Not sure I follow exactly on your heating question, but I'll throw this out there.So, on to the discussion with the heating contractor.
As you can see in the surveillance camera shots the current porch has a crawl space under it. That is why there is a missing block in the foundation. There are additional missing blocks on the other three sides as of today. 2 of those walls are going to be closed off and I am not sure what we need to do for code to maintain ventilation. Because it is dirt under the porch the ventilation is needed in our area because of radon issues though where we are at specifically it isn't much of an issue but code is code. So, this means that this is potentially cold space below the existing porch. As for the addition to the porch the wall with the patio door will move to the new foundation and new trusses go on the porch. However as you can see there is no block between the garage and this new porch space. This is because this new section will have cement poured under it. This is for code of the radon issue. I asked if the cement under the porch will be heated, they said it can be so I said yes. Now we get into our issue.
The plan originally was to cut the subfloor in the porch about 1' from the walls. Pull it up in the middle. Then lay a reflective material above the insulation, staple the heating loops along the floor joists in what will be the corner up against the subfloor and then put it back down. In talking over this plan with the heating guy he said that this won't be very efficient. He recommended pouring 1" of cement over the subfloor with the lines in that. However that will raise the floor an inch plus the thickness of the floor. We are thinking that click together vinyl but would be open to ceramic tile as well. We don't like that idea because then it will cause the floor to be a strange step down into the house. If we want to go that route, we will have to tear up the entire subfloor and lower the floor joists. Of course this isn't in budget.
So, I think I will ask what it will add to the cost. We can tear up the subfloor and possibly prep things. The hard thing is if we have them do it all, does it add how much to the cost and what does it really mean to efficiency? I mean what is the real pay off? If it adds $10/mo to heat it with just the coils under the subfloor vs the cement, but it adds $3-4K to the bill, It is a lot of extra work for something that we may never see a return on. I do get that the cement would provide more of a thermal mass where it provides a more even heat. It might have hot spots with just nailing it to floor joists without the cement.
After they left I thought of another idea. Maybe this doesn't make sense but I will toss it out. I suspect that the subfloor is two sheets of plywood. What if we did one sheet of plywood and one sheet of cement board? Would it just crack because of too much deflection of the plywood? What if we added in additional floor joists? Not sure what the spacing is but we might have room to add even just a 2x4 or 2x6 to beef it up a bit for less flex? It might be a less expensive compromise.
The other idea that the heating contractor had which I dismissed was baseboard heat. We have baseboard heat out there now that is electric so the thought that would be a good option. However the ultimate goal of the porch is to turn half of it into a laundry room with cabinets all the way around that half and the other half a mud room with those build in locker style coat hook/shoe storage that you see in many newer homes. In this use, I don't see how we could do baseboard heat and he agreed. The only other idea was a heat exchanger of some sort but we didn't think that was a good option. Because of this we went back to in floor heat as we originally planned on.
One other thing he mentioned is that he will install a couple extra stubs so that we can run a loop to the furnace. He said that we can install a heat exchanger in there. Since the boiler is running anyhow and is more efficient, the furnace will become more of a backup for us. I need to get a quote on that but I think we will add the PEX run to the basement through another hole through the block for this so we can add it later a little easier.
I think we are going to find out what the cost is for the cement option. The general contractor mentioned around $1000-1300 for the pour, not sure on labor or materials. That is why I said $3K above as a guess.
What are your thoughts? Do you thing cement board might work as a third option?
My parents put heated floors in both levels of the house. Basement it's in the concrete, upper levels its in the joists.
It WAS laid so it would be tight to the subfloor, but when they went to lay the floor they were worried about hitting a pipe with a nail, so they dropped it down. Problem is it never got put back up tight to the subfloor.
It still "works", but the floor doesn't feel warm except for a few spots where the pipe ended up really close yet.