Concrete slabs v timber joist?? Heat loss concerns.

[theReel]

Hi Guys,

I am currently in the planning stage of a bungalow in a rual setting. I am trying to decide between concrete floor slabs or timber joists.

Although it is a bungalow we are planning on having stairs to a room in the roofspace used occasionally as a guest room. There is also space for another room but we have no need for it (yet) so will leave it as an attic room.

So now for the slabs / joists debate.

Price wise I have been told there is not much in it with the slabs being slightly more expensive.

Concrete slabs would be the best for noise but as the room is not going to be used a lot that doesn't bother me too much.

I am restricted height wise (I want maximise height space upstairs) and I believe slabs would take up more room than joists, as they would require service voids and battons/screed for 1st floor.

I have heard that concrete slabs are cold, can be hard to get air tight and can act as a big thermal bridge from all external walls (I have a block build with cavity insulation, parge coat in inner leaf and insulated plasterboard with warm roof.) This worries me but they are used in passive homes so there must be a correct way of installing them.

For the reasons above I was leaning towards timber joists. One last twist - I am also planning on having underfloor heating downstairs and was considering the same for upstairs but I am not sure how effective UFH is with timber joists. (could use low temp rads as alternative)

Has anyone had the same debate, disagree with my points, good/bad experiences, I would love to hear your thoughts.

Thanks

 

[dub_nerd]

I have concrete slab and UFH downstairs and upstairs in a dormer bungalow. I can't really answer your technical points (I'm not very savvy about building), but it is an extremely comfortable style of heating (although very slow to get going from cold -- about 1 degree per hour), and the ceiling is essentially noise proof.

 

[theReel]

Thanks dub_nerd. There is no doubt the slabs are great for noise but I'm not that worried about noise. I like the idea of UFH too but not sure how well it will work with an oil boiler. What do you use to heat the water for your UFH?

 

[lowCO2design]

the slabs / joists debate.

1. Price wise I have been told there is not much in it with the slabs being slightly more expensive.
2. I believe slabs would take up more room than joists, as they would require service voids and battons/screed for 1st floor.
3. I have heard that concrete slabs are cold,
4. can be hard to get air tight
5. and can act as a big thermal bridge from all external walls
6. (I have a block build with cavity insulation, parge coat in inner leaf and insulated plasterboard with warm roof.)
7. This worries me but they are used in passive homes so there must be a correct way of installing them.
8. I am also planning on having underfloor heating downstairs and was considering the same for upstairs but I am not sure how effective UFH is with timber joists. (could use low temp rads as alternative)

1. has your QS costed the extra block walls and foundations of PC slabs into the equation
2. correct
3. they should be inside the heated envelope so why would they be cold?
4. you have to wrap the edge of slab, but its just a detail
5. they shouldn't
6. your arch could provide a better wall build up on your new house
7. exactly, attention to detail is the key
8. i dont see the point in ufh up stairs to be honest

 

[dub_nerd]

Thanks dub_nerd. There is no doubt the slabs are great for noise but I'm not that worried about noise. I like the idea of UFH too but not sure how well it will work with an oil boiler. What do you use to heat the water for your UFH?

Mine's oil. The UFH runs at a much lower temperature than boiler temp, but that's looked after by mixer valves on the UFH manifold.

 

[theReel]

1. has your QS costed the extra block walls and foundations of PC slabs into the equation
2. correct
3. they should be inside the heated envelope so why would they be cold?
4. you have to wrap the edge of slab, but its just a detail
5. they shouldn't
6. your arch could provide a better wall build up on your new house
7. exactly, attention to detail is the key
8. i dont see the point in ufh up stairs to be honest

Thanks for Replies guys,

1) Slabs where on the original plans so foundations are OK.
3) Architect only allowed for 4" block, 4" cavity, 4" block, so looks like part of my heated envelope will have to be insulated plasterboard on the inside and I am afraid that the slabs would breach that albeit only as far as the cavity.
6) I fully agree. My foundations went down 6 years ago but only getting to finish build now and unfortunately I didn't know any better back then. I now understand the importance of increasing the insulation and I am trying to play catch up. They have now recommend I take the dry lining approach to improve the insulation/air tightness - from what I have read this is not easy to get right. I can't afford to switch to new architects so I am trying to come up with a affordable solution to my predicament. Ideally I would love to get a second opinion but I am not sure who to go to for professional advise.
8) Why not go for ufh upstairs? My thinking was if I go for Rads then it wouldn't work as well with geothermal/solar heating as they operate at lower temps.

 

[lowCO2design]

Thanks for Replies guys,

3) Architect only allowed for 4" block, 4" cavity, 4" block, so looks like part of my heated envelope will have to be insulated plasterboard on the inside and I am afraid that the slabs would breach that albeit only as far as the cavity.
6) I fully agree. My foundations went down 6 years ago but only getting to finish build now and unfortunately I didn't know any better back then. I now understand the importance of increasing the insulation and I am trying to play catch up. They have now recommend I take the dry lining approach to improve the insulation/air tightness - from what I have read this is not easy to get right. I can't afford to switch to new architects so I am trying to come up with a affordable solution to my predicament. Ideally I would love to get a second opinion but I am not sure who to go to for professional advise.
8) Why not go for ufh upstairs? My thinking was if I go for Rads then it wouldn't work as well with geothermal/solar heating as they operate at lower temps.

3/ speak to your structural eng. i would not accept the slabs bridging the cavity
6/ if you dry-line you loose the continuity at all internal wall, floor and concrete slab junctions, then there is the problem of where the due point will be, so you need to consider carefully how much dry-lining can be installed. then there's the services, kitchen and air-tightness considerations amongst others.
8/ suit yourself with only 100mm cavity i can only persume the ground floor insulation is light as well. you ideally need lots of insulation for ufh to work efficiently