Closing It In

I've finished cutting the timbers for my 12'x24' cottage, and now I'm planning next steps. My floor joists and rafters are 4"x6" Hemlock, on 2' centers. I plan to do the floor in 2" T&G Kiln dried Hemlock (planed to 1 1/2"). The ceiling will be 1" T&G Hemlock, planed to 3/4". My problem is that I want to insulate the floor and ceiling, and I'm not sure how do to that. Do I effectively have to make two floors and two ceilings, with rigid insulation in between? What about an air space in the roof?

Bryson:

If you live in snow country, a ventilated roof is essential for preventing ice-dams. When there is snow on the roof it forms an insulation sandwich with the installed roof insulation. The R-factor of the snow is about R-1/inch. Thus, you can quickly calculate that it would take about an R-90 roof to insure that the roof deck is always cold enough to prevent melting. Even if you are not in snow country, a ventilated roof will extend the life of your roofing material and prevent condensation between the roofing and the underlying deck.

The insulation in the floor must be applied with attention to vapor barriers (vapor from both the building interior and from the earth below. The goal is to prevent condensation from occurring on the structural timbers. In addition, you need to install some sort of physical barrier so that the insulation does not become a convenient nest for varmints. For the later, you could use metal screening similar to plaster lath.

Brian:

Thanks for your comments. I sure do live in snow country (the cottage is near Peticodiac, New Brunswick, Canada), so roof ventilation is an issue. Here's what I plan to do - cover the ceiling with 3/4" T&G, followed by plastic vapour barrier on top of the T&G, followed by 2" rigid foam, followed by an air space (2" or 4"?), followed by another layer of 3/4" T&G, followed by cedar shingles.

On the floor, to get around the rodent problem, and the fact that my joists are only about eight inches off the ground, I plan to cover the entire floor with 3/4" T&G, followed by 2" rigid foam, followed by vapor barrier, followed by Kiln dried 1 1/2" T&G (the finished floor). Is this overkill? What do you suggest as nailers for the flooring and roof? I'm thinking of 2"x2", laid on top of the joists/rafters, but I'm worried about heat transfer.

Overall, your plan is a sound one. However, 2" of rigid insulation is not much R-factor. I don't know if you are planning to use EPS or Isocyanurate, but assuming that it's the higher R ISO (at about R-7/inch), you'd only end up with maybe R-19 after you add in the other benefits of the deck etc. Thus, I'd recommend at least 4" in the floor and maybe even 6" in the ceiling. There is no need for the 2" to 4" of ventilation channel that you mention. It is only necessary to insure that the cross sectional area of the channel is equal to the inlet area and the exit area at the ridge. From my experience this can be easily accomplished by installing cheap 5/4" strapping on 16" centers to provide the channel. Because your size is small, there won't be much savings from installing rigid foam to avoid the expense of stresskins. So, you might consider just purchasing ISO panels faced on one side with OSB and then nailing the strapping onto the OSB (outer) side.

Good luck,


Brian

Bryson:
I agree with Brian. Also, I wouldn't worry about heat loss through the joists and spacers in the second layer. You will lose some heat no matter what you do. Brian's suggestion to use 2x4s and ISO panels faced with OSB will take care of most of it. Also, the ones I have seen come with lateral recesses in either 16" or 24" OC so you can apply them like sheets of decking and acheive the desired R factor. Then you put on your vapor barrier and roofing materials.
As to the critters, there is no getting around them. I would use some hardware cloth strips wide enough to bend and come up at least a little ways on the sides of the joists on each side and fold up the full height on the bottom and top ends. If you get the 1/4" HWC, it will prevent a lot of the furry little beasts from making your home their home. I would do this in the floor and the roof. I am aware that it is an added expense as well as an expenditure of extra energy and time, but it will pay you dividends in the long run, and that's what timber framing is all about, isn't it?

Keith in Houston

Keith:

Thanks for your suggestions. I'm a novice in this field, so I'm going to have to ask for clarification on a couple of your points.

You mentioned using 2x4's on top of the ISO panels, and Brian talked about using 5/4 strapping. Are you talking about the same things here? Would I nail these through the ISO panels, and then place my roofing on top?

I'm not clear how the laterial recesses you mentioned work. They are on 2'centers - are you suggesting using these recesses for the strapping? What about the vapour barrier. Shouldn't it go on the heated side, i.e. before the ISO panels, not on top of them?

Critter prevention is a major objective for me. I'm not familar with the hardware cloth you mentioned. How wide is this stuff? Would it wrap around the joist and rafters, and then across the floor or roof?

Thanks for your help.


Bryson