Attic flooring, should insulation be covered?

Atlantic Dawn

Looking at flooring my attic in the coming weeks, have been reading up about what it involves, some mention the need to allow the insulation to breath by leaving a gap between the flooring and the roof joists to avoid moisture build up.

I'm looking at using those T&G Chipboard packs you pick up at DIY stores, the joists are around 100mm high, would be looking at renewing insulation to fill this 100mm gap prior to adding the flooring, should I use less than 100mm insulation for the gap.

Anyone done this before, any advice appreciated.

TaraGreene

You can use 100 mm insulation or you can use a smaller one in thickness (say 50mm-70mm) and leave the gap for the insulation to breathe which is also recommended and would save you some money. Anyways before purchasing the joists try asking the seller for advice since he has probably had experience with this questions, also have a look at different websites for tips since you are doing it yourself (e.g. [Snip] and any other DIY sites) or engineering books.

RKQ

Atlantic Dawn wrote: »

Looking at flooring my attic in the coming weeks.

OP you seem to be getting floor insulation confused with roof insulation.
There is no requirement to let insulation breath in a timber floor. Fill your floor the full depth of the joist. Fibre-glass, Rockwool, Polystyrene, PIR etc are inert materials that do not breath.

Ideally an attic would have at least 300mm thick fibre glass insulation. Kingspan or similar PIR boards could be used instead, subject to calculations. 100mm between joists, 100mm x 2 over joist, perpendicular to joists. Again it would depend on your joist support / spans, u-value calculations and your budget.

If the attic floor is only 100mm joist thickness then you need to consider the weight of the proposed flooring. As 100mm joists are not great for a floor. Check your joist support / spans.
Can you describe your house type?

There is a building regulation to maintain 50mm clear air gap between fibre glass insulation and the underside of bitumen roofing felt - but this does not affect your query. See TGD F page 28.

sydthebeat

RKQ wrote: »

OP you seem to be getting floor insulation confused with roof insulation.
There is no requirement to let insulation breath in a timber floor. Fill your floor the full depth of the joist. Fibre-glass, Rockwool, Polystyrene, PIR etc are inert materials that do not breath.
.

The OPs not confused, they definitely should design a solution to allow condensate to exhaust between quilt insulation and a chipboard flooring in a cold attic. If chipboard is laid directly on top of quilt it will turn to mush within a year.

I would suggest building up either with a proprietary raising system like this, or creating something yourself out of wood. Leave min 50 mm gap between insulation and deck.

also, 'breathability' of construction materials has nothing to do with whether they are inert or not..... its a phrase to describe the ability, or inability, of moisture vapour to move through it. Fibreglass and rockwool are actually quite good "breathable" insulations.

RKQ

sydthebeat wrote: »

. If chipboard is laid directly on top of quilt it will turn to mush within a year.

Can you point me towards a BS report or building regulation on this?

Surely a well ventilated attic with prevent condensation in the attic. Otherwise all attics would have water dripping off the inside of its roofing felt.

I have chipboard in my attic for 10 years ands its fine - can you explain this? Its tight on 100mm fibre glass.

The majority of semi's in Ireland have 100 - 120 joists in their attic's, fully insulated with fibre glass. Are you saying that anyone that installs these chipboard flooring planks over their insulation, will see them turn to mush with 12 months?

sydthebeat

RKQ wrote: »

Can you point me towards a BS report or building regulation on this?

Surely a well ventilated attic with prevent condensation in the attic. Otherwise all attics would have water dripping off the inside of its roofing felt.

I have chipboard in my attic for 10 years ands its fine - can you explain this? Its tight on 100mm fibre glass.

The majority of semi's in Ireland have 100 - 120 joists in their attic's, fully insulated with fibre glass. Are you saying that anyone that installs these chipboard flooring planks over their insulation, will see them turn to mush with 12 months?

The reason theres a 50mm gap required on sloped insulation is because vapour condenses to moisture within the insulation plane. A vented gap exhausts this moisture.
If you cover over a quilt insulation with chipboard, the moisture has no where to exhaust to, and the chip board gets wet, obviously.

Wet chipboard is, well, useless to say the least.
I have seen the results and its not pretty.

Id suggest the reason yours is fine is because 100mm quilt isnt enough to allow a dew point be created within the insulation, coupled with the fact that it has probably sagged over the 10 years creating an unintentional vent gap......

todays standards are much higher at 300mm quilt so a dew point is pretty much always guaranteed.
IM not talking about fitting down onto 100mm joists, im talking about allowing 300mm quilt (as you specified above) to vent away, by building up a structure over the existing joists

4Sticks

2.6(a) To reduce the risk of condensation on the
underside of access boarding:
use slatted boarding on the access way, or
support panel flooring at least 25 mm above the top of
the insulation to allow air movement between the
flooring and the insulation

Source Page 11 of BR 262 Thermal Insulation Assessing Risks , BRE 2002 edition.

