Exterior Foam Board in CZ4
I am building a house in KY and I am getting conflicting information from insulation contractors and I was hoping to get some feedback here.
I am building a 1900 sq foot house with a finished basement. The insulator who is giving me the best price so far is planning on 2″ spray on closed cell on the basement walls and band. On the first floor, fill the 2×4 walls with spray open cell foam. In the attic, he quoted four inches of sprayed open cell foam on the floor for an air barrier, and then blown in cellulose to get to R-38. He wanted to spray the roof deck instead of the attic floor and make it unvented, but since there is no ducting of any kind, I sort of preferred vented with insulation on the floor. The garage is 24×24 and he is going to do all blown in cellulose there. The quote seemed reasonable at $9500.
My question is about using foam board on the exterior to help prevent thermal bridging. I have 2×4 walls and went with the Huber Zip System for the “OSB”. I had asked y contractor about installing a 1/2″ (R-3) foam board on the top of that, but he didn’t like the idea (“waste of time and money”). He estimated the cost at $2000-$2500 to add it for material and labor.
So when the insulator came out, I asked him about the foam board. He also said it was a waste of time and money. He said they only think foam board is good for is giving you an even surface to install siding, and that it would never pay for itself.
So I guess my question is, is it worth the time and money to install foam board? If so, how thick does it really need to be? I was under the impression that a 2×4 itself was an R-3.5 or R-4 and the zip around a R .5 or .6. So an R-4 or 4.5 for the stud “short circuit”. The Demilic 500 is R-3.8/in, so R-13.3 in the cavity, so 13.8 or 13.9 for the wall. If I add the R-3 foam board, the cavity goes up to 16.9, but the studs are still just a R-7, which doesn’t seem sufficient.
However, in the publication, “Guide to Building Energy Efficient Homes in Kentucky and Mixed Climate Zone 4″, the seem to suggest that the R-3 (1/2″) is all that is required for this climate zone. this publication also suggests that the annual savings from installing this R-3 foam board over my zip would be $69 per year for CZ4. So at a $2500 cost, that is a 36 year payback, not including the additional interest paid on $2500. Probably takes the true payback up to 50 years or more. I could use $1500 of that money to upgrade my HVAC from a 16 SEER/9 HSPF to a 19 SEER/9.5 HSPF. What I’ve read suggests I’d save about $36 a year in cooling and about the same in heating with the upgrade. So a $72/ year savings for a $1500 investment with HVAC (still a 20 year payback), vs. $69/year with the 1/2” foam for $2500.
Does this seem to be an accurate assessment, or am I missing something? Even looking at thicker rigid foam, what I’ve read would indicate I need to get wall cavities to R-24 with at least R-10 at the studs to save $115 annually. That would suggest 1″ or 1.5″ of foam board in CZ4.
I’m trying to do this house as right as possible within budget (which I’m tracking to be about 5% over budget, so I don’t want to add new things that make that worse, but will if it is really important). I oriented the house to face due North. Only one east facing window, all west facing windows are LoE3, majority of windows are south facing with correct overhangs. I tried to incorporate as much passive stuff as I knew about and do as well with the insulation with what I know.
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Replies
Your insulation and air sealing is perfect IMO. The price is astounding.
Think about sealing up all the areas that the Sprayfoam does not.
Add ventilation.
A low cost split air would work well for you possibly as another option to save along with a HWHP.
And any wood touching concrete in the basement should be pressure treated.
We are using pressure treated lumber against the floor. All the perimeter framing is set in 1/2" from the wall, so when it gets it's 2" of foam, the concrete will actually have a continuous barrier 1/2" thick, and the other 1.5" of foam will be in the framing cavities.
What areas should I look to seal that the sprayfoam won't get? WE've been caulking around doors and windows as those go in, and I guess it wouldn't hurt to put some caulk around where double 2x4's were used to support headers. I guess there is no cavity there for foam to go into. I will say the insulation guys sounds very thourough. He says he uses tons of some kind of special caulk, and he is even foaming the inside of the chimney chase (which gets covered with Type X sheet rock) to help keep it warm.
Yeah, I think I've realized in the past couple of days that I will need a HRV/ERV (even though one HVAC guy said I wouldn't need it, I'm not so sure). I've got a couple of options being priced. The attic will be well vented with 72' of ShingleVent II for exhaust, and soffit intake to balance that (actually a little more intake capacity than exhaust). It gives me a 1'/150' ft^2 ratio. So I think the attic is covered. but I probably do need to exchange house air.
My local Water Furnace dealer threw me a curve today. My HVAC quotes had been coming in about $11,000 for a two-stage 16 SEER system, ~ $12,5000 for a 19 SEER system. I had one Geotherm priced and the quote was about $22,000. So even with the tax break, I had given up on Geotherm. But the Water Furnace rep visited the site today with his quote. Just under $17k for an Envision system with hot water heater. I pay enough in taxes to get a full refund on the 30% credit, so that would get me Geothermal for under $12,000 if I can front the extra cash until tax time. I could always put the $6k on a 0% for 6 month credit card and wipe it out when the refund comes.
Going another $6000 over budget at this point isn't something I really wanted to do, but I sort of feel like I would be stupid if i passed up Geothermal for a final price close to what a 16 SEER system costs. It's about time I got some type of government hand out!:)
Will, you are on top of everything.
Will,
It's good that you are thinking things through.
Many homeowners choose specifications that don't make economic sense right now, because (a) they think that fuel prices will increase in the future, (b) they want to have the increased comfort that accompanies a better thermal envelope, or (c) they know that it will be hard or impossible to retrofit changes to their home in the future.
Remember, if you upgrade your exterior foam from 1/2 inch to 1 inch, you double the R-value of the rigid foam, but you don't increase your labor costs.
Nevertheless, you are probably correct that it's hard to make an economic case for exterior rigid foam in your climate zone.
Will...did you end up doing half inch foam board on the outside of your house? I'm currently building in Finchville, KY and this is something I'm looking at doing. My contractor just started framing. I have 2x6 framing and will have R23 propink insulation. I am also trying to determine if I should use r5 or r10 foam board on my basement walls since we are in zone 4. I currently have r5 below grade exterior drainage board/insulation combo.
Aron
Aron
Sorry I missed the follow up question, but for anyone's future reference, I did not install the foam board. I did go geothermal. I've been heating/cooling since end of February last year and my highest electric bill has been about $135. During the summer, its about $100. We keep the thermostat at 72 F year round.
I'm very pleased so far. My bills seem to be about the same as my 1500 sqft 1.5 story house, except now I have 3800 finished sq ft, upstairs and in the basement, plus a fireplace (yet to have a fire in it) and Jacuzzi tub that gets used daily. I figured my bill would be significantly higher.
The nice thing is that the basement has stayed at 68 F year round, no matter if it is 19 F outside or 95 F. It has sort of made the extra $4000 I payed for a zoned system a bit of a waste. I could have just opened/closed a manual damper. I keep the downstairs t-stat at 65 F heating and have only ran it once, to warm it up to 70 F because my son was having friends over and we wanted to make sure they weren't too cold playing in the basement. That is the one time the basement zone has been on in almost 9 months.