Insulating suspended timber floor

[Van Bergan]

Hi all,
I have spent the better part of a day trying to decide the correct spec to insulate the GF suspended timber floors of my victorian semi-d which over lay my cellar.

Unfortunately, like most building renovation details there is seemingly no build-up which all agree is correct.
Two major points of contention are:

• Promotion for or against a VCL:
• Some recommend using a VCL for the usual wise reason of halting the transmission of warm moist air into the floor unit from the habitable room above.
  However when you think about this it doesn't really make sense as the cellar will almost certainly be more humid than the room above. Furthermore the VCL may even hinder evaporation of moisture in tight areas such as joist ends.

• Insulation type selection
• Some say that PIR is sufficient;
  • others say that PIR being non-breathable should not be used and that mineral wool should instead be used
    • others then say that mineral being non-hydroscopic should not be used, and that a natural fibre insulation should instead be used
      • then others contend that ground floors should not be insulated at all! (which @Toby Newell in a previous post actually makes a good case for).

I find doing the work is the easy part of period renovation; getting the design fixed the harder part by far.
I would appreciate to hear your take on the matter!

 

[Lacan07]

Well, you could probably put me down for the 'no insulation at all' camp, I've never been a fan of the idea but I appreciate that lots of people do it. My neighbour did it and installed under floor heating on top.

 

[a twig]

The only place I’ve done it was above a cellar, installed 100mm of rockwool between joists from beneath (no membrane or anything) secured by insulation netting and cross battens at 600mm intervals. Worked well in that situation but not sure how/if that applies to your situation

 

[Lacan07]

Apologies, it appears I may have misread your post. If its above a cellar and the access is easy then why not install and monitor the performance for a period of time? If you're concerned about moisture I would go with sheets wool batons or wood fibre.

 

[erikwoolf]

We insulated the ground floor. We went for natural insulation (hemp) and only the breathable (airtight) membrane at the end (2 part system). We were not sure about the value of the top membrane that acts as a VCL.
Things to point out: our joists do not go into the walls and we made sure there was healthy DPC in all places that the timber was touching the brick piers. We also cleaned the airbricks (2 of them had to be replaced with new ones) and that we have good ventilation underneath at least 15-20cm in most places.

 

[Van Bergan]

The only place I’ve done it was above a cellar, installed 100mm of rockwool between joists from beneath (no membrane or anything) secured by insulation netting and cross battens at 600mm intervals. Worked well in that situation but not sure how/if that applies to your situation

My floors are also over a cellar so similar in that sense at least.
In such an environment wool/wood based insulation (hydroscoptic) is supposedly preferred over rockwool/mineral wool. Although I don't know how much importance should be placed on that, as you say rockwool has performed fine in your installation.

 

[Van Bergan]

We insulated the ground floor. We went for natural insulation (hemp) and only the breathable (airtight) membrane at the end (2 part system). We were not sure about the value of the top membrane that acts as a VCL.
Things to point out: our joists do not go into the walls and we made sure there was healthy DPC in all places that the timber was touching the brick piers. We also cleaned the airbricks (2 of them had to be replaced with new ones) and that we have good ventilation underneath at least 15-20cm in most places.

I think the one thing that is unanimously agreed is that cross ventilation is key.
For me then, it follows that protecting the insulation from wind washing/thermal bypass is similarly important; therefore the breathable membrane is key component,

 

[Toby Newell]

I would go for a breathable insulation within the bounds of the joist as airflow is key.

Any membrane or foil backed PIR will cause condensation.

You are attempting to reduce heat transmission from the GF to the cellar. That's fine, but the first rule of damp club is to heat and ventilate.

Thus if your cellar is adequately heated and ventillated then there shouldn't be much of a temperature differential and thus not much of a loss (and as heat rises maybe even a gain)

I think the main benefit would be to reduce condensation of the humid rising cellar air onto the timber floor.

 

[Toby Newell]

Well, you could probably put me down for the 'no insulation at all' camp, I've never been a fan of the idea but I appreciate that lots of people do it. My neighbour did it and installed under floor heating on top.

As stated previously, heat rises and moist colder air from below can condense, insulation works better the higher you get, because physics. Putting under UFH is a prerequisite as you want the heat created to travel upwards and not downwards, although UFH in GF over a basement this isn't such an issue as you will just be making the basement warmer, then insulating the basement walls becomes more important...

 

[Toby Newell]

*edit* moist air from below, may not be much colder but probably will be closer to maximum saturation.