Spray versus board foam

[semipro]

I need to insulate and seal some cantilevered overhang areas on my house and am wondering about the best way to do it considering performance and cost. Fiberglass batt was originally installed above soffit board and in between the joists with no air sealing. When you pull the soffit board you can see evidence of air infiltration by way of the dirt-stained fiberglass insulation. I plan to remove the fiberglass (maybe) and install some sort of foam.

I'm sure spray foam would be the best in performance but its a small area and bringing in a contractor would be expensive. I've priced out DIY foams kits but the price seems outrageous, roughly 4x that of sheet foam. I put together a spreadsheet with R values, costs of materials, etc. and found that sheet goods provide much more value if you compare how much volume per dollar you get while taking into account R values per inch thickness.

My question: does anyone see any problem with using sheet foam to fill the joists bays in successive layers while using canned spray foam to seal the installation?

I may even put in a few layers of foam and then fill the remaining bay space with some of the fiberglass that I've removed.

Alternatively, I could pull the original fiberglass down temporarily, seal the seams above it, reinstall the fiberglass and then install foam board below it, in between joists, sealing the gaps with spray foam or caulk. I'd then replace the soffit board. (As of actually typing this post I'm now liking this option the best).

 

[btuser]

That sounds like a lot of work. What's the square footage? if its over 500sqft I'd hire it out to a spray-foam contractor. They could do a better job and probably do it for your cost of material.

 

[velvetfoot]

Are your rim joists insulated, or insulated with fiberglass?
If not, you could get the contractor to do them too.

 

[semipro]

That sounds like a lot of work. What's the square footage? if its over 500sqft I'd hire it out to a spray-foam contractor. They could do a better job and probably do it for your cost of material.

Its only about 200 sq ft. I'd have to do all the prep work and reassembly of the soffit and trim so I'm thinking I'll be doing the bulk of the work anyway.

I'm finding it hard to justify spray foam when its 4 times the cost of board foam.

 

[semipro]

Are your rim joists insulated, or insulated with fiberglass?
If not, you could get the contractor to do them too.

Good thought. The bands are insulated (poorly) with fiberglass. I was thinking as you are until I compared the price of board foam to DIY spray foam.

The only way I can see that spray foam makes sense is if the foam applied by a contractor will end up costing less than DIY foam materials.

 

[Jaugust124]

After having an energy audit done, the contractor quoted me a price of $1875 to insulate 140 sq.ft. of cantilevered floor with cellulose to R-29. I would not have to do any work, they would take down and put back up the soffits.

 

[Swedishchef]

that's not a bad price for the work involved...what do you think of the quote?

Andrew

 

[semipro]

that's not a bad price for the work involved...what do you think of the quote?

Andrew

Thanks JAugust for the price info.

To do this using rigid board I'd have to use 3 layers of 1.5" polyiso foam board at a cost of about $500 plus the cost of several cans of spray foam and maybe a few tubes of caulk (approx $50) plus my labor. I'm thinking this would take about 8 hours to do so my time would be worth about $150/hr.

I'm pretty confident on the labor estimate as a I just finished doing a section.

At this point, for me at least as a consummate DIYer, it makes little sense to do any spray foam where rigid board can be used instead.

Thanks all.

 

[EJL923]

For 200 sq ft, the rigid foam panel is probably your best bet. If i could afford the spray, thats what i would do, but it does seem outrageous. I think they are praying on the people who watch DIY network and the show is paying for it. Cut the rigid foam tight, then seal with spray foam.

 

[gpcollen1]

I Just insulated the bottom of my hot tub before installing it. Used my sawzall and circ saw to cut the 2" XPS - workked just fine. You can use a sheetrock saw to rim them up too. Sealed some with spray foam but most with caulk as the gap was small.

Insulated my cathedral ceiling too. renovated that room and installed a ceiling below the ceiling. Screwed XPS to upper ceiling for insulation and air barrier and then used bats in lower ceiling. Got a good R48 up there now. Many ways to skin the cat in DIY but just use at least one layer of foam board and seal the edges for air barrier and then use additional layers of foam board under or bat insulation under that.

 

[semipro]

I Just insulated the bottom of my hot tub before installing it. Used my sawzall and circ saw to cut the 2" XPS - workked just fine. You can use a sheetrock saw to rim them up too. Sealed some with spray foam but most with caulk as the gap was small.

Insulated my cathedral ceiling too. renovated that room and installed a ceiling below the ceiling. Screwed XPS to upper ceiling for insulation and air barrier and then used bats in lower ceiling. Got a good R48 up there now. Many ways to skin the cat in DIY but just use at least one layer of foam board and seal the edges for air barrier and then use additional layers of foam board under or bat insulation under that.

Somewhat related to this: I only found out recently that you can use foam board to insulate above your original roof deck when replacing your roof, creating a cold roof to prevent ice dams and allowing you to convert your vented roof into an unvented roof.

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view

 

[semipro]

For 200 sq ft, the rigid foam panel is probably your best bet. If i could afford the spray, thats what i would do, but it does seem outrageous. I think they are praying on the people who watch DIY network and the show is paying for it. Cut the rigid foam tight, then seal with spray foam.

Good advice. I like the spray foam concept but the price seems to high. Maybe prices will fall later.

 

[gpcollen1]

I Just insulated the bottom of my hot tub before installing it. Used my sawzall and circ saw to cut the 2" XPS - workked just fine. You can use a sheetrock saw to rim them up too. Sealed some with spray foam but most with caulk as the gap was small.

Insulated my cathedral ceiling too. renovated that room and installed a ceiling below the ceiling. Screwed XPS to upper ceiling for insulation and air barrier and then used bats in lower ceiling. Got a good R48 up there now. Many ways to skin the cat in DIY but just use at least one layer of foam board and seal the edges for air barrier and then use additional layers of foam board under or bat insulation under that.

Somewhat related to this: I only found out recently that you can use foam board to insulate above your original roof deck when replacing your roof, creating a cold roof to prevent ice dams and allowing you to convert your vented roof into an unvented roof.

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view

That is one of the things that inspired the 2 ceiling concept. So essentially, the entire roof rafter bays are vented and the XPS is underneath in mine and the second ceiling is insulated right up to that XPS. Going to do a modified version of that process in teh article on my attic this fall...hopefully. Still waiting for the quote from the roofer...to see if i have any $$ left over...

 

[Jaugust124]

Swedishchef,
As far as the quote for $1875 goes, I'm not sure if its good or bad. One thing I forgot to add was that the contractor was also going to add a plywood barrier. (I think that's what he called it.) I've never worked with soffits before, si I really have no idea how tough it is to take them down and put them back up. I have a friend who used to do some siding work, so I will discuss it with him.

The contractor actually gave me a quote of just about $9000 to add insulation throughout the house where needed, including the attic, basement ceiling, cantilevered floor, spray foam the rim joist, sealing up air leaks throughout the house, and adding the rafter vents in the attic. I am tackling everything that I think I can handle. Right now I am not paying anyone to do anything. When Irene came through it caused my septic tank (which was under the sidewalk) to collapse and we got hit with an unexpected major expense.

I figure if I do a little here and a little there, it will all add up over time.