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Spray Foam Confused

I have an 1,100-sq.-ft. 2-story Cape with two 14 ft. dormers for bedrooms on the 2nd floor. It is 90 years old, currently gutted. I want to insulate it well, but within some equitable reason.

Currently, the house has 1/2-inch rigid foam, R-3.6, on the exterior walls. I want to spray the roof line and enclose the whole envelope.

This is my problem. Three companies have provided estimates now. Two say open-cell foam, 7"-8" on the roof and 3.75" on the walls. One company just quoted 7" open-cell foam on the roof and 2" closed-cell foam on walls. Estimates vary between $4,500 and $5,500.

Will 2 inches of closed-cell foam perform better than 3.75 inches of open-cell foam?

Last question: has anyone had any experience installing rigid foam within the rafter cavities, like the poor man's way?

Asked by Scott Jacobs
Posted Dec 21, 2011 6:39 PM ET
Edited Dec 22, 2011 5:53 AM ET


11 Answers

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Sorry if I am doing this wrong, first time I ever used this. - Let me add I am in climate 6.

Answered by Scott Jacobs
Posted Dec 21, 2011 7:22 PM ET


The advantages of closed-cell foam compared to open-cell foam include its strength, higher R-value, and its greater resistance to the leakage of air or water vapor. The disadvantage of the closed-cell foam is that it is denser, requires more material, and therefore, is more expensive. Even though it has a better R-value, typically the cost per R is still higher than open-cell foam. The choice of foam can also be based on the requirements for the other performance or application specific characteristics such as strength, vapor control, available space, etc. Open-cell SPF has an R-value around 3.5 per inch and typically uses water as the blowing agent. Closed-cell SPF has an R-value of around 6.0 per inch (aged R-value) and uses high R-value blowing agents.

The average cost for Close Cell is 1.20/ sq.ft. and Open Cell is .80/ sq.ft.

Answered by Jean-Paul McGraw
Posted Dec 21, 2011 7:50 PM ET


Scott, what are the dimensions of your framing? Where I work, we never see more than 2x6 rafters in a house like yours.

Answered by Torsten Hansen
Posted Dec 21, 2011 8:27 PM ET


A house in CZ6 requires R49 insulation in the attic. If your intention is to have a conditioned attic, 7.5” OC Foam is R28, then you also need 3” min. rigid insulation on top of the roof decking. If you want to install CC Foam, you would require 7.5”, and it can only be installed in 2” applications. I’m attaching a Demilec chart for reference; it maybe different than the foam you are planning to use. You should check the 2009 IRC - R806.4 code before you continue.

DEMILEC R-Value Table 2011.pdf 123.56 KB
Answered by Armando Cobo
Posted Dec 21, 2011 8:34 PM ET
Edited Dec 21, 2011 8:38 PM ET.


Thank you everyone for your input. I have added width to the 2x6 for a total of 7.5 inches because I am vaulting some of the ceilings. Next summer or the following depending on how my project goes I plan on a new roof. At that time I can add some rigid. What really has thrown me for a loop is this. I have close to 4" dimensional studs.... the old school kind. Two companies have pitched filling them with open cell for R 18 and this afternnon one pitched 2" closed cell. I am having difficulty wrapping around the idea of not using the whole cavity available., but woudl like to hear any thoughts from others. If you have a choice would it be. I really can not afforf another option than the open cell in the roof. Todays estimate was for demilec. The cheapest price I have recieved so far is for Thermoseal 800. Thank you for everyones input. This site is great.

Answered by Scott Jacobs
Posted Dec 21, 2011 10:50 PM ET


The companies that are proposing 7 inches of open-cell spray foam for your roof are only offering R-26. That isn't much. In your climate zone, as Armando points out, you really want at least R-49. Don't let a spray-foam contractor talk you into accepting insulation that is less than the minimum code requirements. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.

Q. "Will 2 inches of closed-cell foam perform better than 3.75 inches of open-cell foam?"

A. These two options are fairly close in R-value. Two inches of closed-cell foam will give you about R-13, while 3.75 inches of open-cell foam will give you about R-13.9.

Q. "Has anyone had any experience installing rigid foam within the rafter cavities, like the poor man's way?"

A. Yes, I've done it. It is slow, fussy work. If you choose this route, be sure that the perimeter of each piece of rigid foam is sealed in place with caulk or canned spray foam.

Q. "I am having difficulty wrapping around the idea of not using the whole [stud] cavity available, but would like to hear any thoughts from others."

A. Closed-cell spray foam is hard to trim, so spray-foam installers usually don't fill the entire stud cavity. Instead, they try to spray only enough foam to almost fill the cavity. In contrast, open-cell spray foam is easy to trim, so installers of open-cell foam usually install enough foam to completely fill the cavity. Then they shave off the excess.

Answered by Martin Holladay
Posted Dec 22, 2011 6:05 AM ET
Edited Dec 22, 2011 6:07 AM ET.


Scott, the spray foam will do a very good job of insulating. Code r values do not factor in how well spray foam works compared to fiberglass of the same r value. Most times spray foam is not installed to code r values and yet it performs much better than glass.

