my parents are converting a portion of their 12.5x20 deck (12.5x12) to living space. my advise to them is to go for 2" of closed cell foam in the walls followed by R15 batts (should be R25-R29, if i'm reading correctly). for the floors, i said they should go for 2-4" of closed cell foam followed by R29 or so batts. then if necessary rigid foam board. the goal is to make the room comfortable at minimal heating cost. i realize that the rooms floor will most likely be cold due to the lack of conditioned (even basement) space underneath. does this sound like a good plan / options? how would you change things? thanks!

I am assuming by Deck, you mean a raised, not covered platform (to differentiate from a patio or porch) Assuming the deck is 2x4 or 2x6 wood construction, I would suggest first, getting underneath the deck to evaluate the wood. Mildew/moss can grow on the underside, even with treated lumber, depending on the age of it. It might be needed to replace the deck based on that inspection. Even if it doesn't need replacement, I would suggest insulating underneath the floor planks with the best foam you can find, to the depth of the support beams. Don't forget the moisture barrier. Then putting plywood under that. Might not be able to do it without re-building the deck, depends on the amount of space you have to crawl under it. If you can't crawl under it, and you have a gap between the deck floor and the house floor (where the house floor is higher) I would suggest building the deck floor to the height of the house floor (plywood over existing planks first) and filling all cavities with the best insulation you can find that will fit into the space you have.

Another question will be the purpose of this room. Will this be a utility room, perhaps an added bedroom? If a bedroom, you might need extra space for a closet. Ripping out the deck might allow you to build a bigger room, should that be needed.

it's a living room / den addition. the house was built in 2004 so it's relatively new. i was under the deck looking at it saturday. everything looks in tip-top shape. it's 2x8's or 2x10's (didn't measure) with plywood decking, covered by the roof of the house, currently at about an inch below finished floor grade, so with carpet and padding, it should be at the same level as the rest of the house. . the ground under the deck slopes away from the house, at about 4 feet closest to the house to 8-9 feet at the end of the deck. plenty of room to get under and take a look.

Sounds like a good project. Something else I would be considering....the footing of the deck. Can it hold all the excess weight without moving? All the extra lumber and drywall might be more than the footings were originally designed to hold or more importantly, considering it's on a slope, more weight than is necessary to make the posts slide. Will the doors to the rest of the living room remain (making this more of a side room) or will the existing wall be cut out? If the wall will be cut out, I would be trying to get the highest R Value I can. Your idea for the 2" closed foam is good, and I don't know which products you had in mind, perhaps some research on the spray polyurethane might help. I don't know if how "closed" that stuff is, but I know it can fill the entire cavity. To give you a little of my background, I am not a contractor/builder. Just a homeowner who likewise has an interest in these discussions. So, take my advice for what it's worth, no matter what I say, you will need to do more research to make sure my suggestions make sense to you. That said, back to the original question. For the floorboards, again, I would try to insulate as much as possible. I don't know if the spray stuff would be best there ( too deep of cavities and the globs could fall off before it sets), but air tightness and moisture barrier are a must. I would try to avoid any batts, unless you put it ontop of something that's already air tight. I also don't know where you live, so you may need less insulation than say someone who lives in IL (like myself). Another piece of advice: Make sure that the attachment point between the deck and the house is also air tight. Dependent on budget and time, it might also be a decent idea to "weatherize" underneath the deck. That is either pour sack concrete or use concrete blocks so wind won't get underneath the deck.

Good Luck!

The deck is supported on two 3 sides by the foundation, and open on the 4th. the deck/porch/whatever you want to call it is really an more of a room with 1 of its 4 walls missing. the joists are the same size / capacity (and probably the same joists) as the rest of the house. i don't see the extra weight as being an issue at all. currently there is a 6' slider, which would be moved to the new exterior wall and where the slider is now would be an open cavity. we're still researching what products to use (and trying to convince my cheapo dad that paying a bit more for insulation now, will save him tons in the future....) and how much, etc.

i'm in northeast connecticut. probably on par with IL.

thanks!