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-   -   Insulate open rafter roof (http://www.houserepairtalk.com/f34/insulate-open-rafter-roof-3126/)

busyflyin 11-21-2007 05:38 PM

Insulate open rafter roof
 
We have a second lake home with an exposed 2X8 dougfir rafter ceiling. Shealthing is 1X10 pine boards with shingles directly on top of that. The building was built in 1956 and intended as a summer cottage but we want to use it year-around. I want to insulate the roof. I'm thinking of removing the shingles and applying 2" foam board, re-shealth with 1/2 OSB and shingle over that. I've also thought about installing an additional 1" on the inside between the rafters and covering that with beadboard and paint to brighten the ceiling while allowing the rafters to show through. Anyone else have better idea?

travelover 11-21-2007 05:46 PM

It seems like it would be a lot easier and more effective to add 6 " fiberglass insulation to the underside of the roof joists and cover with drywall or some other covering to fit your needs. You'd want to add vents to the soffits and ridge.

pgriz 11-22-2007 08:17 AM

You probably need R38 or R40 for your ceiling/roof. Depending on what kind of foam insulation you install, you get anywhere from R2 to R6 per inch. Fiberglas bats are usually around R3-R4. So your first challenge is to get enough insulation into your roof system to avoid heating the outside.

Your second challenge is to ensure you have an appropriate vapour barrier preventing leakage of warm interior air into the cold spaces, where water vapour will condense and create moisture.

Unless you can construct your roof assembly as a sealed unit (ie, completely impremeable to water vapour), you also need to have adequate ventilation to dry out whatever moisture gets trapped in the roof assembly.

Bottom line is that converting a structure that is not intended to be heated or insulated into one that is, is a much bigger job than you are anticipating. I would suggest consulting with a qualified building engineer to understand what changes have to happen. Don't forget that you also have to deal with converting the windows, walls, and maybe even the flooring if you're changing the use from a 3-season cottage to a 4-season home. It can be done, but it is not a trivial undertaking. Better you get the uh-oh reaction now rather than after you've spent the effort and money and found out that there are condensation issues all over the place.

glennjanie 11-22-2007 09:18 AM

Welcome BusyFlyin:
I'm surprised PGriz didn't mention this method.
Cover the roof with 2" (or more) foam board and install steel roofing directly over the foam (forget the extra sheathing). You may want to remove the present shingles to reduce the weight on the roof but the most important part is to get the proper size screws. You want them to go into a solid wood base but not penetrate the inside of your ceiling.
Glenn

pgriz 11-22-2007 05:26 PM

glennjanie, the reason I didn't mention the method is because it wouldn't actually deal with the issues that I brought up in my post.

Yes it can be done, but it won't provide adequate insulation, nor will that method introduce a good vapour barrier to eliminate condensation, or will it provide for ventilation. Busyflyin did not indicate whether the use of the cottage year-round was to be occasional, or frequent. That would determine how stringent are the requirements for vapour barrier, insulation, and ventilation. At this point, not enough information.

inspectorD 11-25-2007 04:47 PM

Yup
 
In order to do it correctly, you need alot of info concerning many things.
Is it a crawl space underneath.
Are there single pain windows.
Is the heat propane or gas. This is a big one.
How much do you use the cottage.
Do you need it just warm or are you going for the 4 season , live there someday solution.
These will get you started, then we may be able to figure out the best solution. Just throwing insulation around an uninsulated house can cause many serious issues with condensation, backdrafting and mold.
But I do like the idea of insulation with metal roofs, works great most of the time, we just need more detailed info.
Post back and ask anything you want....we are her for ya.;)

busyflyin 11-25-2007 07:18 PM

Thanks for all of the feedback. Correction... the rafters are 2x6.. 50 years old and straight as an arrow. See the underside of the roof (ceiling). I'm not exactly sure what would be required to protect against moisture, but if i'm understanding the issue, the moisture would build between the top of the foam board and the sheathing?.. given that it (condensation) occurs when the warm moist air hits the cold air. What changes if I leave the shingles on? yes, we intend for this to be a 4 season cottage.. Michigan where temps will hit zero or below a few days of the winter. A typical winter day is going to be high teens to low 20's. The heat is a propane wall furnace, but we will primarily burn wood in a fireplace insert stove. As for the fiberglass on the underside, the walls are only 7', I think, and covering the rafters may cause the perception of a smaller building and would certainly cover the pine ceiling. At this point, I am trying to understand the issues and find the best solution for us. I appreciate your continued feedback. Will figure out how to reduce the photos I have and send them later. To answer the other questions; slab, not crawl. Single pane now, but upgrading to thermo panes in the spring. We will use the cottage every weekend. thanks!

busyflyin 11-25-2007 07:29 PM

2 Attachment(s)
OK... here are two photos of the underside. Let me know if the detail is not clear enough.

pgriz 11-27-2007 07:38 AM

Busyflyin, you have a beautiful cottage. If that was my place, I'd fill in the space between the rafters with blue styrofoam (5 1/2" should give you about R25-R30), install a vapour barrier across the rafters along the bottom edge, then nail on pine planks as the visible ceiling surface. This way, you get the wood look, and still have insulation in the ceiling. On the outside, I'd get rid of the shingles, install another two inches of blue styrofoam (giving you another R8-R10), vertical battens (2" high) then horizontal battens, then a vertical panel metal roof (with concealed fasteners). The vertical battens will allow vertical movement of air, while the horizontal battens will support the metal roof panels. You'd probably want to put a synthetic membrane like Triflex-30 over the vertical top styrofoam to catch any condensation moisture which may form on the underside of the metal roof and drip down. You would need to provide for venting at the top of the panels and along the lower edge. This way you would have a well-insulated, reasonably well ventilated roof assembly, that still retains a lot of the appeal of the wood interior finish. I like metal both for the looks and for its durability.

If you don't go with a metal roof, then putting on a plywood deck over the vertical battens will give you a good nailing surface for shingles.

That's what I would do. You still need advice regarding the walls, windows, floors. As I noted earlier, changing from a 3-season (essentially unheated) cottage to a 4-season, fully-heated cottage means that you have to re-engineer your dwelling for its new purpose.

On the other hand, as long as you don't mind heating the outdoors and having drafts, you could just use the cottage without making many changes. You'll have lot's of heat loss and you will have condensation on the inside walls and ceilings. But if the cottage is large and the number of people inside is small, then the amount of water vapour that will be added to the inside may not cause a huge condensation issue.

busyflyin 11-27-2007 04:19 PM

pgriz; I'm not sure I want a steel roof... we just build an adjacent garage and used asphalt shingles... so I want to match the look. Also, aslphalt is more economical. So if I understand you, on the top side... 2" blue foam followed by verticle battens to allow for ventilation, then osb, then standard roofing w/ridge vent. This would allow for sufficient ventilation? The walls are no issue for me. We intend to gut the building and wire, insulate, and such, just as a new home. Thanks for all of your advise. It's well taken and appreciated.


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