Ext. foam board after SPF installation?
I recently had 3 inches of Bayseal ccSPF sprayed into my walls and roof from the inside. This summer I was looking at replacing my windows, doors and siding, and was wondering if there is any sense in covering the exposed sheathing with 1/2″ -1″ of rigid foam board insulation.
The spray foam is already a vapor barrier, so the only benefit I can think of is slightly increased R-value and reduced thermal bridging of the wall studs touching the exterior sheathing. Is there any other reason to do this and should I even be wasting my time and money thinking of it?
If I do it, is there any added benefit to using foil faced to help reduce radiant heat transfer, or should I just stick to the unfaced stuff?
thanks in advance for all the coming opinions
Replies
Greetings Nate,
As a first time poster Welcome to Breaktime.
This post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.
Perhaps it will catch someone's attention that can help you with advice.
Cheers
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One of the biggest downsides of SPF, other than cost, is that you still have thermal bridging. I think your plan sounds good, I wouldn't expect any problems with vapor diffusion because, as you mention, you already have a VB with the closed cell foam.
The foil facing won't do a lick of good unless you have an airspace. If you can install it with one you may get some benefit.
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
Hi,
You can get an idea what the thermal bridging on your current setup is costing in R value by using the Whole Wall Calculator here:
http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/rvalueinfo.htm
This is what I get for a 2X4 16OC with PU foam:
Clear Wall R 16.9 (includes bridging due to studs)
Whole Wall R 12.7 (includes all bridging)
So, it looks like thermal bridges cost quite a bit.
If you add 1 inch rigid foam board (R4) under the siding you get:
Clear Wall R 21.15
Whole Wall 17.28
So, your wall heat loss would go down by about 25% -- seems pretty worthwhile?
You can use this calculator to figure what that improvement in R value actually saves you in dollars:
http://www.builditsolar.com/References/Calculators/InsulUpgrd/InsulUpgrade.htm
As an example, for 2000 sqft of wall going from R12.7 to R17.3 in a 6000 heating degree day climate with $2 Propane, adding the foam board outside saves $164 per year, or $2611 over 10 years with 10% fuel price inflation per year.
Gary
That was compelling data to ponder. I went to the sites you cited and tried numbers that more closely match my scenario. Unfortunately, even if I fudge the numbers high, it seems I'd only save $31 per year if I added an inch of foam board (Darn diminishing r-value returns!).
I'll have to doublecheck the r-value to see if this is worthwhile (R4 for foil faced polyiso seems low to me). But at these numbers and my expected cost per sheet, I think this would take up to 20 years to recoup just on material cost vs. gas savings. Even if I forego foil faced since I don't plan an air gap, I think I'm looking at a long, multi-year payback period.
I'm gonna guess that this summer will come down to a question of do I do it anyway since the whole darn wall is exposed. My bank account will probably say no! :-p
Edited 3/2/2009 10:07 am ET by MotorCityNate
Hi,
If you are using polyiso at 1 inch, it should be more like R6.7 -- so that might help some (the R4 per inch came from polystyrene insulation).
I agree that the foil is not going to be doing anything for you unless there is an airspace.In the calculator, when you input your "current" R value, I guess you did use something like the R12.7 that includes thermal bridging?Gary
"In the calculator, when you input your "current" R value, I guess you did use something like the R12.7 that includes thermal bridging?"
I did, but that may be one area where I also messed it up. Does 'clear wall' include thermal bridging or not? If it does, then I guess I want to know what r-value the calculator is using for my PU foam. I specifically chose the BaySystems product b/c at 6.9 per inch aged r-value, it was the highest performing foam I could find.
Hi,
I believe that "clear wall" is a wall that in includes bridging from studs but not form window framing, corner framing, ... If you click the "Clear Wall" link, it shows a picture of wall with studs, insulation, sheating/siding, and sheetrock. I think that this is what they include in the "Clear Wall" calc?Gary
Biggest benefit is thermal bridging like you say. You also point out the diminishing returns of adding R-value to existing R-value ... unless you consider it an extension of your original insulation project ... you can draw the lines of the economics however you please.
I agree w/ others ... foil facing does you no good. Polyiso, BTW is about 6.7-7.0 per inch. Extruded polystyrene R-5.0 per inch; expanded poly ... can't recall 4.0. Price out the various sheets on a per R-value basis and you might get a better idea of the cost benefit (you being the engineer type).
Personally if you only had 3" of foam done ... I'd strongly consider adding the 1 inch to the outside. Relative to your project, it may be very inexpensive while providing some additional thermal value and hedge against ... not just current energy costs, but future as well. Did you consider the escalation rate of energy in your economics?
Once you do the project, you've closed the door toward this option.
If you are doing windows ... consider your window options now. One of the most overlooked options IMO. Consider orientation (SHGC). Consider U-value. For the same reason I stated above ... you have one chance to buy and install windows properly ... make the most of it. Your window heat loss/gain is your biggest loser in the envelope. You can potentially make a huge difference in energy AND comfort with improving your window choices. Like I implied ... and often there are many more choices than people think there are and they impact energy more than people really realize. IMO