I built and take care of a sprayfoamed home that is by far the best performing home that I have ever dealt with.

Just make sure you hire a very experienced crew that contracts to take care of foam issues if blown wrong leading to unwanted smells etc. And make sure the weather is ideal during the spray. Ask questions.

Answered by aj builder, Upstate NY Zone 6a
Posted Dec 22, 2011 9:05 AM ET
Edited Dec 22, 2011 9:07 AM ET.


Thank you for the responses. I have not heard of getting unwanted smells from spray foam. Is this an open/closed cell thing? Craigslist seems to be a great place to pick up some used rigid insulation. If I poor man installed a couple inches and then sprayed the open maybe that would be the best route to get the R value up into the 40's? The roof has nails protruding about 1/2 inch. Do I just shove the foam up, maybe wack it with piece of wood tp press at tightly as I can,.. leave 1/2 inch on sides to intall expanding foam? Seems simple.

Answered by Scott Jacobs
Posted Dec 22, 2011 6:30 PM ET


Go for it Scott, though only your code office would know if you would be ok with firecodes and other pertinent codes. Good luck.

Answered by aj builder, Upstate NY Zone 6a
Posted Dec 23, 2011 12:18 AM ET
Edited Dec 23, 2011 12:21 AM ET.


(CZ6) building an IRC home it is a R-49 to the ceiling........ BUT in the International Energy Conservation Code 2009 an R-20ci is required to the roof deck (but in the new 2012 IECC book it is an R-30ci) the half pound foam to the deck has a diminishing law of return once you get over 8 inches deep in this climate zone 6. People will still think that the R-49 is required but that is a waste of money. Foam May also need a fire coating if an open flame is in the attic space. We are comparing 2 things here, Building a ventilated roof system with the R-49 to the Ceiling or an R-20 ci the the Roof Deck Building the Envelope to the Roof Deck. 2 different R- Values needed in these spots. Green buildings need proper installation. Foam Sprayed Properly does not "SMELL" but foam that is sprayed COLD will have chemicals smelling up during the hot days of summer due to the chemicals still reacting together. Get the Spray Foam Inspected and the building Envelope certified by trained and insured specialist.

Answered by Eric Price
Posted Dec 23, 2011 4:33 PM ET


Eric is right. IMHO the law of diminishing returns really takes a bite out of using more than 2-3" of closed cell (dep on location, I'm in Southern NY Region 4a). Don't trust the codes for telling you what is the most cost effective way to insulate a house. They are written by people wearing suits and working in offices, not contractors. It all depends on your HDDs and cost of insulation. The only way to really determine which insulation is the most cost effective is to do area-weighted average U value calcs for various combinations and plug that result into an Annual BTU Heat Loss calc (Q=U*A*HDD*24), plug that result into your BTU content of fuel and fuel cost and figure out how much each one saves compared to the other. I just did this (today!) comparing an uninsulated roof to 1", 2", 3", 4", 5" of closed cell foam at $1.25/sqft/inch. The low down is that the first 2" are critical (really the first 1"). Going from 2" to 3" saves so little incrementally that it takes roughly 30 years for that extra inch to payback (using a simple cost/savings calc.) My current thinking is use the minimum amount of foam necessary to prevent condensation on the inside surface. You'll have to look that up (and HDDs) for your area.
I read that "OK to skimp" article and totally agree in principal. Yes, R40 transmits 1/2 the amount of heat than R20, but 1/2 of what exactly (pennies/sqft) and for twice the amount of insulation material and therefore cost! Also, done properly you'll get all the air-sealing properties of foam with 2-3", any more won't help in that dept. Re: open-cell. Wouldn't touch it. Too many issues. Look for articles on lawsuits and foam smells. If either type of foam is not applied properly there are ammonia compounds that can off-gas if not consumed during the reaction. I've smelt it on a closed-cell installation. Had to use a fan for two days to help ventilate. Temperature is critical! This is before you even consider the GWP of closed-cell foam due to blowing agents, then going the extra inch or two really becomes an issue.
Personally I only use foam when there are moisture issues to contend with (i.e. crawlspaces, basements). Otherwise I suggest cellulose. Low embodied energy, it "breathes", has high thermal mass, good sound deadening qualities, air-sealing properties when dense-packed, won't separate from the substrate, won't burn like a pile of old tires if you house goes up in smoke (in fact it may prevent that from happening), it's made of renewable and mostly recycled material (not non-renewable oil, and what do future generations do with all that foam if the house is torn down?). Also, consider what happens if your roof has a leak? Open-cell is a sponge and will need to be taken out and replaced, closed-cell won't show the leak and will soak the plywood until something gives. Also, for the repair, who's going to come to your house to do <100 sq ft of open-cell or closed cell for cheap? Cellulose can be replaced fairly easily. If you're a DYI'er then the idea of foam bd + 1 part foam around edges backed by properly installed fiberglass is not a bad idea. Make sure to hit the top-plate and do a complete thermal/pressure boundary. Of course I didn't even get into the issue of venting....(there are articles by Lstiburek on GBA)

Answered by John Pfeiffer
Posted Dec 24, 2011 4:43 PM ET
Edited Dec 28, 2011 11:02 PM ET.

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