Insulating with foam board instead of spray foam.


  #1  
Old 08-01-11, 04:51 PM
D
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,344
Upvotes: 0
Received 1 Upvote on 1 Post
Insulating with foam board instead of spray foam.

My long awaited project of insulating has finally come to the front burner. The details: I am insulating my second floor which is a finished attic. I largely have cathedralized ceilings. The rafters are 5 3/8". The roof is two intersecting gables. This renders the rafters bays unventable so that is the type of roof I will be going for. My original plan was to use spray foam to seal and get maximum R value. I was hoping to do flash and batt, but I don't have the depth. I ran the numbers of CC spray foam and they were large. In an attempt to bring down the cost I am wondering if I can cut foam sheets and stick them between the rafters. Then seal the gaps with CCSF. Is this a poor idea?
 
  #2  
Old 08-01-11, 05:25 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
Planning for a similar project and as you say, spray foam doesn't make the cut. But cutting and fitting all of that foam board isn't fun either. One trick is to cut the foam board narrow to leave a workable gap on each side. You can't get a good fill using can foam on a 1/16" gap. With 1/4" each side, everything goes together quickly and you come back with the can foam and you're done.

The other product I'm looking at is mineral wool. No air will get through it and it can be cut if needed. It's a bit lower in r-value, but you could add 2" of rigid over the everything to add r-value and eliminate some of the thermal bridging from the rafters.

As long as you will be covering the inside with a fire barrier, ie sheetrock, I know of no problems.

Getting your rigid to add up to exactly your rafter depth can be a challenge.

Bud
 
  #3  
Old 08-01-11, 05:36 PM
D
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,344
Upvotes: 0
Received 1 Upvote on 1 Post
I figured as much as far as the gap is concerned. I could probably cut most of it on my table saw. I considered using the two part DIY spray foam for the gap filling. Not sure how the cost compares with can foam. I don't mind all the cutting and fitting. With the CCSF looking like it will cost north of $7k, I'll do what I can to cut that cost in half.

Getting the rigid to add up to 5" shouldn't be too hard. I don't need to fill that final 3/8".

I'm not sure I feel secure enough with just the mineral wool. With not having any roof venting, I'm very concerned about condensation.
 
  #4  
Old 08-01-11, 06:58 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
I cut a bunch of blue stuff and did some on my table saw. Kept melting the foam and would grab the blade. I had to shave the plastic off of the blade before every cut. Ended up cutting half, flip, cut the other half. Then went to a long blade box cutter.

They make a hot knife for cutting foam board. They look like something you could make yourself. I watched a guy on a commercial job use a hot wire, slick.

Keep us up to date.

Bud
 
  #5  
Old 08-02-11, 03:16 AM
D
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,344
Upvotes: 0
Received 1 Upvote on 1 Post
Interesting. Didn't think about it melting. I don't want to gum up my only saw blade. Did you try scoring and snapping it?
 
  #6  
Old 08-02-11, 05:11 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
I did and didn't get a good edge. The utility knives with the small snap-off segments can be extended and used at full length. If you set up a cutting table where the foam board is crowned slightly so that as you cut the cut opens, it works better. The problem is then holding your blade at a 90° angle.

For other projects I have attached a wider base onto my saber saw to hold it perpendicular to my work. Thoughts are that the slower moving blade might cut the foam without melting and the wider base could be run along a guide for straight cuts. That's on my list to test.

I had to remove some 2" foam board where they had meticulously cut every piece to fit tight. They must have used a piece of wood to drive these into place, but they certainly wouldn't have allowed any air to seep in. Since it is a slow process anyway, it might be worth a try to custom cut each piece and press them in. Saves on fastening them and the extra foam. Plus, if you cut it wrong, you can still trim it a bit more and use the foam to seal.

Bud
 
  #7  
Old 08-02-11, 03:52 PM
D
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,344
Upvotes: 0
Received 1 Upvote on 1 Post
I may try the jigsaw myself. They sell extra long blades that should do the trick. I'll give the tight fit a try too. I'm going to have to do some minor tapping in not matter what with the first sheet to get all the roofing nails into it. If I get a good fit like that, I can use caulk to seal the edges to make sure there is no air leakage there. Any known issues with silicone vs XPS?
 
  #8  
Old 08-02-11, 05:21 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
Silicone should be fine. Most tubes will list if there is a problem. I purchased 6 tubes of special foam adhesive once and evidently it is a slow mover. Junk and had to take it all back. They tried to give me a hard time because it had been 10 days and I explained "I WAS BUSY" grrr.

