New Home Begins

   / New Home Begins #391  
I think Dave makes a good point about the need to differentiate between roofs and walls.

Ice dams are probably not a big issue in your climate I am guessing? Around here, they are very common in a normal winter, and do lead to leaks in ceilings when enough melt water backs up behind the ice dam to flood the shingles. Maybe the "cold roof" air gap between the purlins in your case will prevent that.
 
   / New Home Begins #392  
Tom, no problem - and I fully understand. Let me explain a bit.

First things... ice dams are a roof problem only. Air sealing and insulation in the ROOF are the root of ice dam problems. And yes, there are a lot of problems in older houses in this area. You can get away with a lot more in walls, and all it affects is your heating/cooling bill.

The second issue is the type of construction.

Had you built a "typical" truss or rafter roof with blown-in or batt insulation, then you would be in a different realm. What you have is an "insulation contact roof deck" - that is different. With a typical truss roof, you blow in insulation over the ceiling and the roof deck is vented to the outside making it cool. That solves a lot of roof deck issues as there are no thermal bridges to the deck (If it is done right..). With a SIPs type roof (like you have), the issues are different. There is no vent space keeping the roof deck the same temp as outdoors. Therefore you need to keep it cool to avoid melting snow. The only way is by building a deck that is air tight and insulated enough to keep the heat in and the snow happy outside. With thermal bridges like you are showing with the panels, you will have snow melt issues, which may result in ice dam & leak problems (a lot depends on your local climate...)

You can get away with a lot more sloppy construction if you have a vented attic above your ceiling, but you do not. Even the sloppy guys there are learning it is not great to do that too... But the issue here is there is nothing between your heated living space and the outside except the insulation in the roof deck. If there are thermal bridges, they will cause problems by melting snow and making the water run down the roof.

Not sure if I am making my point, but if you held a consistent R30 or R39 then you would be fine in either case. The differential between the R39 foam and the R7 studs, is the problem. Make sense? feel free to ask more if I am not explaining it well enough...

And this is the key point - if you have ice dam problems after building, you will have to strip the shingles and lay down 1-2" of foam. How much fun will that be vs doing it now?


I am glad to see the Superior Walls worked out well. I suspected it was our local outfit that was the problem here, but I wanted to caution you to make sure you were aware.

I am impressed with your progress and really love the look of the T&G ceiling. Please don't take my comments as trying to drag you down, though I know that is easy to do. I'm trying to help by applying accepted Building Science practices here.... The envelope is the critical piece...
 
   / New Home Begins #393  
Tom,

thank you for sharing the link to your build. Great build and progress. I'm looking forward to following your steps as you progress.

Jeremy
 
   / New Home Begins #394  
I too, looked at the roof and thought, this was a weak link in this system. A cold roof system would help with the ice dams, but I would be more concerned with the cold ceiling, at the 2x4s, if moisture might condense and leave water damaged stripes in your ceiling? A friend, bought a home that had T&G ceiling in the dormer, it had vertical stripes 2" wide, but had fiberglass,not foam.
It is a great looking home and building sight.
 
   / New Home Begins
  • Thread Starter
#395  
Dave(s),

Although I am not in the trades professionally, I have been around them most of my life so I do understand the concepts and differences you point out. I also pour over trade journals such as JLC, Builder, Remodeler and FHB to keep up on new products, techniques, trends and best practices.

At least in our area, the temps are not as bad. There are a lot of homes in the area with cathedral ceilings which fall into the same category as what I have. And I know that a cold roof like mine are NOT the norm on these other homes. I have not heard of problems caused by the bridging. Could be I have not asked the specific questions. Could be that at least in our area, the temps are more moderate compared to your areas.

Anyway, you have sufficiently scared the crap out of me to at least ask specific questions of Raycore, Architect, framers, etc before the purlins and decking go on. :D
 
   / New Home Begins #396  
Insulation is a good thing and it's best to maximize it if the cost is reasonable. However, when we are talking in the R40 range, the heat loss from the house is going to dominated by air infiltration and window losses. If it was my house, I would focus on good seals and long term stability. I have a concern that some of the high tech super insulation approaches may work fine when installed but lose a lot of effectiveness 30 or 40 years from now.

Also keep in mind that, because we are building our houses so tight now, many codes require outside air intake. I think any new house in a cold climate now should seriously consider an outside air heat recovery system. How much value is going from R40 to R45 if the code requires a 6 inch hole in your basement wall?
 
   / New Home Begins #397  
Oh well, if you are sufficiently scared, then mission accomplished :laughing:

Joking aside, I have no idea what the optimal balance between building cost and R-value is in your area. I do think insulation is often under-valued. In the long run, insulation allows becoming financially uncoupled from energy markets. Green or otherwise, energy is expensive.

Home building is a funny thing. Practices with known downsides are used for decades. In part to avoid risk, and also just because "that's the way we do things." When forced by economics, changes are experimented with, some of those turn out to be duds, and some succeed and become the new standards that go on to outlive their value.

Generations of Mainers have shoveled their roofs to prevent collapse and ice dam problems. They probably did the same math; knowing that if they spent enough during construction, 95% of that shoveling could be avoided. The same applies to going through 15 cords of wood every winter.

Economically, it boils down to what value is placed on the permanence of homes from two perspectives: how long we typically expect to live in a home, and how long the home and it's neighborhood are expected to maintain value. There are reasons we don't spend enough to build "forever" houses, and we have to live with those reasons--in every sense.
 
   / New Home Begins #398  
KennyG said:
Insulation is a good thing and it's best to maximize it if the cost is reasonable. However, when we are talking in the R40 range, the heat loss from the house is going to dominated by air infiltration and window losses. If it was my house, I would focus on good seals and long term stability. I have a concern that some of the high tech super insulation approaches may work fine when installed but lose a lot of effectiveness 30 or 40 years from now.

Also keep in mind that, because we are building our houses so tight now, many codes require outside air intake. I think any new house in a cold climate now should seriously consider an outside air heat recovery system. How much value is going from R40 to R45 if the code requires a 6 inch hole in your basement wall?
Click to expand...

I have an outside air heat recovery system. It essentially is a joke and if I ran it 24/7 like I supposed to would get zero benefits from my insulation. Could as easily leave a window open. It does strip some heat/cool energy but I would guess in the range of 10%.
 
   / New Home Begins #399  
KennyG said:
Insulation is a good thing and it's best to maximize it if the cost is reasonable. However, when we are talking in the R40 range, the heat loss from the house is going to dominated by air infiltration and window losses. If it was my house, I would focus on good seals and long term stability. I have a concern that some of the high tech super insulation approaches may work fine when installed but lose a lot of effectiveness 30 or 40 years from now.

Also keep in mind that, because we are building our houses so tight now, many codes require outside air intake. I think any new house in a cold climate now should seriously consider an outside air heat recovery system. How much value is going from R40 to R45 if the code requires a 6 inch hole in your basement wall?
Click to expand...

The air tightness of new housing and the low efficiency of heat exchangers are good reasons to take care in what materials are built and brought into a house. Carpet, wood finishes, paints, vinyl anything and radon abatement are all areas where good choices need to be made. Reducing the pollution load in inside air reduces the need for air exchange while still maintaining healthy indoor air quality.

Boosting the cfm's well beyond what is typically installed, and having exhaust vents that actually go outside, for bathroom vents and range hoods can really help. In a tight house, you need to allow in make-up air for those exhaust fans. Cracking a window open works even if it is low-tech.
 
You must log in or register to reply here.
 

Footer Links 1

Top