best way to insulate the roof on a timber frame house

[yarnammurt]

I am looking for the best way to insulate the roof of the house I am building. It is a 36x80 foot home with 24' of the middle timber framed. We will have 6-12 trusses on the end and tie the timber frame into it. So looking for ways to insulate. 

[mikeb1079]

excellent question i'll be following this thread closely....

[witterbound]

I put two layers of three inch polyphysomething on top of my t&g to insulate my roof.  I think the order was t&g, felt, 6 inches of insulation, 2x4s over rafters with 12 inch screws through into rafters, sheathing, felt and metal roof.

[JimXJ2000]

Mine was similar to Witterbounds, but different.

I was/am re-using an old barn with principle rafters, about 12 feet apart.  We reused the old sheathing (which was in rough shape on the top, but very nice on the bottom) which ran eave to ridge.   Connecting the rafters were purlins about 4 feet apart on each bay.   

On top of the old sheathing we made "boxes" of 2X8's with a 3 and a 4 inch sheet of polyisocyanurte inside the boxes.  The boxes were sized to the ends ended on the principle rafters.  We did have to trim all the insulation sheets to make them fit on the insides of the boxes which were 4.5 inches less than 4 feet tall and about 12 feet long.  (We doubled up the 2X8s on the top of the boxes so when assembled we had three 2X8's  every 4 feet going up the roof to make an upper purlin to help the old purlins and transfer loads to the rafters.).   

The boxes then went on as modules and were screwed to the rafters with 12 inch screws.  On top of that was ZIP sheathing and tape.  On top of that went a metal roof. 

For the eave and gable ends we made roof extensions out of sheathing and 2X lumber to get the overhangs we wanted.  Those got screwed the insulation boxes.  The ridge ended up being a "V" shape between the top boxes from either side.  Into this "V" we added a doubled up 2X8 as a ridge beam, blocked and screwed that to the boxes and inside ridge beam.  Filled the gaps with fiberglass and sheathed over. 

The insulation sheets were find on craigslist.  Overall the process when well.  The ZIP sheathing and tape are a bit of $$.  Overall I would say it went on for about half to a third the cost of SIP panels - may not have all the strength advantages of SIPs...but the frame really didn't need that either.  The barn builder installed the modules.  We did most of the insulation cutting and sheathing.    Gives and R value of close to 50. 

I did have to plane down a few of the 2X8s to make things level on the top.  the frame had variations and the 2X8s did as well.  That wasn't a lot of fun to plane things on the roof...but I did justify buying a hand power plane for it. 

I'll find some pictures to post. 

If you think if doing something similar let me know...I might do things a bit different if I was doing it again.

This was the only picture I could find right now ( I have many more at home).  It shows the edge of the 2X8 boxes and one of the gable overhang extensions in place.



 

[Ljohnsaw]

Good thread and interested to follow...

[JimXJ2000]

Better pictures of the insulation "boxes" being made and installed.



  

  

[Brian_Weekley]

Jim,

Are you concerned about any thermal bridging through the decking and wood strips surrounding the foam?  It seems all the seams are in the same place compared to continuous layers of foam that are staggered and taped at the seams.

Brian

[frwinks]

You def want a continuous layers of foam all around the frame. I have some detailed drawings in my gallery of the roof/wall transition, which is usually a tough spot to insulate....rigid foam and REMOTE approach make it easy ;D

   

[JimXJ2000]

There is a weak spot in the insulation where the 2X8's are.   I didn't see an easy way around that - while adding support to the frame.   For the actual insulation sheets we needed roughly 1 and half sheets of each the three and four inch think layers, so the butt joints don't line up there.   Once the panels were in place we injected several cans of spray foam with an injection gun into the edges and gaps.  The foam was pretty easy to cut straight so the gaps were quite small.  With the injection gun it could get several inches into the foam to help seal. 

I had looked at frwinks pictures while we were thinking of ideas.  If we didn't need to add some strength to the 12 ft rafter spans we would have done a similar approach. 

[canopy]

12', wow. I've never heard of anything close to that kind of span. Would be interesting to see a picture of the roof frame.

[D L Bahler]

We use a practice where there is a double roof system in place, essentially separating the solar properties of the roof covering from the thermal performance of the house.

So what we might do is, put sheathing on top of the rafters as normal, then a layer of maybe 4 to 6 inches of insulating foam. Then, a second set of rafters is fastened on top of this foam that are screwed down with heavy lag screws into the structural rafters. To this, then, we apply sheathing and whatever roof covering is used. The space between these rafters is left open and they are vented at the bottom and at the ridge to allow for a very efficient air-exchange. This dramatically improves the performance of any roof, you will even have a cool attic space during the height of the summer.

Really it is similar to witterbound's system. A key is to have a cavity between your roof surface and your insulation so that you have a good ventilation to get the heat away during the summer.

[Brian_Weekley]

Here's my little cartoon depicting how I insulated my roof.  I made this as an exercise when I trying to figure out how I was going to do mine.  Although this small workshop only has 4 inches of insulation on the roof (2 x 2 inch layers, staggered and taped), the principle is the same.  I also included a path for air flow above the insulation and below the upper sheathing as David described.  The air enters the bottom through a Cor-A-Vent S-400 behind the fascia, follows the strapping, and exits through a ridge vent at the top.

https://www.youtube.com/watch?v=DclbGgw1GqE

[witterbound]

Mine is the same as the one D L describes much meter than I did. 

[Myles Franco]

I am curious. Does anyone think it is a good idea to apply a layer of poly between the T&G and foam insulation? This creates a continuous envelope when tied into the poly in the insulated exterior walls. The foam is then strapped above the foam and before the sheeting to allow ventilation.

Does anyone here advocate against a layer of poly?

[Brian_Weekley]

I don't see the poly would be necessary.  The closed cell foam is impermeable and would act as a vapor barrier by itself.  In my case, I did apply a layer of RoofTopGuard-2 synthetic underlayment (breathable) between the boards and foam.  However, I really put it on to protect the wood since it was several weeks before I was able to add the foam and finish the roof.  In the end, it does provide another layer of water protection too.  One more reason to add a layer of felt or synthetic underlayment on top of the T&G is to provide a "dark curtain" in the event you lose knots or have gaps between the boards (so you don't see the foam).

[Woulde]

I'm finally building my timberframe this year and did extensive research on insulation.

I am working with an architect from the timberframe company who has designed the structure and will also cut and assemble the frame for me. They are in the process of completing the construction drawings.

I'm planning to use a SIP manufacture called "Ray-Core".  They are a bit different than most SIP companies, in that they use a molded polyurethane product with a foil radiant vapor barrier in lieu of OSB.  The roof panels will be 8" thick (nominal) which they claim is R-52.  Also the 2" x 6" dimensional lumber used in their molded panel will be "staggard" to reduce thermal bridging.  Google them for more info.  Their price is significantly less than "traditional" SIP cos.

The traditional SIP manufacturer my timberframe company quoted me would only warrant their product if a cold, or ventilated roof was installed.  Meaning stringers had to be installed on top of their OSB panels and then another layer of OSB on top of the stringers.  I'm building a cold roof, but not using a double layer of OSB.  Instead, i'll have stringers installed on top of the Ray-Core and then install OSB or similar product on top.

I'm also planning to use a self sealing 40 mil membrane on the entire roof, not just the first few feet, like most roofers recommend.  This is sometimes called a "rain and ice" guard product. 

I probably would have gone with the timberframe companies recommended SIP product until digging into the warranty and discovering the "cold roof" requirement. 

Just my two cents.  I have no relations with Ray-Core, just think they make an excellent affordable product.