roof insulation upgrade (sorta off topic)

realolman

My wife and I are looking at possibly buying a log house. It obviously has issues with roof leaks.  I believe it is an ice dam issue due to lack of insulation.  Guy says they have the ice shield stuff on there now.



It appears to me that the roof consists of log rafters covered by tongue and groove boards with ( I assume ) a couple inches of foam board and then a roof deck and asphalt shingles.



would anyone have any suggestions / ideas about how to improve upon that?  I thought about possibly removing the shingles and framing out a roof 6-8" above the existing foam board and filling it with spray foam and putting a deck and shingles above that. That would cause a strangely thick facia though.



 Any insights or suggestions would be appreciated.

Gordy

On the right track

 So you got a cathedral ceiling, with little insulation. It would be nice to know exactly what the construction is. 



 Sometimes ice dams are not always caused by insufficient insulation unless of course its a consistant problem.



 Sometimes ice dams occur from weather factors. Usually when there is snow on the roof, and a warming trend with freezing nightly temps. The snow melts runs down the roof, and night fall freezing temps start layering ice up the roof.

Gordy

Gordy

On the right track

 So you got a cathedral ceiling, with little insulation. It would be nice to know exactly what the construction is. 



 Sometimes ice dams are not always caused by insufficient insulation unless of course its a consistant problem.



 Sometimes ice dams occur from weather factors. Usually when there is snow on the roof, and a warming trend with freezing nightly temps. The snow melts runs down the roof, and night fall freezing temps start layering ice up the roof.

Gordy

realolman

yes you are right

That's right a cathedral ceiling.



 from inside they have log rafters maybe 3-4 feet on center. on top of that are looks like tongue and groove pine boards....    looking from the outside, there couldn't possibly be more than a few inches insulation... i assume foam board.



I know you are correct about the possible weather conditions, but I'd like to eliminate the heat loss possibility.  I thought of stripping the existing roof down to the boards, framing in something that would hold some insulation, making sure there is ventilation from soffitt to roof , and then a plywood roof deck and a new roof, ... maybe a  steel roof.

Gordy

roof assembly

 If the log rafters are 3 or 4 foot on center, then I would have to believe that the T and G decking is of 2" nominal dimension to be able to carry between rafters. Could be less but IF there is not a load carrying roof on top of the cathedral roof, then I find it hard to believe the T and G decking is 3/4 inch.



 The house I built some years back had 4 x 8 DF timbers 4' on center then 3/4" t and g  decking nailed to the timbers.Then 2 x 8 rafters on top of the decking with fiberglass insulation between.  we held the rafters 3" above the t and g to allow for more insulation, and to leave a 1 1/2" air space above it to allow for venting circulation of the assembly. This roof was a 12 12 pitch. all this was tied in to a 21" x 6" laminate ridge beam.



I did a quick sketch sorry for being sloppy. Your new assembly needs to carry its own load, and not bear on the existing assembly.  Unless you want to hire an architect to do a load analysis. Your design would depend on pitch, and loading per codes in your area. obviously the less pitch the larger your rafters need to be. You can go larger to allow for more insulation.



As far as the facia you can cut the rafter tails so you have a 6" facia or what ever you like. Hard for me to tell with out pics of existing roof.,

D107

Then there's unvented attics

Building Science has alot of free info on this, but attached are a few pdfs.

Gordy

Vented verses unvented

 David in my opinion the Jury is still out. You can find more information on why you should not build unvented roof assemblies, Than you can on why you should use unvented roof assemblies.



Vented assemblies are more robust, and provide better drying for water permeable materials inside the assembly. Unvented assemblies need to be done with great care, and humidity with in the home tightly controlled during the winter.



 I know Real ole man is considering metal roofing so shingles do not play in. But I don't think you will find one asphalt shingle manufacturer that would condone if not condemn an unvented roof assembly. The unvented pros solution...don't use asphalt shingles yeah okay.



Gordy

D107

Unvented

Thanks Gordy, yes I know unvented is supposed to raise the temperature on those shingles, and thereby shorten their life span. Although Mr. Lstiburek and other proponents claim the percentage of increase temp is relatively low and that they have been tracking such roofs for many years with no real problems.



I think my answer to to the risk factor--since I'm due for a new roof in a few years, is to install an inch or two of Dow styrofoam above the sheathing. I'll have to research to see if that would dissipate the heat to the shingles better. Of course, there may be a limit to how much can be added to the roof before re-installing the gutters might pose a problem --they might have to be built out to catch the water.

D107

double post

JJ_4

Structural Insulated Panels (SIPs)??

How about using SIP roof panels?  Here is some info from the manufactuer's organization.  Some FAQs address roofing use and roofing materials:

http://www.sips.org/content/index.cfm?pageId=269#33

A place I vacation used these on old log cabins that originally were not insulated and they wanted to keep the exposed beam and tongue and groove look inside.  They used standing seam metal roofing.