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Hello,

I am planning to insulate the ground floor in my victorian house. Currently the floor consists of the original floating floor floorboards with a gap to the ground of 30cm covered with laminate. The air flow under the boards i good, which results in very cold floors in wintertime but no damp anywhere..

I read all the disaster stories of damp resulting from insulating in between the joists and the labour intensive ways to prevent this from happening.

I was thinking of not touching the original floor and covering the floorboards with pressure proof insulation boards and then put floor heating on top of that, using spreader plates or screed and then an engineered wooden floor. I do not have a problem with making the floor higher, I can go up to 7cm without any impact on pipes or sockets getting too low to the floor of doors that need to be corrected.

My questions are:

1) How thick does the insulation need to be to effectively stop cold from coming up through the boards into the room.

2) Can I implement a screed floor over the underfloor heating pipes, or will this be too heavy for the standard joists under the floorboards

When I use insulation boards covered in foil, do I need a damp membrane between the insulation boards and the wooden floorboards.

thanks.
 

highup

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I'm no insulation expert, but I would insulate the underside of the house between the joists.
I was going to do work for someone that had an older double wide mobile home. The mobile home has skirting all the way around it but had very tiny vents and not very many of them. Moisture was building up underneath that area and swelled up the particle board flooring in the house.
I told him that he needed to put a vapor barrier on the ground.
I came back at a later date and discovered that he had a helper had stapled a plastic barrier to the floor joists in an attempt to stop moisture from going up towards the floor. That is the worst possible thing a person could do. The insulation needs to breathe so put in that plastic exactly on the joists sealed in any moisture that got in there and prevented any moisture that was already there from getting out. He just made the problem worse. The floor was soft in some spots so he in a contractor put new plywood throughout the entire place on top of that damp particle board. Oh dear that must have been expensive and a total waste of time since it only made things worse.
What I would do if I were you is insulate in between the joists and put down at least a 6 mil polyethylene barrier on the dirt underneath the house. It sounds like you already have plenty of ventilation.
All that said I'm not sure exactly which insulation would be best for you. I'm guessing an insulation that is totally unfaced to allow movement of moisture. They make rolls of craft faced insulation but I don't know if that's the right way to do it under your floor.
I'm trying to understand what you said. If your house is an old Victorian I'm assuming it has either fir or oak floors. You currently have a floating laminate floor and want to install something on top of that?
 

Joost

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Hi,
Thanks for the response!

I was going to take out the laminate and put the insulation on top of the old (pine?) floorboards. I have no damp problem at all, that's why I don't really want to mess with the current floor. Putting insulation on top of the floorboards would not introduce a new thermal barrier, so I would not expect any buildup of moisture under the insulation. This will also save me the hassle of taking up the old floorboards.

My original questions remain of course.
 

highup

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I'm having a hard time understanding this entire buildup. I'm thinking the original floors in a Victorian house would be oak or fir. You said the original floating floor. 🤔
Is is there any way you can draw up a side view of the layers of flooring and heating system etc. I think I'm kind of like a cross-section view.
 

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Sorry for that, of course it doesn't help using the wrong terminology. I have a suspended floor consisting of boards nailed to joists. on top of that I have a floating laminate topping. As anyone can read online, people tend to insulate between the joists and put down membranes as needed. As I do not have a problem with making the floor higher, I was hoping to just leave the floor as it is, take off the laminate and rebuild the floor on top of that, using insulation boards as stated in my first post. My only concern is damp between the insulation and the original floorboards, but that may not even be a concern.
 

highup

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Sorry for that, of course it doesn't help using the wrong terminology.

As I do not have a problem with making the floor higher, I was hoping to just leave the floor as it is, take off the laminate and rebuild the floor on top of that, using insulation boards as stated in my first post. My only concern is damp between the insulation and the original floorboards, but that may not even be a concern.
The wrong terminology just makes it a lot more fun. 😄
This is a new one. If I'm reading this right my math says you can go up as much as 2 and 3/4 in. (7cm)
That would mean the entire house otherwise you'd have a huge height transition in places. The front door and threshold would have to be modified a lot. What would happen at the bathroom. If it was built up also, you need to extend the toilet ring. If a water pipe burst or leaked somewhere, especially in a hidden place like a dishwasher hole, or behind the washing machine, water would somehow find its way down underneath all this insulation that you'd like to put down. It was a slow leak you'd never know it until there was extensive damage. I'm also concerned about what it would do to the value of the home.
Is there enough space under the house to just do standard insulation?
 

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I took all of that into account. The house (1890 build) does not have a downstairs bathroom and the kitchen plus utility room was added to the original house somewhere in the 50s and is concrete on sand. Getting that up to scratch is a separate project. The front door frame is put on top of the floor, so the actual door opening starts 8cm above the current floor. I need to move 3 sockets up a bit and that's it. all radiators go, so no problems there and I don't have any doors, everything is open plan. I added a ground floor plan.

The space under the house is 30cm under the joists and is just sand. All waterpipes for upstairs go straight up from the boiler in the kitchen, so there are no pipes under the suspended floor. No need to ever opening up the floor again. There are a few cables under the floor, but worst case scenario, looking into the future, when for instance rats decide a cable is a nice snack, I will reroute new cables via walls and ceilings.
 

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