siding
I would like to hear other ideas on how thick the AIRspace should be, and what to use for the SPACER
vertical strips of rigid insulation centered over studs as SPACERs would create the AIRspace and reduce 3D thermal bridging where it counts, then mesh spacer along the bottom/top of the wall
Minimize the AIRspace to avoid the need to “build-out” the exterior window/door trim and also avoid an oversized “critter crack” at the bottom of the wall
Another thought would be to just use a “house wrap with RAIN channels” Would that AIRspace be enough in my climate to properly vent Cement siding?
Replies
Look at this as a BUMP for you.
I am in Ky, and I didn't think it important to provide airspace behind the hardie. Wood? I'd think about it. But hardie with good caulk and paint? Nah.
I am shure someone brighter has a better plan for your climate.
Parolee # 40835
You might want to try as a housewrap Tyvek's Thermawrap. It's a new product that gives you an R-2 provided that you have a 3/4 inch airspace. Since you are considering a rain screen wall you might as well get a little more R value. I'd just rip some 3/4 inch OSB for the spacer.
Thanks McCarty,
I will look into the Thermawrap
the fiber cement (Hardie) is typically open at its edges where it laps on the next lower plank- and breathes very well. You do not need anything as an air space. There are lots of rumors about Hardi plank in the South such as "its new and unproven" and on and on- some trades people do not like it because it is in their best interests to put on wood so they can replace the wood after 5 more years with more wood and then paint the wood again in 4 years and then replace the wood. again .......I personally have not seen or heard of wall failures in the South caused by Hardie Lap planks.
Stucco ruins a lot of walls thoNot much water in fiber cement siding- its pretty thinEdited 5/5/2007 8:46 am ET by edwardh1
Edited 5/5/2007 8:53 am ET by edwardh1
Edward,
I think Fiber cement siding is a good idea, I just want to make sure I use it right
Check this link under Building Science Details...Fibercement Siding
http://www.buildingscience.com/bsc/designsthatwork/hothumid/profiles/houston.htm
JB
Thanks I had seen that, also the Hardie installation page (they don't mention it.
The Hardie Lap siding is already open as air come in between the laps- why the building science web site says to use it i have no idea.
They probably "think" its a good idea
You might want to run your design by someone who does Energy Star Certifications. Sounds like some expensive overkill to me, but I'm just a dumb carpenter Outside of the gates the trucks were unloadin',
The weather was hot, a-nearly 90 degrees.
The man standin' next to me, his head was exploding,
Well, I was prayin' the pieces wouldn't fall on me.
homedesign,
I am curious what you are gaining by using the airspace.
I see no purpose for it myself. Install the siding and flashings correctly and no water should be penetrating the siding anyway .
Maybe, maybe at a stretch I can see using one on the south face of a building in your location to prevent solar gain but even that I have my doubts about it's cost effectiveness.
Dovetail,
I am convinced that an air-space is important....I just do not know how thick the space should be. I have heard of wall failures in the South that resulted from moisture remaining in fiber cement after manufacture. I also believe (based on EEBA) that a wall in Hot-Humid climates should have a way to "dry" to the outside and to the inside.
Here is a link for a fiber cement sided wall in a Humid climate:
http://www.buildingscience.com/bsc/designsthatwork/hothumid/profiles/houston.htm
I propose to modify this detail and use rigid insulation as a furring strip as well as a "stud-jacket" to reduce thermal bridging at the studs and headers. Some recent material I have read emphasizes that thermal bridging is a BIG DEAL
Just one more coment. With polystyrene foam and sprayfoam (I've used Corbond with sucess) I don't think you will have a problem with thermal bridging. Rain screen siding is a best practice but I've never seen it done probably due to it's extra cost. That being said I'm going to install rain screen siding on my own house. The labor of putting foam just over the studs doesn't make sense even if you have all day. I'd put polystyrene over the whole house.
McCarty,
How do you plan to do the rain screen on your home...are you still talking about ripping osb? my thought was that it would be at least as easy to slice up the rigid insulation and much easier (lighter) to handle and fasten. I ran this idea past Joe Lstiburek recently at a trade show. He thought it was a good idea...But he is a Building Scientist not a carpenter. I have never even tried to slice the insulation myself so I just do not know if it is "overkill" or a waste of time and material.
I still wonder if the rain-channelling house wrap (forget the thermal bridging)is the easiest way to go.
I have tried to bring up questions like this to the energy star raters around here and it seems like if the question is not an option on their software then they are as clueless as I am
JB
You could fur out just the studs with foam and use that for the airspace. I just like putting an inch of polystyrene around the whole house. It just feels like a nice warm blanket.
There is an interesting product in the last issue of Finehomebuilding (186). Look on pp40, there is a rain screen and air barrier in one product. It looks like it might work. If it doesn't work, at least it's a good concept.
I installed Hardipanel stucco-look on the bottom half of my house. I live in Northern VA and thought a rain screen was a good idea too.
Here's what I did, using the Fine Homebuilding article as a guide.
I used shingle-over ridge vent material (Cobra Nailable Ridge Vent), cut into 2" strips and nailed it at the top and bottom of where my panels would go. This supplied the venting and barrier.
I understand Cor-a-vent makes a special material for this purpose but I needed to save money and It wasn't easily available at the time I was doing it.
Then I ripped 1/2" ply into 2 or 3" lengths, secured it vertically over the housewrap, with the venting material at the top and the bottom, then covered it with tarpaper strips. I then put the panels on top of this.
Just an FYI, the rainscreen is for any moisture that gets behind the siding. This included moisture that migrates from the inside of the house to the outside. Yes, if you build right no rain should get behind the siding. But here in VA, with high heat and humidity and heavy air conditioned houses, houses sweat. Any house that's painted, will peel. and moisture problems lead to mold and algae. Its just a matter of time.
Fixit,
I agree with your comment about moisture. Maybe we should call it a "moisture drain space" instead of a rain screen. In my climate the wall should be able to dry to the outside and the inside.
I have used a product that is made specifically for cement siding, Weather Trek. Has little bumps so water can drain rather than sit on top of the Hardi.
Your plan with the foam on the outside leaves you with nothing to nail your siding to. If you nail it through the foam it will crush it & you will have a wavey mess.
If I wanted a solid sheet of foam to eliminate thermal bridge, I would put it on the inside. Wavey DW would be a lot easier to handle than Hardi.
Reinventing the wheel isn't always worth the effort.
Joe H
Joe, I am considering a Drain Screen type house wrap (like weathertrek or others)
Are you saying that Fiber cement nailed over Rigid Insulation will crush and create waves? Are you talking about expanded polystyrene or extruded or both? I was unaware of a significant "crushing" potential. I was concerned about "waves" with the fiber cement over wood or OSB furring strips if they are spaced too far apart.
Yes, the foam will be crushed.
Not completely, but different from row to row.
Try it, get a piece of foam and nail something through it. You will need some fairly long nails, with an inch of foam probably close to 3" nails to get the penetration Hardi wants.
Wood furring strips won't give like foam so waves will be minimal.
Hardi is flimsy stuff, it will reflect whatever is underneath, If it isn't flat it will show.
Joe H
Joe H
Thank you Joe,
This is the kind of "in the field feedback" I am looking for
Rigid insulation and Fiber-Cement siding are good products.....I would like to hear more comments from those who have "combined" them with success as well as those who have had a poor outcome. JB