(you have to buy it )

RKQ

sydthebeat wrote: »

yours is fine is because 100mm quilt isnt enough to allow a dew point

The OP is using 100mm quilt.:D

I've no gaps in my smooth level chipboard, its perfect because condensation is not a problem.

I've lots of experience myself but I believe in science fact. If I get time I will test your theory in the software.

We could exchange personal experiences all day but this will only sidetrack the tread and do nothing to help the OP.

As sid seems to agree, 100mm thick fibre glass dew point will not cause any condensation. So fill your joist with fibre glass to get the maximum insulation value that you can.

I'm not a structural Engineer so I won't discuss proprietary raising system like this. Please do consider the extra weight of this flooring.

4Sticks

@ RKQ - be careful not to quote selectively- the full quote here with my empahisis for your further guidance.

sydthebeat wrote: »

100mm quilt isnt enough to allow a dew point be created within the insulation, coupled with the fact that it has probably sagged over the 10 years creating an unintentional vent gap......

sydthebeat

RKQ wrote: »

The OP is using 100mm quilt.

I've no gaps in my smooth level chipboard, its perfect because condensation is not a problem.

I've lots of experience myself but I believe in science fact. If I get time I will test your theory in the software.

We could exchange personal experiences all day but this will only sidetrack the tread and do nothing to help the OP.

As sid seems to agree, 100mm thick fibre glass dew point will not cause any condensation. So fill your joist with fibre glass to get the maximum insulation value that you can.

I'm not a structural Engineer so I won't discuss proprietary raising system like this. Please do consider the extra weight of this flooring.

i would suggest they include modern standard of 300mm.

100mm quilt insulation is highly insufficient for an insulation level.

and on the bolded bit, i certainly did not suggest this.

RKQ

I fear we are getting off subject.
The OP asked specifically about 100mm insulation. I answered the query honestly.

@ 4sticks As regards selective quoting, Sid had no reason to assume the boading is sagging. I notice this quote was edited
"
2.6(a) To reduce the risk of condensation on the underside of access boarding: l position the cold water storage tank on a raised platform directly adjacent to the loft hatch, or l use slatted boarding on the access way, or l support panel flooring at least 25 mm above the top of the insulation to allow air movement between the flooring and the insulation"

I've given my honest opinion to the OP and answered the specific question. "100mm quilt isnt enough to allow a dew point be created within the insulation."

The OP asked the question on 5th February yet neither 4sticks or sid answered the OP until today, only after I gave my opinion. Why is that Sid?

sydthebeat

RKQ wrote: »

The OP asked the question on 5th February yet neither 4sticks or sid answered the OP until today, only after I gave my opinion. Why is that Sid?

i had to pick up in the errors and inaccuracies in the two posts after the OP.

Ive also given my opinion, which the OP can take or leave, i would just hope they would not take unnecessary risks, and id hope they would upgrade their proposed specification.

4Sticks

Atlantic Dawn wrote: »

Looking at flooring my attic in the coming weeks, have been reading up about what it involves, some mention the need to allow the insulation to breath by leaving a gap between the flooring and the roof joists to avoid moisture build up.

I'm looking at using those T&G Chipboard packs you pick up at DIY stores, the joists are around 100mm high, would be looking at renewing insulation to fill this 100mm gap prior to adding the flooring, should I use less than 100mm insulation for the gap.

Anyone done this before, any advice appreciated.

There is not a consensus view. Looking at what you have posted here in isolation I would use 100mm quilt but also use 25mm timber battens laid across the top of the joists before laying the boarding over.

DOCARCH

RKQ wrote: »

The OP asked the question on 5th February yet neither 4sticks or sid answered the OP until today, only after I gave my opinion. Why is that Sid?

Mod Note: Let's take it down a notch. Nobody is having a go.

baby fish

What area of floor is actually being covered?
How many boards wide are you hoping to put down?

Is it a truss or cut roof? What centres are the joists at?

With 100mm joists, if its a cut roof the joist spans can't be very wide. it would seem like a very small attic. I cant see that amount of TG flooring causing a problem, I'm only guessing at the moment with knowing the actual attic size to be floored

If its a truss roof, it will be even less of a problem because of less area to floor with even more obstacles

OP, if you are worried, why don't you get 8 X 4 sheets of ply 3/4 inch, rip into 2@ 8 X 2 or 4 @ 4 X 2, put these down with expansion gaps between each board, this will allow moisture through if are concerned about it , should be no problem with this