Boy, don't I wish spray foam was cheap.

Bud
 
  #9  
Old 08-02-11, 07:39 PM
D
Member
Join Date: Aug 2011
Location: US
Posts: 1
Upvotes: 0
Received 0 Upvotes on 0 Posts
i actually just made a post today almost exactly like this on another diy forum. I have gutted 4 rooms down to the studs. My house was built in 1958 and amazingly enough it had insulation but it all compress and fell to the bottom of the wall. I am a fan of spray foam but not the price. I want to use 2" xps foam board then finish of the last 1-1.5" with a diy kit of CC spray foam. that would give me R16-19 plus and air tight wall from the outside.

I actually plan on doing this to the entire house a couple rooms at a time then replacing my 28 year old HVAC system.

The only problem is i cant seem to find anything on this anywhere. I have seen a ton of stuff on flash and batt but i do not want to use fiberglass.

Does anyone know if there can be any major negative side effects from this?

Brad
 
  #10  
Old 08-03-11, 05:19 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
For many years we have heard from the spray foam people that their product performs much better than the standard tested r-values would indicate. My understanding is that much of this improved performance is due to the air sealing and the uniformity (quality) of the install.

Having said that, the down side is air tight can be a problem. You need to maintain approximately one air change every 3 hours, you currently have more than that. There are different ways that number is calculated, but it clearly says super air tight isn't good. But cutting a hole in the side of your home right after you went to extremes to seal it up just does not seem right. The answer is, you want controlled air exchange, when and where it should be. Left to typical construction you get moisture coming and going in places that can be bad.

When done, you will need to determine if your home is a candidate for extra air. When that time comes there are a couple of ways to guess or you can have a blower door test run.

The other down side to foam insulation is combustion by-products. Don't stay inside the house if it is burning. Install several good smoke detectors and proper egress.

One more. I don't like flash and batt. Besides the required fiberglass insulation, the surface of the flash is so irregular that even the fg has trouble forming to it. Lots of unwanted random air pockets. Rigid foam board is uniform and can be fit to the building shell and covered with additional rigid, mineral wool, or even (yuck) fg. If the final layer is spray foamed, it can be trimmed flush.

Bud
 
  #11  
Old 08-03-11, 02:27 PM
D
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,344
Upvotes: 0
Received 1 Upvote on 1 Post
Originally Posted by Bud9051

The other down side to foam insulation is combustion by-products. Don't stay inside the house if it is burning. Install several good smoke detectors and proper egress.
I don't think anyone is going to have a problem with this one.

On the DIY CCSF web site I was looking at, they do explain that the improved performance is from air sealing.

When doing flash and batt, 50% or more of the R value should be in foam. That will prevent condensation issues where your flash and batt meets. You could always opt for a mineral batt over fiberglass. You could even spray your foam and then do the rest with cellulose. I think they have a version that is sticky that you can spray in before you put up the drywall. You could also put up the drywall and then fill your wall cavities. I suppose you would need a certain amount of room for the cellulose to be blown in. There some other insulations that could probably work in this instance as well.
 
  #12  
Old 08-05-11, 03:52 PM
D
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,344
Upvotes: 0
Received 1 Upvote on 1 Post
The CCSF guy recommended just doing 3" of foam for about an R-20. I am trying to maximize my R-value, but will the extra R-5 make much of a difference? I remember seeing a chart somewhere showing the diminished returns of adding more R-value. I wish I could locate that chart again. I think I saw it via a link in the forum.
 
  #13  
Old 08-05-11, 05:58 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
I had such a link, but it returns "page not found". Looks like they moved to facebook.

Anyway, there is a flaw in the economic return analysis and that is, it is based upon a variable energy cost. What may be a poor investment today could well be a valuable one 5 years from now. Add in the fact that you are there now and the extra is easy, I say go for tomorrows energy cost.

I plan on filling my 5.5" cavities almost full and then covering everything with 2 more inches of foil faced rigid.

Bud
 
  #14  
Old 08-06-11, 07:18 AM
D
Member
Thread Starter
Join Date: Aug 2009
Location: USA
Posts: 4,344
Upvotes: 0
Received 1 Upvote on 1 Post
I came across this book online about spray foam. It is written by someone who was spraying polyurethane foam since the 1970's. It discusses everything from R-value to fire hazard. Despite the authors background, I found the book to be honest. http://static.monolithic.com/pdfs/foambook.pdf
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: