Home  5  Books  5  Magazines  5  News  5  GreenPro  5  HelpDesk  5  Your Cart  5  Register  5  Green Living Forum
Not signed in (Sign In)

Categories



green Building Press Book offers
Free UK delivery on all our books...

 Ecohouse 2 by Sue Roaf
Just £26.98

Ecohouse 2 by Sue Roaf


 Building With Cob
Just £25.00

building with cob


More great priced ecobuilding books here





Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthorjwd
    • CommentTimeMar 26th 2010
     
    When insulating a suspended timber joisted floor (with vented space below)if I just put insulation btween the joists will there not be significant cold bridging issues? Surely the timber wont insulate as well as extruded polystyrene.

    In my case the joists will be mounted on dwarf walls but I cant think of a way of providing a thermal break between them and the masonary. There will be wet underfloor heating between the joists.

    Cheers Jw

    Ps reposted from another thread at the suggestion of djh :shamed:
    • CommentAuthordjh
    • CommentTimeMar 26th 2010
     
    Hi JW,

    I'm not an expert on this subject so hopefully somebody else will chime in, but I do know enough to say that yes there are cold bridging issues. Adding extra insulation underneath or on top of the joists is the obvious way to minimise it but you need to be very careful about possible condensation and rot so make sure to find some good details.

    If you're using rigid insulation like XPS you also need to be careful not to leave gaps up the side of the joists that will let cold air bypass the insulation.

    The contacts between the joists and the dwarf walls are basically point bridges, so they're not as bad as a linear bridge would be.
  1.  
    Posted By: jwdWhen insulating a suspended timber joisted floor (with vented space below)if I just put insulation btween the joists will there not be significant cold bridging issues? Surely the timber wont insulate as well as extruded polystyrene.

    Yes, it makes a significant difference.

    For example, using 225mm solid timber joists at 400mm centres with a k value of 0.15 & 225mm of XPS between the joists with a k value of 0.030, the overall U value will be 0.20 compared to 0.13 for the XPS unbridged. This ignores perimeter effects & assumes the underfloor void is at outside air temperature.

    If you want to add another layer to reduce the cold bridging, 75mm of XPS with a k value of 0.030 will bring the U value back to 0.13. I would put this on top, so that it doesn't trap moisture in the joists. Any moisture that makes it through the top layer of XPS will be carried through the joist & safely vented to the underfloor void.

    Likewise, if you want to improve air tightness fit polyethylene on top of the joists/top layer of XPS & breathable roofing membrane under the joists/bottom layer of XPS.

    David
    • CommentAuthorevan
    • CommentTimeMar 29th 2010
     
    Hello, new here!
    I'm just starting a renovation of an old cottage in a village in Fife, with internal floors and walls dating from the 40s.

    I have read many of the previous threads on this, and have the same thought as the original poster.

    I can take up my floorboards relatively easily at this stage and install some sort of insulation and membrane.

    And I was thinking that a thin layer, say 10mm, of rigid board insulation (like that thin blue polystyrene stuff?), on top of the joists and the between-joist insulation, would help a lot to isolate the floorboards from the "cold" joists, and would also make the floors quieter to walk on.

    So my question is - would it actually be worth doing, can you still get that sort of insulation (last seen when my dad was building our house in the 80s), and is it likely to cause any problems?

    Thank you!
    • CommentAuthortony
    • CommentTimeMar 29th 2010
     
    The more insulation you can put in the better -- the stuff that you propose is too thin in my opinion.

    do you have loft insulation? can you do external wall insulation?
    • CommentAuthorevan
    • CommentTimeMar 29th 2010 edited
     
    Hi Tony, thanks for your comments.

    Just to clarify: I'm proposing 10mm on top of the joists *as well* as the usual 150mm of rockwool or similar between the joists. I thought 10mm would be better than none at all, which is the other option! Any more is going to end up making the floors too high. Just not sure if it's a complete waste of effort.

    Loft insulation: there's some flat, tired old glassfibre in the loft, from the 70s, under boards. I'll be lifting the boards and putting in a lot more, between and over the beams - I like that "recycled glass fleece" they sell at B&Q, as it's not itchy like glassfibre. Not sure if it's as good though.

    External wall insulation: this is tricky, the external walls are 1800s rubble wall to about 1m high, and then double-thickness brick above that. So on the outside, the wall is much wider at the base and then gets thinner. It looks like a nightmare to stick insulation to but I'd really welcome any suggestions.

    Of course the internal lath/plaster walls need redoing anyway so I can put insulation on the inside instead or as well.

    I should maybe have started a new thread :)

    edit: I see you can get 12mm celotex for example.
    • CommentAuthortony
    • CommentTimeMar 29th 2010
     
    If it was mine I would do 150 between ans 50mm batts under the floor joists and nothing above the joists but use a thin sheet of poly there as a combined vapour/air sealing barrier.

    glass quilt is OK for the loft -- walls I would build out to a flattish surface with a render mix with high polystyrene bead content then do say 200mm of EPS sheets covered with a mesh and render.
    • CommentAuthorevan
    • CommentTimeMar 29th 2010
     
    OK, fair enough.

    For the external wall insulation I have a couple of concerns:

    I need to read up on this a bit more, but, these walls have no foundation or DPC and it's a cold roof. I'd be concerned about all the heat from the rooms going into the walls and simply radiating up into the loft and conducting down into the ground. Isn't that a problem?

    Also, adding 200mm+ to the outside is going to make the wall extend beyond the roof edge and rain gutter! How is that dealt with normally? Should the windows be moved out into the insulation?

    Is it really worth doing compared to internal wall insulation, I suppose I'm asking.

    Thanks!
    • CommentAuthortony
    • CommentTimeMar 29th 2010
     
    What will the proposed occupancy be?

    You are right to be concerned about heat loss into the ground but there are ways to mitigate this -- up the walls and into the roof should not be happening or the insulation isn't done right
    • CommentAuthorevan
    • CommentTimeMar 29th 2010 edited
     
    Occupancy; 2 people, both working, out during weekdays. Heating will be wood boiler / radiators.

    It's quicker to warm up an internally-insulated space, right? But thermal mass is nice...

    edit: I'm going to start a new thread rather than continue dragging this one off topic, sorry about that.
    • CommentAuthorevan
    • CommentTimeMar 30th 2010 edited
     
    Posted By: tonyIf it was mine I would do 150 between ans 50mm batts under the floor joists and nothing above the joists but use a thin sheet of poly there as a combined vapour/air sealing barrier.


    Coming back to this topic, I think I will follow this advice. I'm considering the 200mm glass-wool (like the stuff currently on offer from B&Q) for between the joists.

    When you say 50mm batts under the joists (I assume cross-ways) what type of product do you mean exactly?
    I was thinking of something fairly rigid, polystyrene board of some sort, which can be nailed up to support the material beneath the boards. Is that right?

    Thanks again.
    • CommentAuthormjansen
    • CommentTimeMar 30th 2010
     
    <When you say 50mm batts under the joists (I assume cross-ways) what type of product do you mean exactly?>

    I had been thinking about the same and though Knauf Space Board might do for underneath
    the joists, perhaps fixed with insulation retaining washers/fixings into bottom of the joists.
    • CommentAuthortony
    • CommentTimeMar 30th 2010
     
    It is nice to mitigate the thermal bridging through the joists by fitting insulation under them#

    I prefer not to mix insulation and although sheet insulation is tempting I would use a rigid or semi rigid form of the same insulation you have between the joists.
  2.  
    Posted By: evanWhen you say 50mm batts under the joists (I assume cross-ways) what type of product do you mean exactly? I was thinking of something fairly rigid, polystyrene board of some sort, which can be nailed up to support the material beneath the boards. Is that right?

    Polystyrene is OK on top of or between the joists, but I wouldn't use it under the joists because it's not vapour open & there's a risk of trapping moisture in the joists. It's better to use one of the semi-rigid mineral wool batts mentioned by Tony below the joists. Take a look at Rockwool & Knauf.

    David
    • CommentAuthorevan
    • CommentTimeMar 30th 2010 edited
     
    Posted By: tonyIt is nice to mitigate the thermal bridging through the joists by fitting insulation under them#

    I prefer not to mix insulation and although sheet insulation is tempting I would use a rigid or semi rigid form of the same insulation you have between the joists.


    OK, got it. Knauff call it 'slab' insulation. I have heard of 'batts' before but assumed it was the american word for polystyrene boards. Just need to find where to buy it now... (edit: Wickes)

    Posted By: mjansen
    I had been thinking about the same and though Knauf Space Board might do for underneath
    the joists, perhaps fixed with insulation retaining washers/fixings into bottom of the joists.


    Have you seen the price of that stuff? There's no way I'd want to use something costing that much mainly to hold other, much cheaper insulation in place :)
    Anyway reading the other comments it seems that anything impermeable isn't a good idea.

    Thanks all
    • CommentAuthordjh
    • CommentTimeMar 30th 2010
     
    Posted By: davidfreeboroughPolystyrene is OK on top of or between the joists, but I wouldn't use it under the joists because it's not vapour open

    Doesn't that depend on the type of PS? I thought EPS is vapour permeable while XPS isn't?
    • CommentAuthorcromar
    • CommentTimeMar 30th 2010
     
    Just make sure you still have a good space between the bottom of the insulation and the solum. Current Scottish Building regs specify 150mm between bottoms of the joists and the solum so this is prob a good figure to go for. Don't fill up the space beneath the joists with insulation. Ventilation is awfy good at preventing rot!
    • CommentAuthorevan
    • CommentTimeMar 30th 2010 edited
     
    Took a floor up and the joists are supported on 3 more joists, which have very little clearance to the soil. I'm not inclined to try to put insulation around those but will dig some more space under it.
    Also should add some ventilation, there isn't any intentional holes I can see.
    Suprisingly little rot except where the roof has let the rain in though.
    • CommentAuthorjwd
    • CommentTimeMar 31st 2010
     
    Sorry not to have participated since my original post (work and small children get in the way) but i am glad to see it has triggered a lively conversation.

    In my case we will use underfloor heating so the idea of insulating above the joist is less attractive - I dont have a lot of height in the room and digging down further is not an option as there are no foundations to the exteral walls -another scottish rubble walled cottage.

    We had been advised of putting panelvent/osb boards down between the dwarf walls and the joists, taping the joins and using it as a breathable air tightness layer /something to stop the insulation from falling out.This however does not solve the cold bridging issue.

    The idea of putting a layer of insulation below the joists/osb sounds good for anti cold bridging but a nightmare of a fiddly job.

    I also realise that I said extruded polystyrene when i meant expanded in my earlier post. Having thought about it I could also use something pourable - warmcell or similar between the joists.

    Still much undecided

    thanks

    Jw
    • CommentAuthorevan
    • CommentTimeMar 31st 2010
     
    Hey JW. I'm currently swayed by the suggestions made in this thread (thank you!) and am hopeful that the "batts" of insulation won't be all that difficult to secure to the joists, if the floor is up anyway.

    I've yet to obtain one of these batts and see what it's actually like though...
    • CommentAuthorjwd
    • CommentTimeApr 1st 2010
     
    A breathable alternative might be something like pavatherm or Steico wood fibre board insulation under the joists. It insulates well - lamda values around 0.04 depending on the exact product. I guess the real question is what is the least amount of insulation required to stop the cold bridging but i dont have the knowledge to work that out.

    Any thoughts any one?

    Jw
    • CommentAuthorevan
    • CommentTimeApr 3rd 2010
     
    I've been looking at this today.

    How essential is the layer of plastic beneath the floorboards?

    I ask because I've discovered that the floorboards in the main rooms are *very* well nailed down, and will be a lot of trouble to take up. They are tightly fitted tongue and groove, and would be varnished / sealed on top. Would that be air-tight enough?

    There's just about enough room to crawl underneath.


    On the same subject, I bought some of the 50mm glassfibre batts to have a look at them. They're a bit floppy but a few wire nails at an angle should hold them up OK. The problem with them is the price. £30 for 7 square metres!

    170mm glasswool roll from Homebase is just £4 for the same area. And has a pretty similar W/mk value. So I'm highly tempted to use that and just figure out a way of holding it up.

    Any thoughts?
    • CommentAuthorevan
    • CommentTimeApr 3rd 2010 edited
     
    Posted By: jwdA breathable alternative might be something like pavatherm or Steico wood fibre board insulation under the joists. It insulates well - lamda values around 0.04 depending on the exact product. I guess the real question is what is the least amount of insulation required to stop the cold bridging but i dont have the knowledge to work that out.


    0.04 is similar to the 50mm batts mentioned so I don't see why not. It is mixin different materials though and it's been suggested that that's a bad idea.
  3.  
    The main reason for ventilating under timber floors is to dry the condensation which occurs on the bottom of the joists which is colder. If you insulate underneath the joists they become warmer and the condensation risk is reduced/eliminated. Then you ask the question, so why do we still need to ventilating under the floor? If we don't need to ventilate then why don't we fill up the void under the floors completly with EPS beads? improving airtightness at the same time. Maybe there is no dpc in the walls so the ventilation is to dry rising damp but a French drain around the house should sort that!

    When you insulate under the joists the joist ends sitting in the wall can show a 8-10 degree lower temperature than the middle of the joists. The joists will assume a similar humidity/moisture level to the internal RH, this moisture moves towards the joist end by capillary and hygroscopic action and will condense as it gets colder, causing joist end rot if this situation persists for over 30 days. The drying mechanism which was the airflow under the joists is eliminated by the insulation under the joists, so this could be a problem!

    Regarding your dwarf walls, which are usually to take out the bounce from the floor and are less structural. You could build these shorter and put a layer of EPS between them and the joists, just a thought!
  4.  
    My understanding is that ventilation is important to help prevent wet and dry rot, not just condensation.
    If a small leak developes ina pipe under the floor for example, you want ventilation to prevent dangerously humid conditions being generated that will encourage unseen mould preoblems. Condensation is one issue but there are others. My instinct is that it is never advisable to completly seal off a void in a building without some form of ventilation, just in case. There are many spaces in a building that might be covered up for years and years without anyone checking until a problem becomes so bad its noticible on the surface, by which time its a very exspensive problem.
    • CommentAuthorevan
    • CommentTimeApr 4th 2010
     
    So if I understand correctly, the main purpose of the plastic layer is to stop humidity from the room soaking through to the joists, where it will make its way to the end, condense and saturate them?

    Again, doesn't a good layer of varnish stop this?

    Anyway I may have thought of a way to get the boards up - a 6mm core drill used around each nail to lift the board off the nail without wrecking the tongue. Ordered the bit, not tried it yet...
  5.  
    • CommentAuthorevan
    • CommentTimeApr 4th 2010
     
    Thanks Mike, interesting article and discussion.

    The article is suggesting that a vapor barrier above the insulation can make matters worse.
    So that sounds like another reason for leaving it out?

    What about radon?
    • CommentAuthorMike George
    • CommentTimeApr 5th 2010 edited
     
    Hi Evan, yes there is a risk with a VB on the warm side, though most [including me] appear to think this is very minimal in a UK climate.

    There is also the issue of what happens to any liquid spilled onto the carpet above the barrier..

    Personally, I prefer to have a suspended floor construction which will allow the vapour to go right through, ie vapour open materials.

    Sometimes t&g is impossible to get up without destroying it. I would try punching the nails in with a nail punch. Then try and raise the boards from the grooved side with a flat edge crowbar or bolster chisel. Running an electric skillsaw along the groove cutting through the tongue is also very effective but laborious. [Obviously you need to set the blade depth at only 10mm or so so that you do not cut through any pipes and/or cables below the boards.

    Use strawberry netting draped over and between then joists to support any of the flexi insulations.

    Airtightness can be achieved using a sheet material such as hardboard tacked on top of the floorboards, which will also cover any minor damage caused during lifting
    • CommentAuthorevan
    • CommentTimeApr 5th 2010
     
    Thanks Mike.

    In my case, under the floor isn't intentionally ventilated. But, it's dry and no signs of mould or rot on the timbers - 90 year old untreated timber.

    There are no homes above the radon action level in this area according to UKRadon (from their sample).

    I can get under there to add insulation, with some difficulty. I can take all the boards up with considerably more difficulty! I could add the plastic layer, I could even add vents.

    Currently I'm inclined to put insulation up from the underside and hope for the best...
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press